linux-kernel - Re: [PATCH] sched/fair: Optimize select_idle

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Message-ID: <20191206123254.GH2844@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Dec 2019 13:32:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/fair: Optimize select_idle_core

On Fri, Dec 06, 2019 at 12:00:00PM +0000, Valentin Schneider wrote:

> Say you have a 4-core SMT2 system with the usual numbering scheme:
> 
> {0, 4}  {1, 5}  {2, 6}  {3, 7}
> CORE0   CORE1   CORE2   CORE3
> 
> 
> Say 'target' is the prev_cpu, in that case let's pick 5. Because we do a
> for_each_cpu_wrap(), our iteration for 'core' would start with 
> 
>   5, 6, 7, ...
> 
> So say CORE2 is entirely idle and CORE1 isn't, we would go through the
> inner loop on CORE1 (with 'core' == 5), then go through CORE2 (with
> 'core' == 6) and return 'core'. I find it a bit unusual that we wouldn't
> return the first CPU in the SMT mask, usually we try to fill sched_groups
> in cpumask order.
> 
> 
> If we could have 'cpus' start with only primary CPUs, that would simplify
> things methinks:
> 
>   for_each_cpu_wrap(core, cpus, target) {
> 	  bool idle = true;
> 
> 	  for_each_cpu(cpu, cpu_smt_mask(core)) {
> 		  if (!available_idle_cpu(cpu)) {
> 			  idle = false;
> 			  break;
> 		  }
> 
> 	  __cpumask_clear_cpu(core, cpus);
> 
> 	  if (idle)
> 		  return core;
> 
> 
> Food for thought; 

See here:

  https://lkml.kernel.org/r/20180530142236.667774973@infradead.org