From: Yuyang Du <yuyang.du@intel.com>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
mingo@kernel.org, linux-kernel@vger.kernel.org,
bsegall@google.com, pjt@google.com, morten.rasmussen@arm.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com
Subject: Re: [RFC PATCH 1/2] sched: Clean up SD_BALANCE_WAKE flags in sched domain build-up
Date: Thu, 2 Jun 2016 06:41:06 +0800 [thread overview]
Message-ID: <20160601224106.GB18670@intel.com> (raw)
In-Reply-To: <1464846623.3766.49.camel@gmail.com>
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. :)
next prev parent reply other threads:[~2016-06-02 6:38 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-31 1:11 [RFC PATCH 0/2] Remove and replace SD_WAKE_AFFINE with SD_BALANCE_WAKE Yuyang Du
2016-05-31 1:11 ` [RFC PATCH 1/2] sched: Clean up SD_BALANCE_WAKE flags in sched domain build-up Yuyang Du
2016-05-31 9:21 ` Peter Zijlstra
2016-05-31 1:31 ` Yuyang Du
2016-05-31 10:41 ` Peter Zijlstra
2016-05-31 18:00 ` Yuyang Du
2016-06-01 5:07 ` Mike Galbraith
2016-06-01 0:01 ` Yuyang Du
2016-06-01 8:32 ` Vincent Guittot
2016-06-01 1:03 ` Yuyang Du
2016-06-01 9:24 ` Vincent Guittot
2016-06-01 19:35 ` Yuyang Du
2016-06-02 6:56 ` Vincent Guittot
2016-06-01 23:19 ` Yuyang Du
2016-06-01 9:36 ` Mike Galbraith
2016-06-01 20:03 ` Yuyang Du
2016-06-02 5:50 ` Mike Galbraith
2016-06-01 22:41 ` Yuyang Du [this message]
2016-06-02 6:44 ` Mike Galbraith
2016-05-31 1:11 ` [RFC PATCH 2/2] sched: Remove SD_WAKE_AFFINE flag and replace it with SD_BALANCE_WAKE Yuyang Du
2016-05-31 9:23 ` Peter Zijlstra
2016-05-31 1:34 ` Yuyang Du
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