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Date: Tue, 17 May 2016 04:11:09 +0800
From: Yuyang Du <yuyang.du@intel.com>
To: Matt Fleming <matt@codeblueprint.co.uk>
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: <20160516201109.GD8790@intel.com>
References: <1463082593-27777-1-git-send-email-matt@codeblueprint.co.uk>
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 <20160515021439.GC8790@intel.com>
 <20160516094638.GB6574@codeblueprint.co.uk>
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On Mon, May 16, 2016 at 10:46:38AM +0100, Matt Fleming wrote:
> > >  
> > > -	rq->clock_skip_update = 0;
> > > +	/* Clear ACT, preserve everything else */
> > > +	rq->clock_update_flags ^= RQCF_ACT_SKIP;
> > 
> > The comment says "Clear ACT", but this is really xor, and I am not sure
> > this is even what you want.
>  
> Urgh, you're right. I'm not sure what I was thinking when I wrote
> that.

It happens, ;)
 
> > In addition, would it be simpler to do this?
> > 
> > update_rq_clock()
> > 	if (flags & RQCF_ACT_SKIP)
> > 		flags <<= 1; /* effective skip is an update */
> > 		return;
> > 
> > 	flags = RQCF_UPDATED;
> 
> 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?).