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    Subject[PATCH] mm, slab: reschedule cache_reap() on the same CPU
    Date
    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.

    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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