linux-kernel - Re: [PATCH 00/11] locking/ww_mutex: Keep sorted wait list to avoid stampedes

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Message-ID: <0e930160-2536-97d6-06a3-07cc0b1df651@gmail.com>
Date:   Wed, 30 Nov 2016 12:52:28 +0100
From:   Nicolai Hähnle <nhaehnle@...il.com>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/11] locking/ww_mutex: Keep sorted wait list to avoid
 stampedes

On 30.11.2016 10:40, Chris Wilson wrote:
> On Mon, Nov 28, 2016 at 01:20:01PM +0100, Nicolai Hähnle wrote:
>> I've included timings taken from a contention-heavy stress test to some of
>> the patches. The stress test performs actual GPU operations which take a
>> good chunk of the wall time, but even so, the series still manages to
>> improve the wall time quite a bit.
>
> In looking at your contention scenarios, what was the average/max list
> size? Just wondering if it makes sense to use an rbtree + first_waiter
> instead of a sorted list from the start.

I haven't measured this with the new series; previously, while I was 
debugging the deadlock on older kernels, I occasionally saw wait lists 
of up to ~20 tasks, spit-balling the average over all the deadlock cases 
I'd say the average was not more than ~5. The average _without_ 
deadlocks should be lower, if anything.

I saw that your test cases go quite a bit higher, but even the rather 
extreme load I was testing with -- which is not quite a load from an 
actual application, though it is related to one -- has 40 threads and so 
a theoretical maximum of 40.

Nicolai

> -Chris
>