Date: Mon, 7 Jul 2014 21:06:37 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Low <jason.low2@hp.com>
Cc: torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com,
        mingo@kernel.org, Waiman.Long@hp.com, davidlohr@hp.com,
        linux-kernel@vger.kernel.org, tglx@linutronix.de, riel@redhat.com,
        rostedt@goodmis.org, akpm@linux-foundation.org, hpa@zytor.com,
        tim.c.chen@linux.intel.com, konrad.wilk@oracle.com, aswin@hp.com,
        scott.norton@hp.com, chegu_vinod@hp.com, david@fromorbit.com
Subject: Re: [PATCH 0/4] MCS spinlocks: Cancellable MCS spinlock rework
Message-ID: <20140707190637.GI19379@twins.programming.kicks-ass.net>
References: <1404759019-4268-1-git-send-email-jason.low2@hp.com>
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On Mon, Jul 07, 2014 at 11:50:15AM -0700, Jason Low wrote:
> The main purpose of this patchset is to reduce the size of the
> cancellable MCS spinlock and reduce the overhead of rwsem
> (currently the largest lock in the kernel).
>=20
> The overhead of the cancellable MCS lock is a pointer to a per-cpu  node
> structure which requires 64 bits on 64 bit systems. Instead of a pointer
> to the per-cpu node, we can instead store the CPU # corresponding to the
> node in atomic_t. This reduces the overhead by 32 bits on 64 bit systems.
>=20
> This then opens the opportunity to reduce the size of the rw_semaphore
> structure (one of the current users of this MCS lock) by moving around so=
me
> of its fields. Due to padding, we would reduce the size of that structure
> by 64 bits (on 64 bit systems). This makes it more in line with the size
> of the mutex structure.

Dave Chinner was very interested in this..

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