linux-kernel - Re: [PATCH] locking/qspinlock: Use this_cpu_ptr instead of this_cpu

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Message-ID: <20160603213549.GB3710@twins.programming.kicks-ass.net>
Date:	Fri, 3 Jun 2016 23:35:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH] locking/qspinlock: Use this_cpu_ptr instead of
 this_cpu_dec

On Fri, Jun 03, 2016 at 05:20:10PM -0400, Waiman Long wrote:
> On 06/03/2016 05:48 AM, Pan Xinhui wrote:
> >queued_spin_lock_slowpath should not worry about interrupt change
> >node->count by accident because ->count is inc and dec when we
> >enter/leave queued_spin_lock_slowpath.
> >
> >So this_cpu_dec() does some no point things here, lets use this_cpu_ptr
> >for a small optimization.
> >
> >Signed-off-by: Pan Xinhui<xinhui.pan@...ux.vnet.ibm.com>
> >---
> >  kernel/locking/qspinlock.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> >diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> >index 99f31e4..2b4daac 100644
> >--- a/kernel/locking/qspinlock.c
> >+++ b/kernel/locking/qspinlock.c
> >@@ -492,7 +492,7 @@ release:
> >  	/*
> >  	 * release the node
> >  	 */
> >-	this_cpu_dec(mcs_nodes[0].count);
> >+	this_cpu_ptr(&mcs_nodes[0])->count--;
> >  }
> >  EXPORT_SYMBOL(queued_spin_lock_slowpath);
> >
> 
> Is this going to generate better code for PPC? For x86, I think it will
> cause more instruction to be issued.

It does; I think he wants __this_cpu_dec() instead, but the Changelog
needs improvement to explain why that is ok.