linux-kernel - Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing update_rq

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Message-ID: <20160922083637.GE2834@quack2.suse.cz>
Date:   Thu, 22 Sep 2016 10:36:37 +0200
From:   Jan Kara <jack@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Ingo Molnar <mingo@...nel.org>,
        Byungchul Park <byungchul.park@....com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Luca Abeni <luca.abeni@...tn.it>,
        Rik van Riel <riel@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Yuyang Du <yuyang.du@...el.com>, Jan Kara <jack@...e.cz>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH v2 7/7] sched/core: Add debug code to catch missing
 update_rq_clock()

On Thu 22-09-16 10:04:36, Peter Zijlstra wrote:
> On Wed, Sep 21, 2016 at 05:58:27PM +0200, Petr Mladek wrote:
> > > +static inline void assert_clock_updated(struct rq *rq)
> > > +{
> > > +#ifdef CONFIG_SCHED_DEBUG
> > > +	/*
> > > +	 * The only reason for not seeing a clock update since the
> > > +	 * last rq_pin_lock() is if we're currently skipping updates.
> > > +	 */
> > > +	WARN_ON_ONCE(rq->clock_update_flags < RQCF_ACT_SKIP);
> > > +#endif
> > > +}
> > 
> > I am afraid that it might eventually create a deadlock.
> > For example, there is the following call chain:
> > 
> 
> Yeah, meh. There's already plenty WARNs in the sched code. The idea of
> course being that they should not trigger. If they do, something
> buggered already, so who bloody cares about a deadlock later ;-)

Yeah, the trouble is that you usually won't see the WARN message
before deadlocking. So WARN_ON in scheduler is usually equivalent to

	if (condition)
		while (1);

;) Not really helping debugging much...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR