From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/rwsem: Make owner store task pointer of last owning reader
Date: Tue, 11 Sep 2018 10:17:55 +0200 [thread overview]
Message-ID: <20180911081755.GO24106@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180910171550.GA3902@linux-r8p5>
On Mon, Sep 10, 2018 at 10:15:50AM -0700, Davidlohr Bueso wrote:
> 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.
Does it really matter though? Last time I looked at something similar
(refcount_t) the "LOCK INCL" vs "LOCK CMPXCHG" was something like 15 vs
23 cycles (and that was with the cmpxchg loop actually doing a lot
more).
Do we really care about the down_read() path _that_ much? I thought that
with the main pain point, pagefaults, the problem was mostly the line
bouncing, not a few extra cycles.
next prev parent reply other threads:[~2018-09-11 8:18 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-06 20:18 [PATCH] locking/rwsem: Make owner store task pointer of last owning reader Waiman Long
2018-09-10 9:31 ` Peter Zijlstra
2018-09-10 15:01 ` Waiman Long
2018-09-10 17:15 ` Davidlohr Bueso
2018-09-10 17:35 ` Waiman Long
2018-09-11 8:17 ` Peter Zijlstra [this message]
2018-09-11 12:56 ` Waiman Long
2018-09-11 19:21 ` Davidlohr Bueso
2018-09-11 19:38 ` Peter Zijlstra
2018-09-10 10:54 ` [tip:locking/core] " tip-bot for Waiman Long
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