linux-kernel - Re: sched: tweak select_idle_sibling to look for idle threads

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Date:	Tue, 12 Apr 2016 06:44:08 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Chris Mason <clm@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: sched: tweak select_idle_sibling to look for idle threads

On Mon, 2016-04-11 at 20:30 -0400, Chris Mason wrote:
> On Mon, Apr 11, 2016 at 06:54:21AM +0200, Mike Galbraith wrote:

> > > Ok, I was able to reproduce this by stuffing tbench_srv and tbench onto
> > > just socket 0.  Version 2 below fixes things for me, but I'm hoping
> > > someone can suggest a way to get task_hot() buddy checks without the rq
> > > lock.
> > > 
> > > I haven't run this on production loads yet, but our 4.0 patch for this
> > > uses task_hot(), so I'd expect it to be on par.  If this doesn't fix it
> > > for you, I'll dig up a similar machine on Monday.
> > 
> > My box stopped caring.  I personally would be reluctant to apply it
> > without a "you asked for it" button or a large pile of benchmark
> > results.  Lock banging or not, full scan existing makes me nervous.
> 
> 
> We can use a bitmap at the socket level to keep track of which cpus are
> idle.  I'm sure there are better places for the array and better ways to
> allocate, this is just a rough cut to make sure the idle tracking works.

See e0a79f529d5b:

      pre   15.22 MB/sec 1 procs
      post 252.01 MB/sec 1 procs

You can make traverse cycles go away, but those cycles, while precious,
are not the most costly cycles.  The above was 1 tbench pair in an
otherwise idle box.. ie it wasn't traverse cycles that demolished it.

	-Mike

(p.s. SCHED_IDLE is dinky bandwidth fair class)