Re: [PULL] cpumask: finally make them variable size w/ CPUMASK_OFFSTACK.

From: KOSAKI Motohiro
Date: Mon May 14 2012 - 21:38:03 EST

This code still slow than original. when calling reclaim path, new allocation is almost always
fail. then, your code almost always invoke all cpu batch invalidation. i.e. many ipi.

I don't know this code. Does that happen often?Do we really need to
optimize the out-of-memory path?

we don't need optimize out-of-memory path. but it's not out-of-memory path. our reclaim code
has two steps 1) purge small file cache (try_to_free_pages) 2) get new page (get_page_from_freelist).
but if you have smp box, it's racy. To success 1) doesn't guarantee to success 2). then, drain_all_pages()
is called frequently than you expected.

But I should have used on_each_cpu_cond() helper which does this for us
(except it falls back to individial IPIs) which would make this code

Ah, yes. that definitely makes sense.

2) When CONFIG_CPUMASK_OFFSTACK=n and NR_CPUS is relatively large, cpumask on stack may
cause stack overflow. because of, alloc_pages() can be called from
very deep call stack.

You can't have large NR_CPUS without CONFIG_CPUMASK_OFFSTACK=y,
otherwise you'll get many other stack overflows, too.

Original code put cpumask bss instead stack then. :-)

Yes, and this is what it looks like if we convert it directly, but I
still don't want to encourage people to do this :(


diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1179,7 +1179,7 @@ void drain_all_pages(void)
* Allocate in the BSS so we wont require allocation in
* direct reclaim path for CONFIG_CPUMASK_OFFSTACK=y
- static cpumask_t cpus_with_pcps;
+ static DECLARE_BITMAP(cpus_with_pcps, NR_CPUS);

* We don't care about racing with CPU hotplug event
@@ -1197,11 +1197,12 @@ void drain_all_pages(void)
if (has_pcps)
- cpumask_set_cpu(cpu,&cpus_with_pcps);
+ cpumask_set_cpu(cpu, to_cpumask(cpus_with_pcps));
- cpumask_clear_cpu(cpu,&cpus_with_pcps);
+ cpumask_clear_cpu(cpu, to_cpumask(cpus_with_pcps));
- on_each_cpu_mask(&cpus_with_pcps, drain_local_pages, NULL, 1);
+ on_each_cpu_mask(to_cpumask(cpus_with_pcps),
+ drain_local_pages, NULL, 1);

Looks good to me. thanks.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>

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