Linux Virtualization - Feb 2017 - [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function

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).
> >> +"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.
> 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.
> 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.
> 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.

On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra
wrote:
> 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).
Maybe:

movsql %edi, %rax;
movq __per_cpu_offset(,%rax,8), %rax;
cmpb $0, %[offset](%rax);
setne %al;

?

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

On 02/13/2017 05:53 AM, Peter Zijlstra wrote:
> On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra wrote:
>> 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).
> Maybe:
>
> movsql %edi, %rax;
> movq __per_cpu_offset(,%rax,8), %rax;
> cmpb $0, %[offset](%rax);
> setne %al;
>
> ?
Yes, that looks good to me.

Cheers,
Longman

On February 13, 2017 2:53:43 AM PST, Peter Zijlstra <peterz at
infradead.org> wrote:
>On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra wrote:
>> 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).
>
>Maybe:
>
>movsql %edi, %rax;
>movq __per_cpu_offset(,%rax,8), %rax;
>cmpb $0, %[offset](%rax);
>setne %al;
>
>?
We could kill the zero or sign extend by changing the calling interface to pass
an unsigned long instead of an int.  It is much more likely that a zero extend
is free for the caller than a sign extend.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.