Re: [PATCH] mm, slab: reschedule cache_reap() on the same CPU

[Minchan Kim]

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@xxxxxxx>
> Fixes: ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask CPUs")
> CC: <stable@xxxxxxxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
> ---
>  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
>