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Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem) causes regression for libvirt/kvm

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Am 15.09.2015 um 18:42 schrieb Paolo Bonzini:
> 
> 
> On 15/09/2015 15:36, Christian Borntraeger wrote:
>> I am wondering why the old code behaved in such fatal ways. Is there
>> some interaction between waiting for a reschedule in the
>> synchronize_sched writer and some fork code actually waiting for the
>> read side to get the lock together with some rescheduling going on
>> waiting for a lock that fork holds? lockdep does not give me an hints
>> so I have no clue :-(
> 
> It may just be consuming too much CPU usage.  kernel/rcu/tree.c warns
> about it:
> 
>  * if you are using synchronize_sched_expedited() in a loop, please
>  * restructure your code to batch your updates, and then use a single
>  * synchronize_sched() instead.
> 
> and you may remember that in KVM we switched from RCU to SRCU exactly to
> avoid userspace-controlled synchronize_rcu_expedited().
> 
> In fact, I would say that any userspace-controlled call to *_expedited()
> is a bug waiting to happen and a bad idea---because userspace can, with
> little effort, end up calling it in a loop.

Right. This also implies that we should fix this for 4.2 - I guess.

Christian

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[4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-15 14:10 +0200
  Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Peter Zijlstra <peterz@infradead.org> - 2015-09-15 15:10 +0200
    Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-15 15:40 +0200
      Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Tejun Heo <tj@kernel.org> - 2015-09-15 16:00 +0200
      Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Paolo Bonzini <pbonzini@redhat.com> - 2015-09-15 18:50 +0200
        Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-15 19:40 +0200
          Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Paolo Bonzini <pbonzini@redhat.com> - 2015-09-16 10:40 +0200
            Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-16 11:00 +0200
              Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Paolo Bonzini <pbonzini@redhat.com> - 2015-09-16 11:20 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes  regression for libvirt/kvm Oleg Nesterov <oleg@redhat.com> - 2015-09-16 14:30 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Paolo Bonzini <pbonzini@redhat.com> - 2015-09-16 14:40 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes  regression for libvirt/kvm Oleg Nesterov <oleg@redhat.com> - 2015-09-16 14:50 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-16 15:00 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Tejun Heo <tj@kernel.org> - 2015-09-16 16:20 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Paolo Bonzini <pbonzini@redhat.com> - 2015-09-16 16:20 +0200
        Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-15 23:20 +0200
          Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Tejun Heo <tj@kernel.org> - 2015-09-15 23:30 +0200
            Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-15 23:40 +0200
              Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Tejun Heo <tj@kernel.org> - 2015-09-16 00:30 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-16 01:40 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Tejun Heo <tj@kernel.org> - 2015-09-16 03:30 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-09-16 06:40 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Tejun Heo <tj@kernel.org> - 2015-09-16 13:10 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-16 09:50 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-16 13:00 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Tejun Heo <tj@kernel.org> - 2015-09-16 13:10 +0200
                Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem) causes regression for  libvirt/kvm Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-16 14:00 +0200
  [PATCH cgroup/for-4.3-fixes 1/2] Revert "cgroup: simplify  threadgroup locking" Tejun Heo <tj@kernel.org> - 2015-09-16 18:00 +0200
    [PATCH cgroup/for-4.3-fixes 2/2] Revert "sched, cgroup: replace  signal_struct->group_rwsem with a global percpu_rwsem" Tejun Heo <tj@kernel.org> - 2015-09-16 18:00 +0200
    Re: [PATCH cgroup/for-4.3-fixes 1/2] Revert "cgroup: simplify  threadgroup locking" Oleg Nesterov <oleg@redhat.com> - 2015-09-16 19:10 +0200
    Re: [PATCH cgroup/for-4.3-fixes 1/2] Revert "cgroup: simplify  threadgroup locking" Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-16 20:50 +0200

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