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SubjectRe: [PATCH v5 7/8] mm: Only IPI CPUs to drain local pages if they exist
(1/2/12 5:24 AM), Gilad Ben-Yossef wrote:
> Calculate a cpumask of CPUs with per-cpu pages in any zone
> and only send an IPI requesting CPUs to drain these pages
> to the buddy allocator if they actually have pages when
> asked to flush.
>
> This patch saves 99% of IPIs asking to drain per-cpu
> pages in case of severe memory preassure that leads
> to OOM since in these cases multiple, possibly concurrent,
> allocation requests end up in the direct reclaim code
> path so when the per-cpu pages end up reclaimed on first
> allocation failure for most of the proceeding allocation
> attempts until the memory pressure is off (possibly via
> the OOM killer) there are no per-cpu pages on most CPUs
> (and there can easily be hundreds of them).
>
> This also has the side effect of shortening the average
> latency of direct reclaim by 1 or more order of magnitude
> since waiting for all the CPUs to ACK the IPI takes a
> long time.
>
> Tested by running "hackbench 400" on a 4 CPU x86 otherwise
> idle VM and observing the difference between the number
> of direct reclaim attempts that end up in drain_all_pages()
> and those were more then 1/2 of the online CPU had any
> per-cpu page in them, using the vmstat counters introduced
> in the next patch in the series and using proc/interrupts.
>
> In the test sceanrio, this saved around 500 global IPIs.
> After trigerring an OOM:
>
> $ cat /proc/vmstat
> ...
> pcp_global_drain 627
> pcp_global_ipi_saved 578
>
> I've also seen the number of drains reach 15k calls
> with the saved percentage reaching 99% when there
> are more tasks running during an OOM kill.
>
> Signed-off-by: Gilad Ben-Yossef<gilad@benyossef.com>
> Acked-by: Christoph Lameter<cl@linux.com>
> CC: Chris Metcalf<cmetcalf@tilera.com>
> CC: Peter Zijlstra<a.p.zijlstra@chello.nl>
> CC: Frederic Weisbecker<fweisbec@gmail.com>
> CC: Russell King<linux@arm.linux.org.uk>
> CC: linux-mm@kvack.org
> CC: Pekka Enberg<penberg@kernel.org>
> CC: Matt Mackall<mpm@selenic.com>
> CC: Sasha Levin<levinsasha928@gmail.com>
> CC: Rik van Riel<riel@redhat.com>
> CC: Andi Kleen<andi@firstfloor.org>
> CC: Mel Gorman<mel@csn.ul.ie>
> CC: Andrew Morton<akpm@linux-foundation.org>
> CC: Alexander Viro<viro@zeniv.linux.org.uk>
> CC: linux-fsdevel@vger.kernel.org
> CC: Avi Kivity<avi@redhat.com>
> ---
> Christopth Ack was for a previous version that allocated
> the cpumask in drain_all_pages().

When you changed a patch design and implementation, ACKs are
should be dropped. otherwise you miss to chance to get a good
review.



> mm/page_alloc.c | 26 +++++++++++++++++++++++++-
> 1 files changed, 25 insertions(+), 1 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2b8ba3a..092c331 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -67,6 +67,14 @@ DEFINE_PER_CPU(int, numa_node);
> EXPORT_PER_CPU_SYMBOL(numa_node);
> #endif
>
> +/*
> + * A global cpumask of CPUs with per-cpu pages that gets
> + * recomputed on each drain. We use a global cpumask
> + * for to avoid allocation on direct reclaim code path
> + * for CONFIG_CPUMASK_OFFSTACK=y
> + */
> +static cpumask_var_t cpus_with_pcps;
> +
> #ifdef CONFIG_HAVE_MEMORYLESS_NODES
> /*
> * N.B., Do NOT reference the '_numa_mem_' per cpu variable directly.
> @@ -1119,7 +1127,19 @@ void drain_local_pages(void *arg)
> */
> void drain_all_pages(void)
> {
> - on_each_cpu(drain_local_pages, NULL, 1);
> + int cpu;
> + struct per_cpu_pageset *pcp;
> + struct zone *zone;
> +

get_online_cpu() ?

> + for_each_online_cpu(cpu)
> + for_each_populated_zone(zone) {
> + pcp = per_cpu_ptr(zone->pageset, cpu);
> + if (pcp->pcp.count)
> + cpumask_set_cpu(cpu, cpus_with_pcps);
> + else
> + cpumask_clear_cpu(cpu, cpus_with_pcps);

cpumask* functions can't be used locklessly?

> + }
> + on_each_cpu_mask(cpus_with_pcps, drain_local_pages, NULL, 1);
> }
>
> #ifdef CONFIG_HIBERNATION
> @@ -3623,6 +3643,10 @@ static void setup_zone_pageset(struct zone *zone)
> void __init setup_per_cpu_pageset(void)
> {
> struct zone *zone;
> + int ret;
> +
> + ret = zalloc_cpumask_var(&cpus_with_pcps, GFP_KERNEL);
> + BUG_ON(!ret);
>
> for_each_populated_zone(zone)
> setup_zone_pageset(zone);



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