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

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Date:	Wed, 18 May 2016 09:41:20 +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 Wed, 18 May, at 03:01:27AM, Yuyang Du wrote:
> On Tue, May 17, 2016 at 01:24:15PM +0100, Matt Fleming wrote:
> > 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();
> > }
>  
> Sigh, you even said "Clear RQCF_ACT_SKIP", but you not only clear it,
> you clear everything.

That was sloppy on my part but intentional because that's what the
code looks like in tip/sched/core today.

It was purely meant to demonstrate that setting RQCF_UPDATE just
because RQCF_ACT_SKIP is set does not make sense. You can replace the
clearing line with the correct bit masking operation.

But I get it, the pseudo-code was confusing. I'll send out a v2.

> And if you clear the RQCF_UPDATE also (maybe you shouldn't, but
> actually it does not matter), of course you will get a warning...

Right, I wouldn't actually clear RQCF_UPDATE in v2 of this patch.

> In addition, it looks like multiple skips are possible, so:
 
I'm not sure what you mean, could you elaborate?

> update_rq_clock() {
>         rq->clock_update_flags |= RQCF_UPDATE;
> 
>         ...
> }
> 
> instead of clearing the skip flag there.

Huh? RQCF_*_SKIP are not cleared in update_rq_clock().