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    SubjectRe: [PATCH] mm, slab: reschedule cache_reap() on the same CPU
    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 <vbabka@suse.cz>
    > Fixes: ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask CPUs")
    > CC: <stable@vger.kernel.org>
    > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    > Cc: David Rientjes <rientjes@google.com>
    > Cc: Pekka Enberg <penberg@kernel.org>
    > Cc: Christoph Lameter <cl@linux.com>
    > Cc: Tejun Heo <tj@kernel.org>
    > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
    > Cc: John Stultz <john.stultz@linaro.org>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: Stephen Boyd <sboyd@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
    >

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