Re: [PATCH v5 7/8] mm: Only IPI CPUs to drain local pages if they exist

[Gilad Ben-Yossef <gilad@benyossef.com>]

2012/1/3 KOSAKI Motohiro <kosaki.motohiro@gmail.com>:
> (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.
>

Got you. Thanks for the 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() ?

I believe this is not needed here as on_each_cpu_mask() (smp_call_function_many
really) later masks the cpumask with the online cpus, so at worst we
are turning on or off
a meaningless bit.

Anyway, If I'm wrong someone should fix show_free_areas() as well :-)

>
>> +     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?

I'm not sure I understand your question ocrrectly. As far as I
understand cpumask_set_cpu and cpumask_set_cpu
are atomic operations that do not require a lock (they might be
implemented using one though).

Thanks!
Gilad

-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@benyossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"Unfortunately, cache misses are an equal opportunity pain provider."
-- Mike Galbraith, LKML

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