linux-kernel - Re: divide by zero bug in find_busiest

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Date:	Fri, 27 Aug 2010 10:39:27 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Chetan Ahuja <chetan.ahuja@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: divide by zero bug in find_busiest_group

On Thu, Aug 26, 2010 at 4:52 PM, Chetan Ahuja <chetan.ahuja@...il.com> wrote:
> On Thu, Aug 26, 2010 at 12:19 PM, Venkatesh Pallipadi <venki@...gle.com> wrote:
>> On Wed, Aug 25, 2010 at 6:17 PM, Chetan Ahuja <chetan.ahuja@...il.com> wrote:
>
>> I tried reproducing the problem with vanilla kernel and couldn't
>> reproduce it. May be your workload is triggering this some how?
>> Also, Suresh did recent fixes to rt_avg and this may not be a problem
>> after that. I Haven't looked at this case closely after that recent
>> change.
>>
>> Thanks,
>> Venki
>>
>
> Venkatesh,
>
>     Thanks for your detailed response. I admit complete ignorance of
> the actual logic of the complicated cpu_power computations (or indeed,
> any other part of the scheduler code). I'm just looking at it from the
> perspective of bug-hunting in an unfamiliar piece of code. I think the
> three ways we've so far identified the cpu_power can be zero so far
> (all three would be extremely low probability events admittedly) are
> warning signs that the code is trying to do  a bit too much. It seems
> to be rather difficult to prove  correctness of the math or the
> synchronization for my taste. Is there a clear and present use-case
> which driving all these complicated computations ?
>
>  For one thing, I don't understand the reason for recomputing
> cpu_power dynamically. How does cpu_power of a gorup (which I believe
> refers to the group of "hyper-threaded" cores on one physical core)
> change while inthe system is in operation ? Is this in support of
> hot-swappable CPU's or something ?
>
>  To be honest, for my use case, I'd  like to just disable the whole
> work-stealing stuff (referred to as load-balancing in the kernel).
> There seems to be no config option that isolates just the dynamic
> load-balancing parts (or is there ?) so maybe I'd just hack away the
> load-balancing functions by hand so as never to worry about those
> denominators again.
>

Chetan,

For the purpose of this bug (assuming you can run a modified kernel in
your environment),
you can add above change and/or add some printk's, WARN_ON_ONCE() to see whether
power is going negative-zero at any point. That will give some more
clue on what may be
happening here.

Thanks,
Venki
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