linux-kernel - Re: percpu related boot crash on x86 (was: Linux 2.6.38-rc1)

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Message-ID: <20110119205353.GA14810@elte.hu>
Date:	Wed, 19 Jan 2011 21:53:53 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: percpu related boot crash on x86 (was: Linux 2.6.38-rc1)


* Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> On Wed, Jan 19, 2011 at 02:56:23PM +0200, Pekka Enberg wrote:
> > Is vmalloc() and vfree() supposed to work with interrupts disabled? I
> > always thought they weren't which would mean something in
> > pcpu_mem_alloc() needs changing...
> 
> They aren't but percpu allocator doesn't call them with interrupts
> disabled normally.  It only happens during early boot before it's
> enabled for the first time but this is an allowed exception.  We'll
> probably need to update the WARN_ON to consider the rare call path.
> It also looks like something in that path is turning off/on IRQ
> instead of saving/restoring it, so that probably needs to be changed
> too but both aren't really critical.
> 
> I think the real problem is a lot of unsigned int percpu allocations
> added by 6c9ae009 (irq: use per_cpu kstat_irqs).  percpu allocator
> should be able to cope but it assumes that it would always be able to
> allocate contiguous allocation map using vmalloc and that might be
> failing, although I cannot readily see how or why that would happen
> frequently on a 64bit machine.

Btw., based on Peter's suggestion i tried the limit-bumping hack below - but it did 
not help, the crash still triggers on 5%-10% of all randconfig bootups.

Thanks,

	Ingo

>From e2c013d28c8fe81cd30976dcd3058cc86c6285d3 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@...e.hu>
Date: Wed, 19 Jan 2011 20:15:20 +0100
Subject: [PATCH] <not for upstream> percpu: Increase allocation limits

Might solve the boot crashes ...

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 mm/percpu.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 3f93001..5cd9cf4 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -74,7 +74,7 @@
 #include <asm/io.h>
 
 #define PCPU_SLOT_BASE_SHIFT		5	/* 1-31 shares the same slot */
-#define PCPU_DFL_MAP_ALLOC		16	/* start a map with 16 ents */
+#define PCPU_DFL_MAP_ALLOC		64	/* start a map with 64 ents */
 
 #ifdef CONFIG_SMP
 /* default addr <-> pcpu_ptr mapping, override in asm/percpu.h if necessary */
--
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