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

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Date:	Mon, 9 May 2016 02:57:47 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Mike Galbraith <mgalbraith@...e.de>
Cc:	Peter Zijlstra <peterz@...radead.org>, Chris Mason <clm@...com>,
	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 Sun, May 08, 2016 at 10:08:55AM +0200, Mike Galbraith wrote:
> > Maybe give the criteria a bit margin, not just wakees tend to equal llc_size,
> > but the numbers are so wild to easily break the fragile condition, like:
> 
> Seems lockless traversal and averages just lets multiple CPUs select
> the same spot.  An atomic reservation (feature) when looking for an
> idle spot (also for fork) might fix it up.  Run the thing as RT,
> push/pull ensures that it reaches box saturation regardless of the
> number of messaging threads, whereas with fair class, any number > 1
> will certainly stack tasks before the box is saturated.

Yes, good idea, bringing order to the race to grab idle CPU is absolutely
helpful.

In addition, I would argue maybe beefing up idle balancing is a more
productive way to spread load, as work-stealing just does what needs
to be done. And seems it has been (sub-unconsciously) neglected in this
case, :)

Regarding wake_wide(), it seems the M:N is 1:24, not 6:6*24, if so,
the slave will be 0 forever (as last_wakee is never flipped).

Basically whenever a waker has more than 1 wakee, the wakee_flips
will comfortably grow very large (with last_wakee alternating),
whereas when a waker has 0 or 1 wakee, the wakee_flips will just be 0.

So recording only the last_wakee seems not right unless you have other
good reason. If not the latter, counting waking wakee times should be
better, and then allow the statistics to happily play.