linux-kernel - Re: [PATCH] sched: Avoid division by zero

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Date:	Thu, 27 Aug 2009 14:32:53 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	jes@....com, jens.axboe@...cle.com, tglx@...utronix.de,
	mingo@...e.hu, Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-tip-commits@...r.kernel.org
Subject: Re: [PATCH] sched: Avoid division by zero - really

On Thu, 2009-08-27 at 14:19 +0200, Eric Dumazet wrote:
> Peter Zijlstra a écrit :
> > When re-computing the shares for each task group's cpu representation we
> > need the ratio of weight on each cpu vs the total weight of the sched
> > domain.
> > 
> > Since load-balancing is loosely (read not) synchronized, the weight of
> > individual cpus can change between doing the sum and calculating the
> > ratio.
> > 
> > The previous patch dealt with only one of the race scenarios, this patch
> > side steps them all by saving a snapshot of all the individual cpu
> > weights, thereby always working on a consistent set.
> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > ---
> >  kernel/sched.c |   50 +++++++++++++++++++++++++++++---------------------
> >  1 files changed, 29 insertions(+), 21 deletions(-)
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 0e76b17..4591054 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -1515,30 +1515,29 @@ static unsigned long cpu_avg_load_per_task(int cpu)
> >  
> >  #ifdef CONFIG_FAIR_GROUP_SCHED
> >  
> > +struct update_shares_data {
> > +	unsigned long rq_weight[NR_CPUS];
> > +};
> > +
> > +static DEFINE_PER_CPU(struct update_shares_data, update_shares_data);
> 
> ouch... thats quite large IMHO, up to 4096*8 = 32768 bytes per cpu...
> 
> Now we have nice dynamic per cpu allocations, we could use it here,
> and use nr_cpus instead of NR_CPUS as the array size ?

Possibly, but I guess that should include stuff like
static_sched_{domain,group} too, since they seem to have the same
problem.

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