linux-kernel - Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath

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Date:   Mon, 9 Apr 2018 15:33:21 -0400
From:   Waiman Long <longman@...hat.com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        peterz@...radead.org, mingo@...nel.org, boqun.feng@...il.com,
        paulmck@...ux.vnet.ibm.com, catalin.marinas@....com
Subject: Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop
 from locking slowpath

On 04/09/2018 10:54 AM, Will Deacon wrote:
>
>>> I am not against this patch, but we certainly need to find out a way to
>>> bring the performance number up closer to what it is before applying
>>> the patch.
>> We certainly need to *understand* where the drop is coming from, because
>> the two-threaded case is still just a CAS on x86 with and without this
>> patch series. Generally, there's a throughput cost when ensuring fairness
>> and forward-progress otherwise we'd all be using test-and-set.
> Whilst I think we still need to address my questions above, I've had a
> crack at the diff below. Please can you give it a spin? It sticks a trylock
> on the slowpath before setting pending and replaces the CAS-based set
> with an xchg (which I *think* is safe, but will need to ponder it some
> more).
>
> Thanks,
>
> Will
>

Unfortunately, this patch didn't help.

pending count = 777
queuing count = 9,991,272
locking rate = 4,087 kop/s

-Longman