On Thu, Jan 12, 2012 at 4:51 PM, Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx> wrote:
> On Wed, Jan 11, 2012 at 12:11 PM, Mel Gorman <mgorman@xxxxxxx> wrote:
> <SNIP>
>> Rather than making it safe to call get_online_cpus() from the page
>> allocator, this patch simply removes the page allocator call to
>> drain_all_pages(). To avoid impacting high-order allocation success
>> rates, it still drains the local per-cpu lists for high-order
>> allocations that failed. As a side effect, this reduces the number
>> of IPIs sent during low memory situations.
>>
>> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
>> ---
>> mm/page_alloc.c | 16 ++++++++++++----
>> 1 files changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 2b8ba3a..b6df6fc 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1119,7 +1119,9 @@ void drain_local_pages(void *arg)
>> */
>> void drain_all_pages(void)
>> {
>> + get_online_cpus();
>> on_each_cpu(drain_local_pages, NULL, 1);
>> + put_online_cpus();
>> }
>>
>> #ifdef CONFIG_HIBERNATION
>> @@ -1982,11 +1984,17 @@ retry:
>> migratetype);
>>
>> /*
>> - * If an allocation failed after direct reclaim, it could be because
>> - * pages are pinned on the per-cpu lists. Drain them and try again
>> + * If a high-order allocation failed after direct reclaim, there is a
>> + * possibility that it is because the necessary buddies have been
>> + * freed to the per-cpu list. Drain the local list and try again.
>> + * drain_all_pages is not used because it is unsafe to call
>> + * get_online_cpus from this context as it is possible that kthreadd
>> + * would block during thread creation and the cost of sending storms
>> + * of IPIs in low memory conditions is quite high.
>> */
>> - if (!page && !drained) {
>> - drain_all_pages();
>> + if (!page && order && !drained) {
>> + drain_pages(get_cpu());
>> + put_cpu();
>> drained = true;
>> goto retry;
>> }
>> --
>> 1.7.3.4
>>
>
> I very much like the judo like quality of relying on the fact that in
> memory pressure conditions most
> of the cpus will end up in the direct reclaim path to drain them all
> without IPIs.
>
> What I can't figure out is why we don't need get/put_online_cpus()
> pair around each and every call
> to on_each_cpu everywhere? and if we do, perhaps making it a part of
> on_each_cpu is the way to go?
>
> Something like:
>
> diff --git a/kernel/smp.c b/kernel/smp.c
> index f66a1b2..cfa3882 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -691,11 +691,15 @@ void on_each_cpu(void (*func) (void *info), void
> *info, int wait)
> {
> unsigned long flags;
>
> + BUG_ON(in_atomic());
> +
> + get_online_cpus();
> preempt_disable();
> smp_call_function(func, info, wait);
> local_irq_save(flags);
> func(info);
> local_irq_restore(flags);
> preempt_enable();
> + put_online_cpus();
> }
> EXPORT_SYMBOL(on_each_cpu);
>
> Does that makes?
Well, it dies on boot (after adding the needed include file), so it's
obviously wrong, but I'm still guessing
other users of on_each_cpu will need an get/put_online_cpus() wrapper too.
Maybe -
on_each_cpu(...)
{
get_online_cpus();
__on_each_cpu(...);
put_online_cpus();
}
We'll need to audit all callers.
Gilad
--
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@xxxxxxxxxxxxx
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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