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

On 02/08/2017 02:05 PM, Peter Zijlstra wrote:

On Wed, Feb 08, 2017 at 01:00:24PM -0500, Waiman Long wrote:

quoted

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