linux-kernel - Re: lots of brief rcu stalls.

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Message-ID: <20131205025147.GN15492@linux.vnet.ibm.com>
Date:	Wed, 4 Dec 2013 18:51:47 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Joe Perches <joe@...ches.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Josh Triplett <josh@...edesktop.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: lots of brief rcu stalls.

On Wed, Dec 04, 2013 at 06:36:05PM -0800, Joe Perches wrote:
> On Wed, 2013-12-04 at 18:18 -0800, Eric Dumazet wrote:
> > On Wed, 2013-12-04 at 16:16 -0800, Paul E. McKenney wrote:
> > > +	    ULONG_CMP_GE(ACCESS_ONCE(jiffies), rdp->rsp->jiffies_resched)) {
> 
> perhaps time_before_eq
> 
> > jiffies should not need ACCESS_ONCE(), right ?
> > 
> > It is one of the few variables marked with volatile keyword.
> 
> It does seem redundant
> 
> $ git grep -n "ACCESS_ONCE(jiffies)"
> kernel/rcu/torture.c:1354:      jiffies_snap = ACCESS_ONCE(jiffies);
> kernel/rcu/torture.c:1363:              jiffies_snap = ACCESS_ONCE(jiffies);
> kernel/rcu/tree.c:820:  unsigned long j = ACCESS_ONCE(jiffies);
> kernel/rcu/tree.c:975:  j = ACCESS_ONCE(jiffies);

I took care of the first four, plus one more that I have in my local tree.

							Thanx, Paul

> kernel/sched/core.c:2324:       unsigned long next, now = ACCESS_ONCE(jiffies);
> kernel/sched/proc.c:536:        unsigned long curr_jiffies = ACCESS_ONCE(jiffies);
> kernel/sched/proc.c:558:        unsigned long curr_jiffies = ACCESS_ONCE(jiffies);
> 
> 

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