linux-kernel - Re: [PATCH v6] sched: fix init NOHZ

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Date:	Mon, 22 Apr 2013 14:05:59 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Paul Turner <pjt@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH v6] sched: fix init NOHZ_IDLE flag

On 22 April 2013 13:39, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2013-04-22 at 13:01 +0200, Vincent Guittot wrote:
>> > I'm not quite getting things.. what's wrong with adding this flags
>> > thing to sched_domain itself? That's already RCU destroyed so why
>> add a
>> > second RCU layer?
>>
>> We need one flags for all sched_domain so if we add it into
>> sched_domain struct, we have to define which one will handle the flags
>> for all other and find it in the sched_domain tree when we need it.
>
> Just pick rq->sd -- if the root_domain thing doesn't work out.
>
>>  In
>> addition, the flags in other sched_domain will be a waste of space.
>> The RCU in sched_domain might become useless as it is protected by the
>> one that is in sched_domain_rq
>
> I'm all for wasting space instead over adding extra pointer chasing all
> over the place. But also, look at pahole -C sched_domain, there's
> plenty of 4 byte holes in there where we can stuff a single bit.

Ok, I'm going to move the flags in sched_domain struct.
This should make the fix simpler

>
>> > We also have the root_domain for things that don't need to go in a
>> > hierarchy but are once per cpu -- it sounds like this is one of
>> those
>> > things; iirc the root_domain life-time is the same as the entire
>> > sched_domain tree so adding it to the root_domain is also an option.
>>
>> AFAICT, it doesn't share the same RCU object and as a result the same
>> lifecycle than sched_domain so there is a time window where
>> sched_domain and flags could lost their synchronization.
>> Nevertheless, i'm going to have a look at root_domain
>
> They're set under the same write side lock at the same time rq->sd it
> set, but yes I suppose that since its a separate pointer there might be
> a tiny window where we could go wrong.
>
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