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Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function

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On 02/13/2017 05:47 AM, Peter Zijlstra wrote:
> On Fri, Feb 10, 2017 at 12:00:43PM -0500, Waiman Long wrote:
>
>>>> +asm(
>>>> +".pushsection .text;"
>>>> +".global __raw_callee_save___kvm_vcpu_is_preempted;"
>>>> +".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
>>>> +"__raw_callee_save___kvm_vcpu_is_preempted:"
>>>> +FRAME_BEGIN
>>>> +"push %rdi;"
>>>> +"push %rdx;"
>>>> +"movslq  %edi, %rdi;"
>>>> +"movq    $steal_time+16, %rax;"
>>>> +"movq    __per_cpu_offset(,%rdi,8), %rdx;"
>>>> +"cmpb    $0, (%rdx,%rax);"
> Could we not put the $steal_time+16 displacement as an immediate in the
> cmpb and save a whole register here?
>
> That way we'd end up with something like:
>
> asm("
> push %rdi;
> movslq %edi, %rdi;
> movq __per_cpu_offset(,%rdi,8), %rax;
> cmpb $0, %[offset](%rax);
> setne %al;
> pop %rdi;
> " : : [offset] "i" (((unsigned long)&steal_time) + offsetof(struct steal_time, preempted)));
>
> And if we could get rid of the sign extend on edi we could avoid all the
> push-pop nonsense, but I'm not sure I see how to do that (then again,
> this asm foo isn't my strongest point).

Yes, I think that can work. I will try to ran this patch to see how
thing goes.

>>>> +"setne   %al;"
>>>> +"pop %rdx;"
>>>> +"pop %rdi;"
>>>> +FRAME_END
>>>> +"ret;"
>>>> +".popsection");
>>>> +
>>>> +#endif
>>>> +
>>>>  /*
>>>>   * Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
>>>>   */
>>> That should work for now. I have done something similar for
>>> __pv_queued_spin_unlock. However, this has the problem of creating a
>>> dependency on the exact layout of the steal_time structure. Maybe the
>>> constant 16 can be passed in as a parameter offsetof(struct
>>> kvm_steal_time, preempted) to the asm call.
> Yeah it should be well possible to pass that in. But ideally we'd have
> GCC grow something like __attribute__((callee_saved)) or somesuch and it
> would do all this for us.

That will be really nice too. I am not too fond of working in assembly.

>> One more thing, that will improve KVM performance, but it won't help Xen.
> People still use Xen? ;-) In any case, their implementation looks very
> similar and could easily crib this.

In Red Hat, my focus will be on KVM performance. I do believe that there
are still Xen users out there. So we still need to keep their interest
into consideration. Given that, I am OK to make it work better in KVM
first and then think about Xen later.

>> I looked into the assembly code for rwsem_spin_on_owner, It need to save
>> and restore 2 additional registers with my patch. Doing it your way,
>> will transfer the save and restore overhead to the assembly code.
>> However, __kvm_vcpu_is_preempted() is called multiple times per
>> invocation of rwsem_spin_on_owner. That function is simple enough that
>> making __kvm_vcpu_is_preempted() callee-save won't produce much compiler
>> optimization opportunity.
> This is because of that noinline, right? Otherwise it would've been
> folded and register pressure would be much higher.

Yes, I guess so. The noinline is there so that we know what the CPU time
is for spinning rather than other activities within the slowpath.

>
>> The outer function rwsem_down_write_failed()
>> does appear to be a bit bigger (from 866 bytes to 884 bytes) though.
> I suspect GCC is being clever and since all this is static it plays
> games with the calling convention and pushes these clobbers out.
>
>

Cheers,
Longman

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[PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function Waiman Long <longman@redhat.com> - 2017-02-10 17:00 +0100
  Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Paolo Bonzini <pbonzini@redhat.com> - 2017-02-10 17:30 +0100
  Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-10 17:30 +0100
    Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-10 18:10 +0100
      Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-13 11:50 +0100
        Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-13 12:00 +0100
          Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-13 20:50 +0100
            Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-13 21:20 +0100
              Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-13 23:00 +0100
                Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function hpa@zytor.com - 2017-02-13 23:10 +0100
                Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function hpa@zytor.com - 2017-02-13 23:20 +0100
                Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-13 23:40 +0100
                Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function hpa@zytor.com - 2017-02-13 23:50 +0100
                Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-14 10:50 +0100
                Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-14 15:50 +0100
                Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-14 17:10 +0100
                Re: [Xen-devel] [PATCH v2] x86/paravirt: Don't make  vcpu_is_preempted() a callee-save function Andrew Cooper <andrew.cooper3@citrix.com> - 2017-02-14 17:20 +0100
          Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function hpa@zytor.com - 2017-02-13 21:20 +0100
            Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-13 23:00 +0100
            Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-13 23:30 +0100
              Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Peter Zijlstra <peterz@infradead.org> - 2017-02-13 23:40 +0100
        Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-13 20:50 +0100
    Re: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a  callee-save function Waiman Long <longman@redhat.com> - 2017-02-10 19:10 +0100

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