linux-kernel - Re: [PULL] cpumask: finally make them variable size w/ CPUMASK

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Message-ID: <87zk9btqmu.fsf@rustcorp.com.au>
Date:	Mon, 14 May 2012 12:28:49 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	LKML <linux-kernel@...r.kernel.org>, anton@...ba.org,
	Arnd Bergmann <arnd@...db.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PULL] cpumask: finally make them variable size w/ CPUMASK_OFFSTACK.

On Thu, 10 May 2012 02:42:43 -0400, KOSAKI Motohiro <kosaki.motohiro@...il.com> wrote:
> (5/10/12 12:54 AM), Rusty Russell wrote:
> > On Wed, 09 May 2012 22:43:39 -0400, KOSAKI Motohiro<kosaki.motohiro@...il.com>  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
neater.

> >> 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 :(

Cheers,
Rusty.

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));
 		else
-			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);
 }
 
 #ifdef CONFIG_HIBERNATION
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