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

Peter Zijlstra
2017-02-08T19:09:49Z

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.