linux-kernel - Re: [PATCH v1] sched: steer waking task to empty cfs

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Date:	Thu, 03 May 2012 11:13:24 +0530
From:	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] sched: steer waking task to empty cfs_rq for better latencies

On Wed, 2 May 2012 19:31:17 +0530, Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com> wrote:
> * Peter Zijlstra <peterz@...radead.org> [2012-04-24 19:09:14]:
> 
> > On Tue, 2012-04-24 at 18:58 +0200, Peter Zijlstra wrote:
> > > On Tue, 2012-04-24 at 22:26 +0530, Srivatsa Vaddagiri wrote:
> > > > Steer a waking task towards a cpu where its cgroup has zero tasks (in
> > > > order to provide it better sleeper credits and hence reduce its wakeup
> > > > latency). 
> > > 
> > > That's just vile.. pjt could you post your global vruntime stuff so
> > > vatsa can have a go at that?
> > 
> > That is, you're playing a deficiency we should fix, not exploit.
> > 
> > Also, you do a second loop over all those cpus right after we've already
> > iterated them..
> > 
> > furthermore, that 100%+ gain is still way insane, what else is broken?
> > Did you try those paravirt tlb-flush patches and other such weirdness?
> 
> I got around to try pv-tlb-flush patches and it showed >100%
> improvement for sysbench (without the balance-on-wake patch on host). This
> was what readprofile showed (when pv-tlb-flush patches were absent in
> guest):
> 
> 1135704 total                                   0.3265
> 636737 native_cpuid                             18192.4857
> 283201 __bitmap_empty                           2832.0100
> 137853 flush_tlb_others_ipi                     569.6405
> 
> I will try out how much they help Trade workload (which got me started
> on this originally) and report back (part of the problem in trying out
> is that pv-tlb-flush platches are throwing wierd problems - which Nikunj
> is helping investigate). 
> 
A bug, the below patch should cure it. kvm_mmu_flush_tlb will queue the
request, but we have already passed the phase of checking that. I will
fold this in my next version.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6c42056..b114411 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1550,7 +1550,7 @@ static void kvm_set_vcpu_state(struct kvm_vcpu *vcpu)
 
        vs->state = 1;
        if (vs->flush_on_enter) {
-               kvm_mmu_flush_tlb(vcpu);
+               kvm_x86_ops->tlb_flush(vcpu);
                vs->flush_on_enter = 0;
        }
 

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