linux-kernel - Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems

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Message-ID: <20151211133612.GG6373@twins.programming.kicks-ass.net>
Date:	Fri, 11 Dec 2015 14:36:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	yuyang.du@...el.com, Morten Rasmussen <morten.rasmussen@....com>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems

On Fri, Dec 11, 2015 at 02:25:51PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 03:55:18PM +0300, Andrey Ryabinin wrote:
> > Make 'r' 64-bit type to avoid overflow in 'r * LOAD_AVG_MAX'
> > on 32-bit systems:
> > 	UBSAN: Undefined behaviour in kernel/sched/fair.c:2785:18
> > 	signed integer overflow:
> > 	87950 * 47742 cannot be represented in type 'int'
> > 
> > Fixes: 9d89c257dfb9 ("sched/fair: Rewrite runnable load and utilization average tracking")
> > Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> > ---
> >  kernel/sched/fair.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index e3266eb..733f0b8 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -2780,14 +2780,14 @@ static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> >  	int decayed, removed = 0;
> >  
> >  	if (atomic_long_read(&cfs_rq->removed_load_avg)) {
> > -		long r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
> > +		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);
> 
> This makes sense, because sched_avg::load_sum is u64.
> 
> >  		removed = 1;
> >  	}
> >  
> >  	if (atomic_long_read(&cfs_rq->removed_util_avg)) {
> > -		long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0);
> > +		s64 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);
> >  	}
> 
> However sched_avg::util_sum is u32, so this is still wrecked.

I seems to have wrecked that in:

  006cdf025a33 ("sched/fair: Optimize per entity utilization tracking")

maybe just make util_load u64 too?
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