linux-kernel - Re: [RFC][PATCH 5/5] sched/core: Add debug code to catch missing update_rq

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Message-ID: <20160516094638.GB6574@codeblueprint.co.uk>
Date:	Mon, 16 May 2016 10:46:38 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Luca Abeni <luca.abeni@...tn.it>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [RFC][PATCH 5/5] sched/core: Add debug code to catch missing
 update_rq_clock()

On Sun, 15 May, at 10:14:39AM, Yuyang Du wrote:
> Hi Matt,
> 
> Thanks for Ccing me.
> 
> I am indeed interested, because I recently encountered an rq clock
> issue, which is that the clock jumps about 200ms when I was
> experimenting the "flat util hierarchy" patches, which really annoyed
> me, and I had to stop to figure out what is wrong (but haven't yet
> figured out ;))
> 
> First, this patchset does not solve my problem, but never mind, by
> reviewing your patches, I have some comments:
 
Thanks for the review. One gap that this patch series doesn't address
is that some callers of update_rq_clock() do not pin rq->lock, which
makes the diagnostic checks useless in that case.

I plan on handling that next, but I wanted to get this series out as
soon as possible for review.

> On Thu, May 12, 2016 at 08:49:53PM +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.

> 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.