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Message-ID: <20130916055202.GL21832@twins.programming.kicks-ass.net>
Date: Mon, 16 Sep 2013 07:52:02 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Vladimir Davydov <vdavydov@...allels.com>
Cc: Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
linux-kernel@...r.kernel.org, devel@...nvz.org
Subject: Re: [PATCH 1/2] sched: calculate_imbalance: Fix local->avg_load >
sds->avg_load case
On Sun, Sep 15, 2013 at 05:49:13PM +0400, Vladimir Davydov wrote:
> In busiest->group_imb case we can come to calculate_imbalance() with
> local->avg_load >= busiest->avg_load >= sds->avg_load. This can result
> in imbalance overflow, because it is calculated as follows
>
> env->imbalance = min(
> max_pull * busiest->group_power,
> (sds->avg_load - local->avg_load) * local->group_power
> ) / SCHED_POWER_SCALE;
>
> As a result we can end up constantly bouncing tasks from one cpu to
> another if there are pinned tasks.
>
> Fix this by skipping the assignment and assuming imbalance=0 in case
> local->avg_load > sds->avg_load.
> --
> The bug can be caught by running 2*N cpuhogs pinned to two logical cpus
> belonging to different cores on an HT-enabled machine with N logical
> cpus: just look at se.nr_migrations growth.
>
> Signed-off-by: Vladimir Davydov <vdavydov@...allels.com>
> ---
> kernel/sched/fair.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9b3fe1c..507a8a9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4896,7 +4896,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
> * max load less than avg load(as we skip the groups at or below
> * its cpu_power, while calculating max_load..)
> */
> - if (busiest->avg_load < sds->avg_load) {
> + if (busiest->avg_load <= sds->avg_load ||
> + local->avg_load >= sds->avg_load) {
> env->imbalance = 0;
> return fix_small_imbalance(env, sds);
> }
Why the = part? Surely 'busiest->avg_load < sds->avg_load ||
local->avg_load > sds->avg_load' avoids both underflows?
--
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