Re: [PATCH 1/2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function

Waiman Long
2017-02-08T20:42:19Z

On 02/08/2017 02:05 PM, Peter Zijlstra wrote:
> On Wed, Feb 08, 2017 at 01:00:24PM -0500, Waiman Long wrote:
>> It was found when running fio sequential write test with a XFS ramdisk
>> on a 2-socket x86-64 system, the %CPU times as reported by perf were
>> as follows:
>>
>> 71.27% 0.28% fio [k] down_write
>> 70.99% 0.01% fio [k] call_rwsem_down_write_failed
>> 69.43% 1.18% fio [k] rwsem_down_write_failed
>> 65.51% 54.57% fio [k] osq_lock
>> 9.72% 7.99% fio [k] __raw_callee_save___kvm_vcpu_is_preempted
>> 4.16% 4.16% fio [k] __kvm_vcpu_is_preempted
>>
>> So making vcpu_is_preempted() a callee-save function has a pretty high
>> cost associated with it. As vcpu_is_preempted() is called within the
>> spinlock, mutex and rwsem slowpaths, there isn't much to gain by making
>> it callee-save. So it is now changed to a normal function call instead.
>>
> Numbers for bare metal too please.

I will run the test on bare metal, but I doubt there will be noticeable
difference.

Cheers,
Longman