On Tue, Feb 07, 2017 at 10:23:31AM +0100, Vlastimil Babka wrote: > > cpu offlining. I have to check the code but my impression was that WQ > > code will ignore the cpu requested by the work item when the cpu is > > going offline. If the offline happens while the worker function already > > executes then it has to wait as we run with preemption disabled so we > > should be safe here. Or am I missing something obvious? > > Tejun suggested an alternative solution to avoiding get_online_cpus() in > this thread: > https://lkml.kernel.org/r/<20170123170329.GA7820@xxxxxxxxxxxxxxx> But it would look like the following as it could be serialised against pcpu_drain_mutex as the cpu hotplug teardown callback is allowed to sleep. diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 3b93879990fd..8cd8b1bbe00c 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2319,9 +2319,17 @@ static void drain_pages(unsigned int cpu) { struct zone *zone; + /* + * A per-cpu drain via a workqueue from drain_all_pages can be + * rescheduled onto an unrelated CPU. That allows the hotplug + * operation and the drain to potentially race on the same + * CPU. Serialise hotplug versus drain using pcpu_drain_mutex + */ + mutex_lock(&pcpu_drain_mutex); for_each_populated_zone(zone) { drain_pages_zone(cpu, zone); } + mutex_unlock(&pcpu_drain_mutex); } /* @@ -2377,13 +2385,10 @@ void drain_all_pages(struct zone *zone) mutex_lock(&pcpu_drain_mutex); } - get_online_cpus(); - /* - * We don't care about racing with CPU hotplug event - * as offline notification will cause the notified - * cpu to drain that CPU pcps and on_each_cpu_mask - * disables preemption as part of its processing + * We don't care about racing with CPU hotplug event as offline + * notification will cause the notified cpu to drain that CPU pcps + * and it is serialised against here via pcpu_drain_mutex. */ for_each_online_cpu(cpu) { struct per_cpu_pageset *pcp; @@ -2418,7 +2423,6 @@ void drain_all_pages(struct zone *zone) for_each_cpu(cpu, &cpus_with_pcps) flush_work(per_cpu_ptr(&pcpu_drain, cpu)); - put_online_cpus(); mutex_unlock(&pcpu_drain_mutex); } -- Mel Gorman SUSE Labs -- 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>
