Date: Wed, 2 Jul 2014 18:27:49 +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, riel@redhat.com,
        akpm@linux-foundation.org, hpa@zytor.com, andi@firstfloor.org,
        James.Bottomley@hansenpartnership.com, rostedt@goodmis.org,
        tim.c.chen@linux.intel.com, aswin@hp.com, scott.norton@hp.com,
        chegu_vinod@hp.com
Subject: Re: [RFC] Cancellable MCS spinlock rework
Message-ID: <20140702162749.GP19379@twins.programming.kicks-ass.net>
References: <1404318070-2856-1-git-send-email-jason.low2@hp.com>
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On Wed, Jul 02, 2014 at 09:21:10AM -0700, Jason Low wrote:
> The cancellable MCS spinlock is currently used to queue threads that are
> doing optimistic spinning. It uses per-cpu nodes, where a thread obtaining
> the lock would access and queue the local node corresponding to the CPU t=
hat
> it's running on. Currently, the cancellable MCS lock is implemented by us=
ing
> pointers to these nodes.
>=20
> In this RFC patch, instead of operating on pointers to the per-cpu nodes,=
 we
> store the CPU numbers in which the per-cpu nodes correspond to in atomic_=
t.
> A similar concept is used with the qspinlock.
>=20
> We add 1 to the CPU number to retrive an "encoded value" representing the=
 node
> of that CPU. By doing this, 0 can represent "no CPU", which allows us to
> keep the simple "if (CPU)" and "if (!CPU)" checks. In this patch, the nex=
t and
> prev pointers in each node were also modified to store encoded CPU values.
>=20
> By operating on the CPU # of the nodes using atomic_t instead of pointers
> to those nodes, this can reduce the overhead of the cancellable MCS spinl=
ock
> by 32 bits (on 64 bit systems).

Still struggling to figure out why you did this.


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