linux-kernel - Re: [PATCH 3.6-rc6] cpufreq/powernow-k8: workqueue user shouldn't migrate the kworker to another CPU

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Date:	Mon, 17 Sep 2012 22:36:54 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	Duncan <1i5t5.duncan@....net>,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [PATCH 3.6-rc6] cpufreq/powernow-k8: workqueue user shouldn't
 migrate the kworker to another CPU

On Mon, Sep 17, 2012 at 01:17:21PM -0700, Tejun Heo wrote:
> powernowk8_target() runs off a per-cpu work item and if the
> cpufreq_policy->cpu is different from the current one, it migrates the
> kworker to the target CPU by manipulating current->cpus_allowed.  The
> function migrates the kworker back to the original CPU but this is
> still broken.  Workqueue concurrency management requires the kworkers
> to stay on the same CPU and powernowk8_target() ends up triggerring
> BUG_ON(rq != this_rq()) in try_to_wake_up_local() if it contends on
> fidvid_mutex and sleeps.
> 
> It is unclear why this bug is being reported now.  Duncan says it
> appeared to be a regression of 3.6-rc1 and couldn't reproduce it on
> 3.5.  Bisection seemed to point to 63d95a91 "workqueue: use @pool
> instead of @gcwq or @cpu where applicable" which is an non-functional
> change.  Given that the reproduce case sometimes took upto days to
> trigger, it's easy to be misled while bisecting.  Maybe something made
> contention on fidvid_mutex more likely?  I don't know.
> 
> This patch fixes the bug by punting to another per-cpu work item on
> the target CPU if it isn't the same as the current one.  The code
> assumes that cpufreq_policy->cpu is kept online by the caller, which
> Rafael tells me is the case.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Duncan <1i5t5.duncan@....net>
> Cc: Rafael J. Wysocki <rjw@...k.pl>
> Cc: Andreas Herrmann <andreas.herrmann3@....com>
> Cc: stable@...nel.org
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=47301
> ---
> 
> While it's very late in the merge cycle, the fix is limited in scope
> and fairly safe, so it wouldn't be too crazy to merge but then again
> this can go through the next -rc1 and then -stable.  Linus, Rafael,
> what do you guys think?

Wouldn't it be much simpler to carve out the piece after
set_cpus_allowed_ptr(), put it in a sub-function called
__powernowk8_target() and call it with smp_call_function_single instead
of defining another work item?

Would the workqueue code handle that or are there any other issues?

>  drivers/cpufreq/powernow-k8.c |   89 +++++++++++++++++++++++++-----------------
>  1 file changed, 53 insertions(+), 36 deletions(-)

If it can, the diffstat should look much slimmer.

Thanks.

-- 
Regards/Gruss,
    Boris.
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