On Wed, 25 Jan 2017 08:30:38 +0000 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
> As suggested by Vlastimil Babka and Tejun Heo, this patch uses a static
> work_struct to co-ordinate the draining of per-cpu pages on the workqueue.
> Only one task can drain at a time but this is better than the previous
> scheme that allowed multiple tasks to send IPIs at a time.
>
> One consideration is whether parallel requests should synchronise against
> each other. This patch does not synchronise for a global drain as the common
> case for such callers is expected to be multiple parallel direct reclaimers
> competing for pages when the watermark is close to min. Draining the per-cpu
> list is unlikely to make much progress and serialising the drain is of
> dubious merit. Drains are synchonrised for callers such as memory hotplug
> and CMA that care about the drain being complete when the function returns.
>
> ...
>
> @@ -2402,24 +2415,16 @@ void drain_all_pages(struct zone *zone)
> cpumask_clear_cpu(cpu, &cpus_with_pcps);
> }
>
> - 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);
> - }
> - 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);
> - }
> + for_each_cpu(cpu, &cpus_with_pcps) {
> + struct work_struct *work = per_cpu_ptr(&pcpu_drain, cpu);
> + INIT_WORK(work, drain_local_pages_wq);
It's strange to repeatedly run INIT_WORK() in this fashion.
Overwriting an atomic_t which should already be zero, initializing a
list_head which should already be in the initialized state...
Can we instead do this a single time in init code?
> + schedule_work_on(cpu, work);
> }
> + for_each_cpu(cpu, &cpus_with_pcps)
> + flush_work(per_cpu_ptr(&pcpu_drain, cpu));
> +
> put_online_cpus();
> + mutex_unlock(&pcpu_drain_mutex);
> }
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