On Wed, Apr 11, 2018 at 09:00:07AM +0200, Vlastimil Babka wrote:
> cache_reap() is initially scheduled in start_cpu_timer() via
> schedule_delayed_work_on(). But then the next iterations are scheduled via
> schedule_delayed_work(), i.e. using WORK_CPU_UNBOUND.
>
> Thus since commit ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on
> wq_unbound_cpumask CPUs") there is no guarantee the future iterations will run
> on the originally intended cpu, although it's still preferred. I was able to
> demonstrate this with /sys/module/workqueue/parameters/debug_force_rr_cpu.
> IIUC, it may also happen due to migrating timers in nohz context. As a result,
> some cpu's would be calling cache_reap() more frequently and others never.
>
> This patch uses schedule_delayed_work_on() with the current cpu when
> scheduling
> the next iteration.
Could you write down part about "so what's the user effect on some condition?".
It would really help to pick up the patch.
>
> Signed-off-by: Vlastimil Babka <vba...@suse.cz>
> Fixes: ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on
> wq_unbound_cpumask CPUs")
> CC: <sta...@vger.kernel.org>
> Cc: Joonsoo Kim <iamjoonsoo....@lge.com>
> Cc: David Rientjes <rient...@google.com>
> Cc: Pekka Enberg <penb...@kernel.org>
> Cc: Christoph Lameter <c...@linux.com>
> Cc: Tejun Heo <t...@kernel.org>
> Cc: Lai Jiangshan <jiangshan...@gmail.com>
> Cc: John Stultz <john.stu...@linaro.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Stephen Boyd <sb...@kernel.org>
> ---
> mm/slab.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index 9095c3945425..a76006aae857 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -4074,7 +4074,8 @@ static void cache_reap(struct work_struct *w)
> next_reap_node();
> out:
> /* Set up the next iteration */
> - schedule_delayed_work(work, round_jiffies_relative(REAPTIMEOUT_AC));
> + schedule_delayed_work_on(smp_processor_id(), work,
> + round_jiffies_relative(REAPTIMEOUT_AC));
> }
>
> void get_slabinfo(struct kmem_cache *cachep, struct slabinfo *sinfo)
> --
> 2.16.3
>
