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tip.today - scheduler bam boom crash (cpu hotplug)

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  tip.today - scheduler  bam boom crash (cpu hotplug) Mike Galbraith <efault@gmx.de> - 2017-01-19 08:40 +0100
    Re: tip.today - scheduler  bam boom crash (cpu hotplug) Peter Zijlstra <peterz@infradead.org> - 2017-01-19 11:40 +0100
      Re: tip.today - scheduler  bam boom crash (cpu hotplug) Mike Galbraith <efault@gmx.de> - 2017-01-19 12:50 +0100
      Re: tip.today - scheduler  bam boom crash (cpu hotplug) Peter Zijlstra <peterz@infradead.org> - 2017-01-19 14:40 +0100
        Re: tip.today - scheduler  bam boom crash (cpu hotplug) Ingo Molnar <mingo@kernel.org> - 2017-01-19 18:00 +0100
          Re: tip.today - scheduler  bam boom crash (cpu hotplug) Peter Zijlstra <peterz@infradead.org> - 2017-01-19 18:30 +0100
        Re: sched/clock: Fix hotplug issue kbuild test robot <lkp@intel.com> - 2017-01-19 21:40 +0100
        Re: sched/clock: Fix hotplug issue kbuild test robot <lkp@intel.com> - 2017-01-19 21:50 +0100
        [tip:sched/core] sched/clock: Fix hotplug crash tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2017-01-20 08:50 +0100

#1562441 — tip.today - scheduler bam boom crash (cpu hotplug)

Mindless testing only, too sick to work, not sick enough to be immune
to boredom.  Was verifying first warning wasn't somehow rt inspired,
but while doing so, plain nopreempt (and no rt patch set) went boom.

[  203.088255] smpboot: CPU 1 is now offline
[  203.168181] smpboot: CPU 2 is now offline
[  203.221461] x86: Booting SMP configuration:
[  203.221464] smpboot: Booting Node 0 Processor 1 APIC 0x2
[  203.221728] ------------[ cut here ]------------
[  203.221733] WARNING: CPU: 1 PID: 0 at kernel/sched/clock.c:149 set_sched_clock_stable+0x43/0x50
[  203.221733] Modules linked in: nls_utf8(E) isofs(E) ebtable_filter(E) ebtables(E) fuse(E) nf_log_ipv6(E) xt_pkttype(E) xt_physdev(E) br_netfilter(E) nf_log_ipv4(E) nf_log_common(E) xt_LOG(E) xt_limit(E) af_packet(E) bridge(E) stp(E) llc(E) iscsi_ibft(E) iscsi_boot_sysfs(E) ip6t_REJECT(E) xt_tcpudp(E) nf_conntrack_ipv6(E) nf_defrag_ipv6(E) ip6table_raw(E) ipt_REJECT(E) iptable_raw(E) xt_CT(E) iptable_filter(E) ip6table_mangle(E) nf_conntrack_netbios_ns(E) nf_conntrack_broadcast(E) nf_conntrack_ipv4(E) nf_defrag_ipv4(E) ip_tables(E) xt_conntrack(E) nf_conntrack(E) ip6table_filter(E) ip6_tables(E) x_tables(E) nls_iso8859_1(E) snd_hda_codec_hdmi(E) nls_cp437(E) intel_rapl(E) x86_pkg_temp_thermal(E) intel_powerclamp(E) coretemp(E) kvm_intel(E) snd_hda_codec_realtek(E) snd_hda_codec_generic(E) kvm(E) snd_hda_intel(E)
[  203.221748]  snd_hda_codec(E) irqbypass(E) crct10dif_pclmul(E) snd_hda_core(E) snd_hwdep(E) nfsd(E) crc32_pclmul(E) crc32c_intel(E) ghash_clmulni_intel(E) pcbc(E) snd_pcm(E) auth_rpcgss(E) aesni_intel(E) aes_x86_64(E) snd_timer(E) nfs_acl(E) joydev(E) crypto_simd(E) snd(E) lockd(E) grace(E) iTCO_wdt(E) iTCO_vendor_support(E) lpc_ich(E) mei_me(E) i2c_i801(E) mei(E) pcspkr(E) glue_helper(E) mfd_core(E) shpchp(E) intel_smartconnect(E) sunrpc(E) soundcore(E) tpm_infineon(E) fan(E) thermal(E) battery(E) cryptd(E) efivarfs(E) sr_mod(E) cdrom(E) hid_logitech_hidpp(E) hid_logitech_dj(E) uas(E) usb_storage(E) hid_generic(E) usbhid(E) nouveau(E) wmi(E) i2c_algo_bit(E) drm_kms_helper(E) ahci(E) syscopyarea(E) sysfillrect(E) sysimgblt(E) fb_sys_fops(E) libahci(E) xhci_pci(E) ehci_pci(E) xhci_hcd(E) ehci_hcd(E)
[  203.221765]  ttm(E) libata(E) r8169(E) mii(E) drm(E) usbcore(E) fjes(E) video(E) button(E) sd_mod(E) vfat(E) fat(E) ext4(E) crc16(E) jbd2(E) mbcache(E) dm_mod(E) loop(E) sg(E) scsi_mod(E) autofs4(E)
[  203.221773] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G            E   4.10.0-tip-default #29
[  203.221774] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[  203.221774] Call Trace:
[  203.221778]  dump_stack+0x63/0x90
[  203.221780]  __warn+0xd1/0xf0
[  203.221782]  warn_slowpath_null+0x1d/0x20
[  203.221782]  set_sched_clock_stable+0x43/0x50
[  203.221784]  early_init_intel+0x225/0x360
[  203.221785]  init_intel+0x18/0x2d0
[  203.221786]  identify_cpu+0x2d1/0x4d0
[  203.221786]  identify_secondary_cpu+0x18/0x80
[  203.221789]  smp_store_cpu_info+0x3e/0x40
[  203.221790]  start_secondary+0x53/0x180
[  203.221791]  start_cpu+0x14/0x14
[  203.221792] ---[ end trace 262c7e4b746d5a76 ]---
....
[  207.525918] smpboot: CPU 2 is now offline
[  207.586516] smpboot: CPU 4 is now offline
[  207.642988] smpboot: CPU 6 is now offline
[  207.682207] x86: Booting SMP configuration:
[  207.682210] smpboot: Booting Node 0 Processor 1 APIC 0x2
[  207.682505] sched_clock: Marking stable (207412639708, 0)->(207410993286, 1646422)
[  207.706220] smpboot: Booting Node 0 Processor 2 APIC 0x4
[  207.706502] sched_clock: Marking stable (207524564558, 0)->(207522917721, 1646837)
[  207.730376] smpboot: Booting Node 0 Processor 4 APIC 0x1
[  207.730644] sched_clock: Marking stable (207585448402, 0)->(207583801309, 1647093)
[  207.754593] smpboot: Booting Node 0 Processor 6 APIC 0x5
[  207.754881] sched_clock: Marking stable (207641939733, 0)->(207640292394, 1647339)
[  207.802195] smpboot: CPU 3 is now offline
[  207.862396] smpboot: CPU 4 is now offline
[  207.927159] ------------[ cut here ]------------
[  207.927163] WARNING: CPU: 6 PID: 45 at kernel/sched/sched.h:807 assert_clock_updated.isra.62.part.63+0x25/0x27
[  207.927164] rq->clock_update_flags < RQCF_ACT_SKIP
[  207.927164] Modules linked in: nls_utf8(E) isofs(E) ebtable_filter(E) ebtables(E) fuse(E) nf_log_ipv6(E) xt_pkttype(E) xt_physdev(E) br_netfilter(E) nf_log_ipv4(E) nf_log_common(E) xt_LOG(E) xt_limit(E) af_packet(E) bridge(E) stp(E) llc(E) iscsi_ibft(E) iscsi_boot_sysfs(E) ip6t_REJECT(E) xt_tcpudp(E) nf_conntrack_ipv6(E) nf_defrag_ipv6(E) ip6table_raw(E) ipt_REJECT(E) iptable_raw(E) xt_CT(E) iptable_filter(E) ip6table_mangle(E) nf_conntrack_netbios_ns(E) nf_conntrack_broadcast(E) nf_conntrack_ipv4(E) nf_defrag_ipv4(E) ip_tables(E) xt_conntrack(E) nf_conntrack(E) ip6table_filter(E) ip6_tables(E) x_tables(E) nls_iso8859_1(E) snd_hda_codec_hdmi(E) nls_cp437(E) intel_rapl(E) x86_pkg_temp_thermal(E) intel_powerclamp(E) coretemp(E) kvm_intel(E) snd_hda_codec_realtek(E) snd_hda_codec_generic(E) kvm(E) snd_hda_intel(E)
[  207.927186]  snd_hda_codec(E) irqbypass(E) crct10dif_pclmul(E) snd_hda_core(E) snd_hwdep(E) nfsd(E) crc32_pclmul(E) crc32c_intel(E) ghash_clmulni_intel(E) pcbc(E) snd_pcm(E) auth_rpcgss(E) aesni_intel(E) aes_x86_64(E) snd_timer(E) nfs_acl(E) joydev(E) crypto_simd(E) snd(E) lockd(E) grace(E) iTCO_wdt(E) iTCO_vendor_support(E) lpc_ich(E) mei_me(E) i2c_i801(E) mei(E) pcspkr(E) glue_helper(E) mfd_core(E) shpchp(E) intel_smartconnect(E) sunrpc(E) soundcore(E) tpm_infineon(E) fan(E) thermal(E) battery(E) cryptd(E) efivarfs(E) sr_mod(E) cdrom(E) hid_logitech_hidpp(E) hid_logitech_dj(E) uas(E) usb_storage(E) hid_generic(E) usbhid(E) nouveau(E) wmi(E) i2c_algo_bit(E) drm_kms_helper(E) ahci(E) syscopyarea(E) sysfillrect(E) sysimgblt(E) fb_sys_fops(E) libahci(E) xhci_pci(E) ehci_pci(E) xhci_hcd(E) ehci_hcd(E)
[  207.927212]  ttm(E) libata(E) r8169(E) mii(E) drm(E) usbcore(E) fjes(E) video(E) button(E) sd_mod(E) vfat(E) fat(E) ext4(E) crc16(E) jbd2(E) mbcache(E) dm_mod(E) loop(E) sg(E) scsi_mod(E) autofs4(E)
[  207.927230] CPU: 6 PID: 45 Comm: migration/6 Tainted: G        W   E   4.10.0-tip-default #29
[  207.927231] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[  207.927231] Call Trace:
[  207.927235]  dump_stack+0x63/0x90
[  207.927238]  __warn+0xd1/0xf0
[  207.927240]  ? load_balance+0xa10/0xa10
[  207.927242]  warn_slowpath_fmt+0x4f/0x60
[  207.927244]  ? find_next_bit+0x19/0x20
[  207.927245]  ? cpumask_next_and+0x35/0x50
[  207.927246]  assert_clock_updated.isra.62.part.63+0x25/0x27
[  207.927247]  update_load_avg+0x855/0x950
[  207.927249]  ? load_balance+0xa10/0xa10
[  207.927250]  set_next_entity+0xa6/0x210
[  207.927252]  ? load_balance+0xa10/0xa10
[  207.927252]  pick_next_task_fair+0x78/0x550
[  207.927255]  ? sched_clock+0x9/0x10
[  207.927256]  ? sched_clock_cpu+0x11/0xc0
[  207.927257]  ? load_balance+0xa10/0xa10
[  207.927258]  sched_cpu_dying+0x251/0x2a0
[  207.927260]  ? fini_debug_store_on_cpu+0x34/0x40
[  207.927261]  ? sched_cpu_starting+0x60/0x60
[  207.927263]  cpuhp_invoke_callback+0x90/0x440
[  207.927265]  take_cpu_down+0x5e/0xa0
[  207.927267]  multi_cpu_stop+0xc4/0xf0
[  207.927268]  ? cpu_stop_queue_work+0xb0/0xb0
[  207.927269]  cpu_stopper_thread+0x96/0x120
[  207.927270]  smpboot_thread_fn+0x11a/0x1e0
[  207.927272]  kthread+0x10c/0x140
[  207.927273]  ? sort_range+0x30/0x30
[  207.927274]  ? kthread_parkme+0x40/0x40
[  207.927276]  ret_from_fork+0x2c/0x40
[  207.927277] ---[ end trace 262c7e4b746d5a77 ]---
[  207.927287] BUG: unable to handle kernel NULL pointer dereference at 0000000000000150
[  207.927289] IP: pick_next_task_fair+0x397/0x550
[  207.927289] PGD 0 
[  207.927290] 
[  207.927291] Oops: 0000 [#1] SMP
[  207.927294] Dumping ftrace buffer:
[  207.927296]    (ftrace buffer empty)
[  207.927296] Modules linked in: nls_utf8(E) isofs(E) ebtable_filter(E) ebtables(E) fuse(E) nf_log_ipv6(E) xt_pkttype(E) xt_physdev(E) br_netfilter(E) nf_log_ipv4(E) nf_log_common(E) xt_LOG(E) xt_limit(E) af_packet(E) bridge(E) stp(E) llc(E) iscsi_ibft(E) iscsi_boot_sysfs(E) ip6t_REJECT(E) xt_tcpudp(E) nf_conntrack_ipv6(E) nf_defrag_ipv6(E) ip6table_raw(E) ipt_REJECT(E) iptable_raw(E) xt_CT(E) iptable_filter(E) ip6table_mangle(E) nf_conntrack_netbios_ns(E) nf_conntrack_broadcast(E) nf_conntrack_ipv4(E) nf_defrag_ipv4(E) ip_tables(E) xt_conntrack(E) nf_conntrack(E) ip6table_filter(E) ip6_tables(E) x_tables(E) nls_iso8859_1(E) snd_hda_codec_hdmi(E) nls_cp437(E) intel_rapl(E) x86_pkg_temp_thermal(E) intel_powerclamp(E) coretemp(E) kvm_intel(E) snd_hda_codec_realtek(E) snd_hda_codec_generic(E) kvm(E) snd_hda_intel(E)
[  207.927317]  snd_hda_codec(E) irqbypass(E) crct10dif_pclmul(E) snd_hda_core(E) snd_hwdep(E) nfsd(E) crc32_pclmul(E) crc32c_intel(E) ghash_clmulni_intel(E) pcbc(E) snd_pcm(E) auth_rpcgss(E) aesni_intel(E) aes_x86_64(E) snd_timer(E) nfs_acl(E) joydev(E) crypto_simd(E) snd(E) lockd(E) grace(E) iTCO_wdt(E) iTCO_vendor_support(E) lpc_ich(E) mei_me(E) i2c_i801(E) mei(E) pcspkr(E) glue_helper(E) mfd_core(E) shpchp(E) intel_smartconnect(E) sunrpc(E) soundcore(E) tpm_infineon(E) fan(E) thermal(E) battery(E) cryptd(E) efivarfs(E) sr_mod(E) cdrom(E) hid_logitech_hidpp(E) hid_logitech_dj(E) uas(E) usb_storage(E) hid_generic(E) usbhid(E) nouveau(E) wmi(E) i2c_algo_bit(E) drm_kms_helper(E) ahci(E) syscopyarea(E) sysfillrect(E) sysimgblt(E) fb_sys_fops(E) libahci(E) xhci_pci(E) ehci_pci(E) xhci_hcd(E) ehci_hcd(E)
[  207.927332]  ttm(E) libata(E) r8169(E) mii(E) drm(E) usbcore(E) fjes(E) video(E) button(E) sd_mod(E) vfat(E) fat(E) ext4(E) crc16(E) jbd2(E) mbcache(E) dm_mod(E) loop(E) sg(E) scsi_mod(E) autofs4(E)
[  207.927347] CPU: 6 PID: 45 Comm: migration/6 Tainted: G        W   E   4.10.0-tip-default #29
[  207.927347] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[  207.927348] task: ffff88017df703c0 task.stack: ffffc90001a4c000
[  207.927349] RIP: 0010:pick_next_task_fair+0x397/0x550
[  207.927350] RSP: 0018:ffffc90001a4fdb8 EFLAGS: 00010006
[  207.927350] RAX: 000000000b7eefb7 RBX: ffff88041ed98580 RCX: ffffffff8203e080
[  207.927351] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000800
[  207.927351] RBP: ffffc90001a4fe18 R08: 0000000000000001 R09: 0000000000000001
[  207.927352] R10: 00000030696e879b R11: 00000000001b6d69 R12: 0000000000000000
[  207.927352] R13: ffff88041ed985f0 R14: 0000000000000000 R15: 0000000000000000
[  207.927353] FS:  0000000000000000(0000) GS:ffff88041ed80000(0000) knlGS:0000000000000000
[  207.927354] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  207.927354] CR2: 0000000000000150 CR3: 0000000001c09000 CR4: 00000000001406e0
[  207.927355] Call Trace:
[  207.927358]  __schedule+0x745/0x940
[  207.927359]  schedule+0x3d/0x90
[  207.927360]  __kthread_parkme+0x4e/0xa0
[  207.927362]  kthread_parkme+0x25/0x40
[  207.927363]  smpboot_thread_fn+0x96/0x1e0
[  207.927364]  kthread+0x10c/0x140
[  207.927365]  ? sort_range+0x30/0x30
[  207.927366]  ? kthread_parkme+0x40/0x40
[  207.927366]  ret_from_fork+0x2c/0x40
[  207.927367] Code: 38 85 c9 74 c4 4c 89 e7 e8 d7 8e ff ff eb bd 4c 8b 75 b8 48 8d 48 80 48 89 4d c0 49 39 ce 0f 84 8b 00 00 00 49 83 ee 80 49 89 c4 <4d> 8b ac 24 50 01 00 00 49 8b be 50 01 00 00 49 39 fd 74 4d 41 
[  207.927377] RIP: pick_next_task_fair+0x397/0x550 RSP: ffffc90001a4fdb8
[  207.927377] CR2: 0000000000000150

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#1562583

On Thu, Jan 19, 2017 at 08:31:09AM +0100, Mike Galbraith wrote:
> Mindless testing only, too sick to work, not sick enough to be immune
> to boredom.  Was verifying first warning wasn't somehow rt inspired,
> but while doing so, plain nopreempt (and no rt patch set) went boom.
> 
> [  203.088255] smpboot: CPU 1 is now offline
> [  203.168181] smpboot: CPU 2 is now offline
> [  203.221461] x86: Booting SMP configuration:
> [  203.221464] smpboot: Booting Node 0 Processor 1 APIC 0x2
> [  203.221728] ------------[ cut here ]------------
> [  203.221733] WARNING: CPU: 1 PID: 0 at kernel/sched/clock.c:149 set_sched_clock_stable+0x43/0x50
> [  203.221733] Modules linked in: nls_utf8(E) isofs(E) ebtable_filter(E) ebtables(E) fuse(E) nf_log_ipv6(E) xt_pkttype(E) xt_physdev(E) br_netfilter(E) nf_log_ipv4(E) nf_log_common(E) xt_LOG(E) xt_limit(E) af_packet(E) bridge(E) stp(E) llc(E) iscsi_ibft(E) iscsi_boot_sysfs(E) ip6t_REJECT(E) xt_tcpudp(E) nf_conntrack_ipv6(E) nf_defrag_ipv6(E) ip6table_raw(E) ipt_REJECT(E) iptable_raw(E) xt_CT(E) iptable_filter(E) ip6table_mangle(E) nf_conntrack_netbios_ns(E) nf_conntrack_broadcast(E) nf_conntrack_ipv4(E) nf_defrag_ipv4(E) ip_tables(E) xt_conntrack(E) nf_conntrack(E) ip6table_filter(E) ip6_tables(E) x_tables(E) nls_iso8859_1(E) snd_hda_codec_hdmi(E) nls_cp437(E) intel_rapl(E) x86_pkg_temp_thermal(E) intel_powerclamp(E) coretemp(E) kvm_intel(E) snd_hda_codec_realtek(E) snd_hda_codec_generic(E) kvm(E) snd_hda_intel(E)
> [  203.221748]  snd_hda_codec(E) irqbypass(E) crct10dif_pclmul(E) snd_hda_core(E) snd_hwdep(E) nfsd(E) crc32_pclmul(E) crc32c_intel(E) ghash_clmulni_intel(E) pcbc(E) snd_pcm(E) auth_rpcgss(E) aesni_intel(E) aes_x86_64(E) snd_timer(E) nfs_acl(E) joydev(E) crypto_simd(E) snd(E) lockd(E) grace(E) iTCO_wdt(E) iTCO_vendor_support(E) lpc_ich(E) mei_me(E) i2c_i801(E) mei(E) pcspkr(E) glue_helper(E) mfd_core(E) shpchp(E) intel_smartconnect(E) sunrpc(E) soundcore(E) tpm_infineon(E) fan(E) thermal(E) battery(E) cryptd(E) efivarfs(E) sr_mod(E) cdrom(E) hid_logitech_hidpp(E) hid_logitech_dj(E) uas(E) usb_storage(E) hid_generic(E) usbhid(E) nouveau(E) wmi(E) i2c_algo_bit(E) drm_kms_helper(E) ahci(E) syscopyarea(E) sysfillrect(E) sysimgblt(E) fb_sys_fops(E) libahci(E) xhci_pci(E) ehci_pci(E) xhci_hcd(E) ehci_hcd(E)
> [  203.221765]  ttm(E) libata(E) r8169(E) mii(E) drm(E) usbcore(E) fjes(E) video(E) button(E) sd_mod(E) vfat(E) fat(E) ext4(E) crc16(E) jbd2(E) mbcache(E) dm_mod(E) loop(E) sg(E) scsi_mod(E) autofs4(E)
> [  203.221773] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G            E   4.10.0-tip-default #29
> [  203.221774] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
> [  203.221774] Call Trace:
> [  203.221778]  dump_stack+0x63/0x90
> [  203.221780]  __warn+0xd1/0xf0
> [  203.221782]  warn_slowpath_null+0x1d/0x20
> [  203.221782]  set_sched_clock_stable+0x43/0x50
> [  203.221784]  early_init_intel+0x225/0x360
> [  203.221785]  init_intel+0x18/0x2d0
> [  203.221786]  identify_cpu+0x2d1/0x4d0
> [  203.221786]  identify_secondary_cpu+0x18/0x80
> [  203.221789]  smp_store_cpu_info+0x3e/0x40
> [  203.221790]  start_secondary+0x53/0x180
> [  203.221791]  start_cpu+0x14/0x14
> [  203.221792] ---[ end trace 262c7e4b746d5a76 ]---


OK, you also forgot to tell what you did to trigger this, but a little
playing around seems enough to reproduce. All that was required was
offline + online and *boom*.

I'll go have a prod. Thanks!

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#1562648

On Thu, 2017-01-19 at 11:19 +0100, Peter Zijlstra wrote:

> OK, you also forgot to tell what you did to trigger this, but a little
> playing around seems enough to reproduce. All that was required was
> offline + online and *boom*.

Oh, sorry, it was Steven's hotplug stress script.  I thought the first
couple lines of kernel output (+$subject) told the tale.

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#1562728

On Thu, Jan 19, 2017 at 11:19:24AM +0100, Peter Zijlstra wrote:

> I'll go have a prod. Thanks!

This seems to cure it for me.

---
Subject: sched/clock: Fix hotplug issue

Mike reported that he could trigger the WARN_ON_ONCE() in
set_sched_clock_stable() using hotplug.

This exposed a fundamental problem with the interface, we should never
mark the TSC stable if we ever find it to be unstable. Therefore
set_sched_clock_stable() is a broken interface.

The reason it existed is that not having it is a pain, it means all
relevant architecture code needs to call clear_sched_clock_stable()
where appropriate.

Of the three architectures that select HAVE_UNSTABLE_SCHED_CLOCK ia64
and parisc are trivial in that they never called
set_sched_clock_stable(), so add an unconditional call to
clear_sched_clock_stable() to them.

For x86 the story is a lot more involved, and what this patch tries to
do is ensure we preserve the status quo. So even is Cyrix or Transmeta
have usable TSC they never called set_sched_clock_stable() so they now
get an explicit mark unstable.

XXX: what about Xen ?

Fixes: 9881b024b7d7 ("sched/clock: Delay switching sched_clock to stable")
Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/ia64/kernel/setup.c        |  2 ++
 arch/parisc/kernel/setup.c      |  2 ++
 arch/x86/kernel/cpu/amd.c       |  6 ++++--
 arch/x86/kernel/cpu/centaur.c   |  2 ++
 arch/x86/kernel/cpu/common.c    |  3 +++
 arch/x86/kernel/cpu/cyrix.c     |  2 ++
 arch/x86/kernel/cpu/intel.c     |  6 ++++--
 arch/x86/kernel/cpu/transmeta.c |  3 +++
 arch/x86/kernel/kvmclock.c      |  2 +-
 include/linux/sched.h           |  1 -
 kernel/sched/clock.c            | 29 ++++++++---------------------
 11 files changed, 31 insertions(+), 27 deletions(-)

diff --git a/arch/ia64/kernel/setup.c b/arch/ia64/kernel/setup.c
index 7ec7acc..c483ece 100644
--- a/arch/ia64/kernel/setup.c
+++ b/arch/ia64/kernel/setup.c
@@ -619,6 +619,8 @@ setup_arch (char **cmdline_p)
 	check_sal_cache_flush();
 #endif
 	paging_init();
+
+	clear_sched_clock_stable();
 }
 
 /*
diff --git a/arch/parisc/kernel/setup.c b/arch/parisc/kernel/setup.c
index 2e66a88..068ed36 100644
--- a/arch/parisc/kernel/setup.c
+++ b/arch/parisc/kernel/setup.c
@@ -36,6 +36,7 @@
 #undef PCI_DEBUG
 #include <linux/proc_fs.h>
 #include <linux/export.h>
+#include <linux/sched.h>
 
 #include <asm/processor.h>
 #include <asm/sections.h>
@@ -176,6 +177,7 @@ void __init setup_arch(char **cmdline_p)
 	conswitchp = &dummy_con;	/* we use do_take_over_console() later ! */
 #endif
 
+	clear_sched_clock_stable();
 }
 
 /*
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 1d31672..80e657e 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -541,8 +541,10 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (!check_tsc_unstable())
-			set_sched_clock_stable();
+		if (check_tsc_unstable())
+			clear_sched_clock_stable();
+	} else {
+		clear_sched_clock_stable();
 	}
 
 	/* Bit 12 of 8000_0007 edx is accumulated power mechanism. */
diff --git a/arch/x86/kernel/cpu/centaur.c b/arch/x86/kernel/cpu/centaur.c
index 1661d8e..c4bab8c 100644
--- a/arch/x86/kernel/cpu/centaur.c
+++ b/arch/x86/kernel/cpu/centaur.c
@@ -104,6 +104,8 @@ static void early_init_centaur(struct cpuinfo_x86 *c)
 #ifdef CONFIG_X86_64
 	set_cpu_cap(c, X86_FEATURE_SYSENTER32);
 #endif
+
+	clear_sched_clock_stable();
 }
 
 static void init_centaur(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 9bab7a8..0bdb1ab 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -83,6 +83,7 @@ static void default_init(struct cpuinfo_x86 *c)
 			strcpy(c->x86_model_id, "386");
 	}
 #endif
+	clear_sched_clock_stable();
 }
 
 static const struct cpu_dev default_cpu = {
@@ -1055,6 +1056,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	 */
 	if (this_cpu->c_init)
 		this_cpu->c_init(c);
+	else
+		clear_sched_clock_stable();
 
 	/* Disable the PN if appropriate */
 	squash_the_stupid_serial_number(c);
diff --git a/arch/x86/kernel/cpu/cyrix.c b/arch/x86/kernel/cpu/cyrix.c
index bd9dcd6..47416f9 100644
--- a/arch/x86/kernel/cpu/cyrix.c
+++ b/arch/x86/kernel/cpu/cyrix.c
@@ -9,6 +9,7 @@
 #include <asm/pci-direct.h>
 #include <asm/tsc.h>
 #include <asm/cpufeature.h>
+#include <linux/sched.h>
 
 #include "cpu.h"
 
@@ -183,6 +184,7 @@ static void early_init_cyrix(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_CYRIX_ARR);
 		break;
 	}
+	clear_sched_clock_stable();
 }
 
 static void init_cyrix(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 203f860..026c728 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -119,8 +119,10 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (!check_tsc_unstable())
-			set_sched_clock_stable();
+		if (check_tsc_unstable())
+			clear_sched_clock_stable();
+	} else {
+		clear_sched_clock_stable();
 	}
 
 	/* Penwell and Cloverview have the TSC which doesn't sleep on S3 */
diff --git a/arch/x86/kernel/cpu/transmeta.c b/arch/x86/kernel/cpu/transmeta.c
index 3417856..c1ea5b9 100644
--- a/arch/x86/kernel/cpu/transmeta.c
+++ b/arch/x86/kernel/cpu/transmeta.c
@@ -1,4 +1,5 @@
 #include <linux/kernel.h>
+#include <linux/sched.h>
 #include <linux/mm.h>
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
@@ -14,6 +15,8 @@ static void early_init_transmeta(struct cpuinfo_x86 *c)
 		if (xlvl >= 0x80860001)
 			c->x86_capability[CPUID_8086_0001_EDX] = cpuid_edx(0x80860001);
 	}
+
+	clear_sched_clock_stable();
 }
 
 static void init_transmeta(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 2a5cafd..542710b 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -107,12 +107,12 @@ static inline void kvm_sched_clock_init(bool stable)
 {
 	if (!stable) {
 		pv_time_ops.sched_clock = kvm_clock_read;
+		clear_sched_clock_stable();
 		return;
 	}
 
 	kvm_sched_clock_offset = kvm_clock_read();
 	pv_time_ops.sched_clock = kvm_sched_clock_read;
-	set_sched_clock_stable();
 
 	printk(KERN_INFO "kvm-clock: using sched offset of %llu cycles\n",
 			kvm_sched_clock_offset);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 68b35dd..8a9bbb6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2522,7 +2522,6 @@ extern void sched_clock_init_late(void);
  * is reliable after all:
  */
 extern int sched_clock_stable(void);
-extern void set_sched_clock_stable(void);
 extern void clear_sched_clock_stable(void);
 
 extern void sched_clock_tick(void);
diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 7713b2b..ad64efe 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -83,8 +83,15 @@ void sched_clock_init(void)
 }
 
 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+/*
+ * We must start with !__sched_clock_stable because the unstable -> stable
+ * transition is accurate, while the stable -> unstable transition is not.
+ *
+ * Similarly we start with __sched_clock_stable_early, thereby assuming we
+ * will become stable, such that there's only a single 1 -> 0 transition.
+ */
 static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
-static int __sched_clock_stable_early;
+static int __sched_clock_stable_early = 1;
 
 /*
  * We want: ktime_get_ns() + gtod_offset == sched_clock() + raw_offset
@@ -132,24 +139,6 @@ static void __set_sched_clock_stable(void)
 	tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
 }
 
-void set_sched_clock_stable(void)
-{
-	__sched_clock_stable_early = 1;
-
-	smp_mb(); /* matches sched_clock_init_late() */
-
-	/*
-	 * This really should only be called early (before
-	 * sched_clock_init_late()) when guestimating our sched_clock() is
-	 * solid.
-	 *
-	 * After that we test stability and we can negate our guess using
-	 * clear_sched_clock_stable, possibly from a watchdog.
-	 */
-	if (WARN_ON_ONCE(sched_clock_running == 2))
-		__set_sched_clock_stable();
-}
-
 static void __clear_sched_clock_stable(struct work_struct *work)
 {
 	struct sched_clock_data *scd = this_scd();
@@ -199,8 +188,6 @@ void sched_clock_init_late(void)
 
 	if (__sched_clock_stable_early)
 		__set_sched_clock_stable();
-	else
-		__clear_sched_clock_stable(NULL);
 }
 
 /*

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#1562929

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Jan 19, 2017 at 11:19:24AM +0100, Peter Zijlstra wrote:
> 
> > I'll go have a prod. Thanks!
> 
> This seems to cure it for me.
> 
> ---
> Subject: sched/clock: Fix hotplug issue

JFYI:

 arch/x86/kernel/cpu/centaur.c: In function ‘early_init_centaur’:
 arch/x86/kernel/cpu/centaur.c:108:2: error: implicit declaration of function ‘clear_sched_clock_stable’ [-Werror=implicit-function-declaration]

x86-64 defconfig-ish.

Thanks,

	Ingo

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#1562956

On Thu, Jan 19, 2017 at 05:54:55PM +0100, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Thu, Jan 19, 2017 at 11:19:24AM +0100, Peter Zijlstra wrote:
> > 
> > > I'll go have a prod. Thanks!
> > 
> > This seems to cure it for me.
> > 
> > ---
> > Subject: sched/clock: Fix hotplug issue
> 
> JFYI:
> 
>  arch/x86/kernel/cpu/centaur.c: In function ‘early_init_centaur’:
>  arch/x86/kernel/cpu/centaur.c:108:2: error: implicit declaration of function ‘clear_sched_clock_stable’ [-Werror=implicit-function-declaration]
> 
> x86-64 defconfig-ish.

Duh.. that'd be a missing: #include <linux/sched.h>

Clearly that's not selected with the .config for the textbox ;-)

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#1563067 — Re: sched/clock: Fix hotplug issue

[Multipart message — attachments visible in raw view] — view raw

Hi Peter,

[auto build test ERROR on tip/sched/core]
[also build test ERROR on next-20170119]
[cannot apply to tip/x86/core v4.10-rc4]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Peter-Zijlstra/sched-clock-Fix-hotplug-issue/20170120-034727
config: parisc-c3000_defconfig (attached as .config)
compiler: hppa-linux-gnu-gcc (Debian 6.1.1-9) 6.1.1 20160705
reproduce:
        wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=parisc 

All errors (new ones prefixed by >>):

   arch/parisc/kernel/setup.c: In function 'setup_arch':
>> arch/parisc/kernel/setup.c:180:2: error: implicit declaration of function 'clear_sched_clock_stable' [-Werror=implicit-function-declaration]
     clear_sched_clock_stable();
     ^~~~~~~~~~~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors

vim +/clear_sched_clock_stable +180 arch/parisc/kernel/setup.c

   174	#endif
   175	
   176	#if defined(CONFIG_VT) && defined(CONFIG_DUMMY_CONSOLE)
   177		conswitchp = &dummy_con;	/* we use do_take_over_console() later ! */
   178	#endif
   179	
 > 180		clear_sched_clock_stable();
   181	}
   182	
   183	/*

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

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#1563075 — Re: sched/clock: Fix hotplug issue

[Multipart message — attachments visible in raw view] — view raw

Hi Peter,

[auto build test ERROR on tip/sched/core]
[also build test ERROR on next-20170119]
[cannot apply to tip/x86/core v4.10-rc4]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Peter-Zijlstra/sched-clock-Fix-hotplug-issue/20170120-034727
config: i386-randconfig-x003-201703 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   arch/x86/kernel/cpu/centaur.c: In function 'early_init_centaur':
>> arch/x86/kernel/cpu/centaur.c:108:2: error: implicit declaration of function 'clear_sched_clock_stable' [-Werror=implicit-function-declaration]
     clear_sched_clock_stable();
     ^~~~~~~~~~~~~~~~~~~~~~~~
   cc1: some warnings being treated as errors

vim +/clear_sched_clock_stable +108 arch/x86/kernel/cpu/centaur.c

   102			break;
   103		}
   104	#ifdef CONFIG_X86_64
   105		set_cpu_cap(c, X86_FEATURE_SYSENTER32);
   106	#endif
   107	
 > 108		clear_sched_clock_stable();
   109	}
   110	
   111	static void init_centaur(struct cpuinfo_x86 *c)

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

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#1563341 — [tip:sched/core] sched/clock: Fix hotplug crash

Commit-ID:  acb04058de49458010c44bb35b849d45113fd668
Gitweb:     http://git.kernel.org/tip/acb04058de49458010c44bb35b849d45113fd668
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 19 Jan 2017 14:36:33 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 20 Jan 2017 02:38:46 +0100

sched/clock: Fix hotplug crash

Mike reported that he could trigger the WARN_ON_ONCE() in
set_sched_clock_stable() using hotplug.

This exposed a fundamental problem with the interface, we should never
mark the TSC stable if we ever find it to be unstable. Therefore
set_sched_clock_stable() is a broken interface.

The reason it existed is that not having it is a pain, it means all
relevant architecture code needs to call clear_sched_clock_stable()
where appropriate.

Of the three architectures that select HAVE_UNSTABLE_SCHED_CLOCK ia64
and parisc are trivial in that they never called
set_sched_clock_stable(), so add an unconditional call to
clear_sched_clock_stable() to them.

For x86 the story is a lot more involved, and what this patch tries to
do is ensure we preserve the status quo. So even is Cyrix or Transmeta
have usable TSC they never called set_sched_clock_stable() so they now
get an explicit mark unstable.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 9881b024b7d7 ("sched/clock: Delay switching sched_clock to stable")
Link: http://lkml.kernel.org/r/20170119133633.GB6536@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/ia64/kernel/setup.c        |  2 ++
 arch/parisc/kernel/setup.c      |  2 ++
 arch/x86/kernel/cpu/amd.c       |  6 ++++--
 arch/x86/kernel/cpu/centaur.c   |  6 ++++--
 arch/x86/kernel/cpu/common.c    |  3 +++
 arch/x86/kernel/cpu/cyrix.c     |  2 ++
 arch/x86/kernel/cpu/intel.c     |  6 ++++--
 arch/x86/kernel/cpu/transmeta.c |  3 +++
 arch/x86/kernel/kvmclock.c      |  2 +-
 include/linux/sched.h           |  1 -
 kernel/sched/clock.c            | 29 ++++++++---------------------
 11 files changed, 33 insertions(+), 29 deletions(-)

diff --git a/arch/ia64/kernel/setup.c b/arch/ia64/kernel/setup.c
index 7ec7acc..c483ece 100644
--- a/arch/ia64/kernel/setup.c
+++ b/arch/ia64/kernel/setup.c
@@ -619,6 +619,8 @@ setup_arch (char **cmdline_p)
 	check_sal_cache_flush();
 #endif
 	paging_init();
+
+	clear_sched_clock_stable();
 }
 
 /*
diff --git a/arch/parisc/kernel/setup.c b/arch/parisc/kernel/setup.c
index 2e66a88..068ed36 100644
--- a/arch/parisc/kernel/setup.c
+++ b/arch/parisc/kernel/setup.c
@@ -36,6 +36,7 @@
 #undef PCI_DEBUG
 #include <linux/proc_fs.h>
 #include <linux/export.h>
+#include <linux/sched.h>
 
 #include <asm/processor.h>
 #include <asm/sections.h>
@@ -176,6 +177,7 @@ void __init setup_arch(char **cmdline_p)
 	conswitchp = &dummy_con;	/* we use do_take_over_console() later ! */
 #endif
 
+	clear_sched_clock_stable();
 }
 
 /*
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 71cae73..1bb253a 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -548,8 +548,10 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (!check_tsc_unstable())
-			set_sched_clock_stable();
+		if (check_tsc_unstable())
+			clear_sched_clock_stable();
+	} else {
+		clear_sched_clock_stable();
 	}
 
 	/* Bit 12 of 8000_0007 edx is accumulated power mechanism. */
diff --git a/arch/x86/kernel/cpu/centaur.c b/arch/x86/kernel/cpu/centaur.c
index 1661d8e..2c234a6 100644
--- a/arch/x86/kernel/cpu/centaur.c
+++ b/arch/x86/kernel/cpu/centaur.c
@@ -1,5 +1,5 @@
-#include <linux/bitops.h>
-#include <linux/kernel.h>
+
+#include <linux/sched.h>
 
 #include <asm/cpufeature.h>
 #include <asm/e820.h>
@@ -104,6 +104,8 @@ static void early_init_centaur(struct cpuinfo_x86 *c)
 #ifdef CONFIG_X86_64
 	set_cpu_cap(c, X86_FEATURE_SYSENTER32);
 #endif
+
+	clear_sched_clock_stable();
 }
 
 static void init_centaur(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index dc1697c..3457186 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -83,6 +83,7 @@ static void default_init(struct cpuinfo_x86 *c)
 			strcpy(c->x86_model_id, "386");
 	}
 #endif
+	clear_sched_clock_stable();
 }
 
 static const struct cpu_dev default_cpu = {
@@ -1055,6 +1056,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	 */
 	if (this_cpu->c_init)
 		this_cpu->c_init(c);
+	else
+		clear_sched_clock_stable();
 
 	/* Disable the PN if appropriate */
 	squash_the_stupid_serial_number(c);
diff --git a/arch/x86/kernel/cpu/cyrix.c b/arch/x86/kernel/cpu/cyrix.c
index bd9dcd6..47416f9 100644
--- a/arch/x86/kernel/cpu/cyrix.c
+++ b/arch/x86/kernel/cpu/cyrix.c
@@ -9,6 +9,7 @@
 #include <asm/pci-direct.h>
 #include <asm/tsc.h>
 #include <asm/cpufeature.h>
+#include <linux/sched.h>
 
 #include "cpu.h"
 
@@ -183,6 +184,7 @@ static void early_init_cyrix(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_CYRIX_ARR);
 		break;
 	}
+	clear_sched_clock_stable();
 }
 
 static void init_cyrix(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index fcd484d..26eaff4 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -124,8 +124,10 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (!check_tsc_unstable())
-			set_sched_clock_stable();
+		if (check_tsc_unstable())
+			clear_sched_clock_stable();
+	} else {
+		clear_sched_clock_stable();
 	}
 
 	/* Penwell and Cloverview have the TSC which doesn't sleep on S3 */
diff --git a/arch/x86/kernel/cpu/transmeta.c b/arch/x86/kernel/cpu/transmeta.c
index 3417856..c1ea5b9 100644
--- a/arch/x86/kernel/cpu/transmeta.c
+++ b/arch/x86/kernel/cpu/transmeta.c
@@ -1,4 +1,5 @@
 #include <linux/kernel.h>
+#include <linux/sched.h>
 #include <linux/mm.h>
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
@@ -14,6 +15,8 @@ static void early_init_transmeta(struct cpuinfo_x86 *c)
 		if (xlvl >= 0x80860001)
 			c->x86_capability[CPUID_8086_0001_EDX] = cpuid_edx(0x80860001);
 	}
+
+	clear_sched_clock_stable();
 }
 
 static void init_transmeta(struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 2a5cafd..542710b 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -107,12 +107,12 @@ static inline void kvm_sched_clock_init(bool stable)
 {
 	if (!stable) {
 		pv_time_ops.sched_clock = kvm_clock_read;
+		clear_sched_clock_stable();
 		return;
 	}
 
 	kvm_sched_clock_offset = kvm_clock_read();
 	pv_time_ops.sched_clock = kvm_sched_clock_read;
-	set_sched_clock_stable();
 
 	printk(KERN_INFO "kvm-clock: using sched offset of %llu cycles\n",
 			kvm_sched_clock_offset);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a8daed9..69e6852 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2547,7 +2547,6 @@ extern void sched_clock_init_late(void);
  * is reliable after all:
  */
 extern int sched_clock_stable(void);
-extern void set_sched_clock_stable(void);
 extern void clear_sched_clock_stable(void);
 
 extern void sched_clock_tick(void);
diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 7713b2b..ad64efe 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -83,8 +83,15 @@ void sched_clock_init(void)
 }
 
 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+/*
+ * We must start with !__sched_clock_stable because the unstable -> stable
+ * transition is accurate, while the stable -> unstable transition is not.
+ *
+ * Similarly we start with __sched_clock_stable_early, thereby assuming we
+ * will become stable, such that there's only a single 1 -> 0 transition.
+ */
 static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
-static int __sched_clock_stable_early;
+static int __sched_clock_stable_early = 1;
 
 /*
  * We want: ktime_get_ns() + gtod_offset == sched_clock() + raw_offset
@@ -132,24 +139,6 @@ static void __set_sched_clock_stable(void)
 	tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
 }
 
-void set_sched_clock_stable(void)
-{
-	__sched_clock_stable_early = 1;
-
-	smp_mb(); /* matches sched_clock_init_late() */
-
-	/*
-	 * This really should only be called early (before
-	 * sched_clock_init_late()) when guestimating our sched_clock() is
-	 * solid.
-	 *
-	 * After that we test stability and we can negate our guess using
-	 * clear_sched_clock_stable, possibly from a watchdog.
-	 */
-	if (WARN_ON_ONCE(sched_clock_running == 2))
-		__set_sched_clock_stable();
-}
-
 static void __clear_sched_clock_stable(struct work_struct *work)
 {
 	struct sched_clock_data *scd = this_scd();
@@ -199,8 +188,6 @@ void sched_clock_init_late(void)
 
 	if (__sched_clock_stable_early)
 		__set_sched_clock_stable();
-	else
-		__clear_sched_clock_stable(NULL);
 }
 
 /*

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