linux-kernel - Re: [PATCH] kernel/sched: run nohz idle load balancer on HK_FLAG

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Date:   Fri, 26 Apr 2019 14:40:40 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] kernel/sched: run nohz idle load balancer on HK_FLAG_MISC
 CPUs

Peter Zijlstra's on April 25, 2019 9:56 pm:
> On Fri, Apr 12, 2019 at 02:26:13PM +1000, Nicholas Piggin wrote:
>> The nohz idle balancer runs on the lowest idle CPU. This can
>> interfere with isolated CPUs, so confine it to HK_FLAG_MISC
>> housekeeping CPUs.
>> 
>> HK_FLAG_SCHED is not used for this because it is not set anywhere
>> at the moment. This could be folded into HK_FLAG_SCHED once that
>> option is fixed.
> 
> Frederic? Anyway, I thnk I'll take this patch as is.

That would be great, thanks. We've been testing it in a staging
environment (this is where they noticed the noise in the first
place), and results have been as expected:

  I've been able to test Nick's idle-loop load balancer (ILB) patch, 
  with and without the TEO cpuidle governor. With the ILB patch (and 
  nohz_full) I get a very quiet noise profile with either cpuidle 
  governor (menu or teo). For my tests, I don't see a meaningful 
  difference between the two governors.

  [...]

  Bottom line: Nick's patch that constrains the ILB to run on non-nohz 
  cores has a noticeable noise-reduction effect. For this type of 
  workload, the choice of cpuidle governor, menu or teo, is immaterial.

This is against a slightly backported RHEL kernel they are using, but
no significant differences from upstream in these areas.

Thanks,
Nick