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 > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>