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>
> ---
> 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);
BTW I wonder, even with GFP_ATOMIC, is this a good idea to do for a
temporary allocation like this one? pcpu_alloc() seems rather involved
to me and I've glanced at the other usages and they seem much more
long-lived. Maybe it would be really better to have single static
"works" and serialize the callers as you suggest in the changelog?
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