Re: [patch 10/15] sched/migration: Move calc_load_migrate() into CPU_DYING

Shilpasri G Bhat
2016-07-12T20:05:46Z

Hi,

On 07/12/2016 10:03 PM, Thomas Gleixner wrote:
> Anton,
>
> On Tue, 12 Jul 2016, Anton Blanchard wrote:
>>> It really does not matter when we fold the load for the outgoing cpu.
>>> It's almost dead anyway, so there is no harm if we fail to fold the
>>> few microseconds which are required for going fully away.
>>
>> We are seeing the load average shoot up when hot unplugging CPUs (+1
>> for every CPU we offline) on ppc64. This reproduces on bare metal as
>> well as inside a KVM guest. A bisect points at this commit.
>>
>> As an example, a completely idle box with 128 CPUS and 112 hot
>> unplugged:
>>
>> # uptime
>> 04:35:30 up 1:23, 2 users, load average: 112.43, 122.94, 125.54
>
> Yes, it's an off by one as we now call that from the task which is tearing
> down the cpu. Does the patch below fix it?

Tested your patch to see that on an idle box on offlinig the cpus I dont see
increase in loadaverage.

# uptime
01:27:44 up 10 min, 1 user, load average: 0.00, 0.18, 0.18

# lscpu | grep -Ei "on-line|off-line"
On-line CPU(s) list: 0-127

# ppc64_cpu --cores-on=2

# lscpu | grep -Ei "on-line|off-line"
On-line CPU(s) list: 0-15
Off-line CPU(s) list: 16-127

# sleep 60
# uptime
01:28:52 up 11 min, 1 user, load average: 0.11, 0.19, 0.18

Thanks and Regards,
Shilpa

>
> Thanks,
>
> tglx
>
> 8<----------------------
>
> Subject: sched/migration: Correct off by one in load migration
> From: Thomas Gleixner <tglx@linutronix.de>
>
> The move of calc_load_migrate() from CPU_DEAD to CPU_DYING did not take into
> account that the function is now called from a thread running on the outgoing
> CPU. As a result a cpu unplug leakes a load of 1 into the global load
> accounting mechanism.
>
> Fix it by adjusting for the currently running thread which calls
> calc_load_migrate().
>
> Fixes: e9cd8fa4fcfd: "sched/migration: Move calc_load_migrate() into CPU_DYING"
> Reported-by: Anton Blanchard <anton@samba.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 51d7105f529a..97ee9ac7e97c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5394,13 +5394,15 @@ void idle_task_exit(void)
> /*
> * Since this CPU is going 'away' for a while, fold any nr_active delta
> * we might have. Assumes we're called after migrate_tasks() so that the
> - * nr_active count is stable.
> + * nr_active count is stable. We need to take the teardown thread which
> + * is calling this into account, so we hand in adjust = 1 to the load
> + * calculation.
> *
> * Also see the comment "Global load-average calculations".
> */
> static void calc_load_migrate(struct rq *rq)
> {
> - long delta = calc_load_fold_active(rq);
> + long delta = calc_load_fold_active(rq, 1);
> if (delta)
> atomic_long_add(delta, &calc_load_tasks);
> }
> diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
> index b0b93fd33af9..a2d6eb71f06b 100644
> --- a/kernel/sched/loadavg.c
> +++ b/kernel/sched/loadavg.c
> @@ -78,11 +78,11 @@ void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
> loads[2] = (avenrun[2] + offset) << shift;
> }
>
> -long calc_load_fold_active(struct rq *this_rq)
> +long calc_load_fold_active(struct rq *this_rq, long adjust)
> {
> long nr_active, delta = 0;
>
> - nr_active = this_rq->nr_running;
> + nr_active = this_rq->nr_running - adjust;
> nr_active += (long)this_rq->nr_uninterruptible;
>
> if (nr_active != this_rq->calc_load_active) {
> @@ -188,7 +188,7 @@ void calc_load_enter_idle(void)
> * We're going into NOHZ mode, if there's any pending delta, fold it
> * into the pending idle delta.
> */
> - delta = calc_load_fold_active(this_rq);
> + delta = calc_load_fold_active(this_rq, 0);
> if (delta) {
> int idx = calc_load_write_idx();
>
> @@ -389,7 +389,7 @@ void calc_global_load_tick(struct rq *this_rq)
> if (time_before(jiffies, this_rq->calc_load_update))
> return;
>
> - delta = calc_load_fold_active(this_rq);
> + delta = calc_load_fold_active(this_rq, 0);
> if (delta)
> atomic_long_add(delta, &calc_load_tasks);
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 7cbeb92a1cb9..898c0d2f18fe 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -28,7 +28,7 @@ extern unsigned long calc_load_update;
> extern atomic_long_t calc_load_tasks;
>
> extern void calc_global_load_tick(struct rq *this_rq);
> -extern long calc_load_fold_active(struct rq *this_rq);
> +extern long calc_load_fold_active(struct rq *this_rq, long adjust);
>
> #ifdef CONFIG_SMP
> extern void cpu_load_update_active(struct rq *this_rq);
>
>
>