Message-ID: <1366630759.4077.6.camel@laptop>
Subject: Re: [PATCH v6] sched: fix init NOHZ_IDLE flag
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
        "linaro-kernel@lists.linaro.org" <linaro-kernel@lists.linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        =?ISO-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
        Paul Turner <pjt@google.com>, Steven Rostedt <rostedt@goodmis.org>,
        Mike Galbraith <efault@gmx.de>
Date: Mon, 22 Apr 2013 13:39:19 +0200
In-Reply-To: <CAKfTPtBMG6KPSZsMFnJqEAajg2qjRXowKSfrWHED-EhUCLB2+Q@mail.gmail.com>
References: <1366377044-23989-1-git-send-email-vincent.guittot@linaro.org>
	 <1366623052.2721.6.camel@laptop>
	 <CAKfTPtBMG6KPSZsMFnJqEAajg2qjRXowKSfrWHED-EhUCLB2+Q@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1858
Lines: 41

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.

> > 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.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/