Re: [PATCH 3/4] mm, page_alloc: Drain per-cpu pages from workqueue context

[Vlastimil Babka]

On 01/17/2017 10:29 AM, Mel Gorman wrote:
> The per-cpu page allocator can be drained immediately via drain_all_pages()
> which sends IPIs to every CPU. In the next patch, the per-cpu allocator
> will only be used for interrupt-safe allocations which prevents draining
> it from IPI context. This patch uses workqueues to drain the per-cpu
> lists instead.
> 
> This is slower but no slowdown during intensive reclaim was measured and
> the paths that use drain_all_pages() are not that sensitive to performance.
> This is particularly true as the path would only be triggered when reclaim
> is failing. It also makes a some sense to avoid storming a machine with IPIs
> when it's under memory pressure. Arguably, it should be further adjusted
> so that only one caller at a time is draining pages but it's beyond the
> scope of the current patch.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

I'm not a workqueue expert (CC Petr Mladek) but I compared this to
lru_add_drain_all() and have some questions...

> ---
>  mm/page_alloc.c | 42 +++++++++++++++++++++++++++++++++++-------
>  1 file changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d15527a20dce..9c3a0fcf8c13 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2341,19 +2341,21 @@ void drain_local_pages(struct zone *zone)
>  		drain_pages(cpu);
>  }
>  
> +static void drain_local_pages_wq(struct work_struct *work)
> +{
> +	drain_local_pages(NULL);
> +}
> +
>  /*
>   * Spill all the per-cpu pages from all CPUs back into the buddy allocator.
>   *
>   * When zone parameter is non-NULL, spill just the single zone's pages.
>   *
> - * Note that this code is protected against sending an IPI to an offline
> - * CPU but does not guarantee sending an IPI to newly hotplugged CPUs:
> - * on_each_cpu_mask() blocks hotplug and won't talk to offlined CPUs but
> - * nothing keeps CPUs from showing up after we populated the cpumask and
> - * before the call to on_each_cpu_mask().
> + * Note that this can be extremely slow as the draining happens in a workqueue.
>   */
>  void drain_all_pages(struct zone *zone)
>  {
> +	struct work_struct __percpu *works;
>  	int cpu;
>  
>  	/*
> @@ -2362,6 +2364,16 @@ void drain_all_pages(struct zone *zone)
>  	 */
>  	static cpumask_t cpus_with_pcps;
>  
> +	/* Workqueues cannot recurse */
> +	if (current->flags & PF_WQ_WORKER)
> +		return;
> +
> +	/*
> +	 * As this can be called from reclaim context, do not reenter reclaim.
> +	 * An allocation failure can be handled, it's simply slower
> +	 */
> +	works = alloc_percpu_gfp(struct work_struct, GFP_ATOMIC);
> +
>  	/*
>  	 * We don't care about racing with CPU hotplug event
>  	 * as offline notification will cause the notified
> @@ -2392,8 +2404,24 @@ void drain_all_pages(struct zone *zone)
>  		else
>  			cpumask_clear_cpu(cpu, &cpus_with_pcps);
>  	}
> -	on_each_cpu_mask(&cpus_with_pcps, (smp_call_func_t) drain_local_pages,
> -								zone, 1);
> +
> +	if (works) {
> +		for_each_cpu(cpu, &cpus_with_pcps) {
> +			struct work_struct *work = per_cpu_ptr(works, cpu);
> +			INIT_WORK(work, drain_local_pages_wq);
> +			schedule_work_on(cpu, work);

This translates to queue_work_on(), which has the comment of "We queue
the work to a specific CPU, the caller must ensure it can't go away.",
so is this safe? lru_add_drain_all() uses get_online_cpus() around this.

schedule_work_on() also uses the generic system_wq, while lru drain has
its own workqueue with WQ_MEM_RECLAIM so it seems that would be useful
here as well?

> +		}
> +		for_each_cpu(cpu, &cpus_with_pcps)
> +			flush_work(per_cpu_ptr(works, cpu));
> +	} else {
> +		for_each_cpu(cpu, &cpus_with_pcps) {
> +			struct work_struct work;
> +
> +			INIT_WORK(&work, drain_local_pages_wq);
> +			schedule_work_on(cpu, &work);
> +			flush_work(&work);

Totally out of scope, but I wonder if schedule_on_each_cpu() could use
the same fallback that's here?

> +		}
> +	}
>  }
>  
>  #ifdef CONFIG_HIBERNATION
>