linux-kernel - Re: [PATCH 4.4 043/146] sched/fair: Fix cfs

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Date:	Tue, 2 Aug 2016 13:24:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	bsegall@...gle.com
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yuyang Du <yuyang.du@...el.com>, kernel@...p.com,
	morten.rasmussen@....com, pjt@...gle.com, steve.muckle@...aro.org,
	Ingo Molnar <mingo@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 4.4 043/146] sched/fair: Fix cfs_rq avg tracking underflow

On Mon, Jul 25, 2016 at 03:12:24PM -0700, bsegall@...gle.com wrote:
> > @@ -2690,15 +2707,15 @@ static inline int update_cfs_rq_load_avg
> >  
> >  	if (atomic_long_read(&cfs_rq->removed_load_avg)) {
> >  		s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
> > -		sa->load_avg = max_t(long, sa->load_avg - r, 0);
> > -		sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0);
> > +		sub_positive(&sa->load_avg, r);
> > +		sub_positive(&sa->load_sum, r * LOAD_AVG_MAX);
> >  		removed = 1;
> >  	}
> >  
> >  	if (atomic_long_read(&cfs_rq->removed_util_avg)) {
> >  		long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0);
> > -		sa->util_avg = max_t(long, sa->util_avg - r, 0);
> > -		sa->util_sum = max_t(s32, sa->util_sum - r * LOAD_AVG_MAX, 0);
> > +		sub_positive(&sa->util_avg, r);
> > +		sub_positive(&sa->util_sum, r * LOAD_AVG_MAX);
> >  	}
> >  
> >  	decayed = __update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,

> I missed this the first time around, and I have no problem with this
> backport, but it's not remotely obvious that update_cfs_rq_h_load() is
> intended to be something approaching racing-safe when not under
> rq->lock. (And given only somewhat-adversarial compilers I agree that it
> probably won't do any worse than skip updates, though I certainly won't
> swear to it)

Right, this is somewhat 'new' because we now use the avg load value for
the load-balancer, which is entirely unserialized.