Message-ID: <1429151341.7039.162.camel@j-VirtualBox>
Subject: Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the
 scheduler
From: Jason Low <jason.low2@hp.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Mel Gorman <mgorman@suse.de>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>, hideaki.kimura@hp.com,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>, jason.low2@hp.com
Date: Wed, 15 Apr 2015 19:29:01 -0700
In-Reply-To: <20150414224059.061ec5bf@grimm.local.home>
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On Tue, 2015-04-14 at 22:40 -0400, Steven Rostedt wrote:
> You are correct. Now I'm thinking that the WRITE_ONCE() is not needed,
> and just a:
> 
> 	p->mm->numa_scan_seq = READ_ONCE(p->numa_scan_seq) + 1;

Just to confirm, is this a typo? Because there really is a numa_scan_seq
in the task_struct itself too :)

p->mm->numa_scan_seq is read in task_numa_placement() with
ACCESS_ONCE(), and so the benefit that I do see with it is that it makes
it consistent by doing the updates with ACCESS_ONCE too (for
documentation purposes).

If that's really the case:

    WRITE_ONCE(p->mm->numa_scan_seq, p->mm->numa_scan_seq + 1)

should be enough for that.

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