Re: [PULL] cpumask: finally make them variable size w/ CPUMASK_OFFSTACK.
From: Rusty Russell
Date: Mon May 14 2012 - 21:27:55 EST
On Thu, 10 May 2012 02:42:43 -0400, KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> wrote:
> (5/10/12 12:54 AM), Rusty Russell wrote:
> > On Wed, 09 May 2012 22:43:39 -0400, KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxx> wrote:
> >>> Or is there a reason we shouldn't even try to allocate here?
> >> 1) your code always use GFP_KERNEL. it is trouble maker when alloc_pages w/ GFP_ATOMIC.
> > Oh :(
> > How about the below instead?
> 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?
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
> >> 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
@@ -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)
- 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);
+ drain_local_pages, NULL, 1);
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