Re: [RFC][PATCH v2 5/5] mutex: Give spinners a chance to spin_on_ownerif need_resched() triggered while queued

From: Waiman Long
Date: Thu Feb 06 2014 - 13:46:08 EST

On 02/06/2014 09:04 AM, Peter Zijlstra wrote:
On Wed, Feb 05, 2014 at 04:44:34PM -0500, Waiman Long wrote:
I have an alternative way of breaking out of the MCS lock waiting queue when
need_resched() is set. I overload the locked flag to indicate a skipped node
if negative.
I'm not quite seeing how it works (then again, I've not really read the
patch carefully).

Suppose you break out; at that point you get queued and go to sleep.
Suppose you got woken up while you MCS entry is still 'pending' and
magically win the race and acquire the lock.

At that point your MCS entry can be re-used while its still part of the

Actually the MCS node entry cannot be reused until an MCS queue head task traverses that entry and clear the locked flag. So it is possible that the affected CPU won't be able to participate in optimistic spinning for a while if the mutex is heavily contended.

Its a fantastically small race window, but I don't see anything that
makes it impossible.

I will send out an official patch with some performance data to solicit further feedback.

I run the patch through the AIM7 high-systime workload on a
4-socket server and it seemed to run fine.
How do people run this AIM7 piece of shit? I let it run for over an hour
and it generated exactly 0 numbers, it just sits there eating cpu-time
and creating a racket from my pantry.

AIM7 can be tricky to set up. Fortunately, someone in our team had done the ground work so I can just grab and run it.

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