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 04:03:25 +0800 [thread overview]
Message-ID: <20160601200325.GA18670@intel.com> (raw)
In-Reply-To: <1464773799.4023.72.camel@gmail.com>
On Wed, Jun 01, 2016 at 11:36:39AM +0200, Mike Galbraith wrote:
> > Yup. Up to this point, we don't have any disagreement. And I don't think we
> > have any disagreement conceptually. What the next patch really does is:
> >
> > (1) we don't remove SD_BALANCE_WAKE as an important sched_domain flag, on
> > the contrary, we strengthen it.
> >
> > (2) the semantic of SD_BALANCE_WAKE is currently represented by SD_WAKE_AFFINE,
> > we actually remove this representation.
>
> 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.
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,
> > (3) regarding the semantic of SD_WAKE_AFFINE, it is really not about selecting
> > waker CPU or about the fast path. Conceptually, it is just saying the waker
> > CPU is a valid and important candidate if SD_BALANCE_WAKE, which is just so
> > obvious, so I don't think it deserves to be a separate sched_domain flag.
>
> SD_WAKE_AFFINE being a separate domain flag, the user can turn it
> on/off... separately :)
Sure, that is very true, :) But turning it off for what, that is a big question mark.
We don't want a flag unless the flag is for goodness, and not a flag with big question
mark.
> > (4) the outcome is, if SD_BALANCE_WAKE, we definitely will/should try waker CPU,
> > and if !SD_BALANCE_WAKE, we don't try waker CPU. So nothing functional is
> > changed.
>
> 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. :)
next prev parent reply other threads:[~2016-06-02 4:00 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 [this message]
2016-06-02 5:50 ` Mike Galbraith
2016-06-01 22:41 ` Yuyang Du
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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