linux-kernel - Re: [PATCH] locking/rwsem: Make owner store task pointer of last owning reader

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Message-ID: <20180910171550.GA3902@linux-r8p5>
Date:   Mon, 10 Sep 2018 10:15:50 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/rwsem: Make owner store task pointer of last
 owning reader

On Mon, 10 Sep 2018, Waiman Long wrote:

>One major issue with a combined count/owner is that we may have to use
>cmpxchg for reader lock which will certainly impact reader-heavy
>workloads. I have also thought about ways to compress the task pointer
>address so that it can use fewer bits and leave the rests for reader
>count. It is probably doable on 64-bit systems, but likely not on 32-bit
>system given that there are less bits to play around.

Yeah we've discussed this before. As a cleanup it would obviously be good,
but I fear about raw performance loss when using cmpxchg instead of xadd.

Thanks,
Davidlohr