linux-kernel - 3.0-git15 Atomic scheduling in pidmap

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Message-ID: <20110801154636.GA2096@zod.bos.redhat.com>
Date:	Mon, 1 Aug 2011 11:46:37 -0400
From:	Josh Boyer <jwboyer@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>
Subject: 3.0-git15 Atomic scheduling in pidmap_init

We're seeing a scheduling while atomic backtrace in rawhide from pidmap_init
(https://bugzilla.redhat.com/show_bug.cgi?id=726877).  While this seems
mostly harmless given that there isn't anything else to schedule to at
this point, I do wonder why things are marked as needing rescheduled so
early.

We get to might_sleep through the might_sleep_if call in
slab_pre_alloc_hook because both kzalloc and KMEM_CACHE are called with
GFP_KERNEL.  That eventually has a call chain like:

might_resched->_cond_resched->should_resched

which apparently returns true.  Why the initial thread says it should
reschedule at this point, I'm not sure.

I tried cheating by making the kzalloc call in pidmap_init use GFP_IOFS
instead of GFP_KERNEL to avoid the might_sleep_if call, and that worked
but I can't do the same for the kmalloc calls in kmem_cache_create, so
getting to the bottom of why should_resched is returning true seems to
be a better approach.

josh
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