linux-kernel - Re: [RFC PATCH 1/2] sched: Clean up SD_BALANCE

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Date:	Thu, 2 Jun 2016 06:41:06 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, bsegall@...gle.com, pjt@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com
Subject: Re: [RFC PATCH 1/2] sched: Clean up SD_BALANCE_WAKE flags in sched
 domain build-up

On Thu, Jun 02, 2016 at 07:50:23AM +0200, Mike Galbraith wrote:
> > > Nope, those two have different meanings.  We pass SD_BALANCE_WAKE to
> > > identify a ttwu() wakeup, just as we pass SD_BALANCE_FORK to say we're
> > > waking a child.  SD_WAKE_AFFINE means exactly what it says, but is only
> > > applicable to ttwu() wakeups.
> >  
> > I don't disagree, but want to add that, SD_WAKE_AFFINE has no meaning that is so
> > special and so important for anyone to use the flag to tune anything. If you want
> > to do any SD_BALANCE_*, waker CPU is a valid candidate if allowed, that is it.
> 
> That flag lets the user specifically tell us that he doesn't want us to
> bounce his tasks around the box, cache misses be damned.  The user may
> _know_ that say cross node migrations hurt his load more than help, and
> not want us to do that, thus expresses himself by turning the flag off
> at whatever level.  People do that.  You can force them to take other
> measures, but why do that?
 
Agreed, and with this patch, just disable SD_BALANCE_WAKE.

> > IIUC your XXX mark and your comment "Prefer wake_affine over balance flags", you
> > said the same thing: SD_WAKE_AFFINE should be part of SD_BALANCE_WAKE, and should
> > be part of all SD_BALANCE_* flags,
> 
> Peter wrote that, but I don't read it the way you do.  I read as if the
> user wants the benefits of affine wakeups, he surely doesn't want us to
> send the wakee off to god know where on every wakeup _instead_ of
> waking affine, he wants to balance iff he can't have an affine wakeup.

That is another matter within SD_BALANCE_WAKE we may further define: how
much effort to scan or how frequent bouncing etc the user wants. This is now
defined by SD_WAKE_AFFINE flag, which I certainly don't think is good.
 
> > > If wake_wide() says we do not want an affine wakeup, but you apply
> > > SD_WAKE_AFFINE meaning to SD_BALANCE_WAKE and turn it on in ->flags,
> > > we'll give the user a free sample of full balance cost, no?
> >  
> > Yes, and otherwise we don't select anything? That is just bad engough whether worse
> > or not. So the whole fuss I made is really that this is a right thing to start with. :)
> 
> Nope, leaving tasks where they were is not a bad thing.  Lots of stuff
> likes the scheduler best when it leaves them the hell alone :)  That
> works out well all around, balance cycles are spent in userspace
> instead, scheduler produces wins by doing nothing, perfect.
> 
Again, agreed, and with this patch, just disable SD_BALANCE_WAKE. :)