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Date: Tue, 17 May 2016 13:24:15 +0100
From: Matt Fleming <matt@codeblueprint.co.uk>
To: Yuyang Du <yuyang.du@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel@vger.kernel.org, Byungchul Park <byungchul.park@lge.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Luca Abeni <luca.abeni@unitn.it>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Rik van Riel <riel@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Galbraith <umgwanakikbuti@gmail.com>
Subject: Re: [RFC][PATCH 5/5] sched/core: Add debug code to catch missing
 update_rq_clock()
Message-ID: <20160517122415.GD21993@codeblueprint.co.uk>
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On Tue, 17 May, at 04:11:09AM, Yuyang Du wrote:
> On Mon, May 16, 2016 at 10:46:38AM +0100, Matt Fleming wrote:
> > 
> > No because if someone calls rq_clock() immediately after __schedule(),
> > or even immediately after we clear RQCF_ACT_SKIP in __schedule(), we
> > should trigger a warning since the clock has not actually been
> > updated.
> 
> Well, I don't know how concurrent it can be, but aren't both update
> and read synchronized by rq->lock? So I don't understand the latter
> case, and the former should be addressed (missing its own update?).

I'm not talking about concurrency; when I said "someone" above, I was
referring to code.

So, if the code looks like the following, either now or in the future,

static void __schedule(bool preempt)
{
	...
	/* Clear RQCF_ACT_SKIP */
	rq->clock_update_flags = 0;
	...
	delta = rq_clock();
}

I would expect to see a warning triggered, because we've read the rq
clock outside of the code area where we know it's safe to do so
without a clock update.

The solution for that bug may be as simple as rearranging the code,

	delta = rq_clock();
	...
	rq->clock_update_flags = 0;

but we definitely want to catch such bugs in the first instance.