Messages in this thread Patch in this message | | | Date | Mon, 28 Jul 2014 00:04:44 +0000 (UTC) | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads |
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----- Original Message ----- > From: "Pranith Kumar" <bobby.prani@gmail.com> > To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "Josh Triplett" <josh@joshtriplett.org>, "Steven Rostedt" > <rostedt@goodmis.org>, "Lai Jiangshan" <laijs@cn.fujitsu.com>, "open list:READ-COPY UPDATE..." > <linux-kernel@vger.kernel.org> > Sent: Sunday, July 27, 2014 7:58:40 PM > Subject: Re: [PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads > > On Sun, Jul 27, 2014 at 7:49 PM, Mathieu Desnoyers > <mathieu.desnoyers@efficios.com> wrote: > > ----- Original Message ----- > >> From: "Pranith Kumar" <bobby.prani@gmail.com> > >> To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "Josh Triplett" > >> <josh@joshtriplett.org>, "Steven Rostedt" > >> <rostedt@goodmis.org>, "Mathieu Desnoyers" > >> <mathieu.desnoyers@efficios.com>, "Lai Jiangshan" <laijs@cn.fujitsu.com>, > >> "open list:READ-COPY UPDATE..." <linux-kernel@vger.kernel.org> > >> Sent: Sunday, July 27, 2014 7:37:29 PM > >> Subject: [PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up grace > >> period kthreads > >> > >> The rcu_gp_kthread_wake() function checks for three conditions before > >> waking > >> up > >> grace period kthreads: > >> > >> * Is the thread we are trying to wake up the current thread? > >> * Are the gp_flags zero? (all threads wait on non-zero gp_flags > >> condition) > >> * Is there no thread created for this flavour, hence nothing to wake up? > >> > >> If any one of these condition is true, we do not call wake_up(). > >> > >> It was found that there are quite a few avoidable wake ups both during > >> idle > >> time and under stress induced by rcutorture. > >> > >> Idle: > >> > >> Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0 > >> Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0 > >> > >> rcutorture: > >> > >> Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0 > >> Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0 > >> > >> Here case{1-3} are the cases listed above. We can avoid these wake ups by > >> using > >> rcu_gp_kthread_wake() to conditionally wake up the grace period kthreads. > >> > >> Hence this commit tries to avoid calling wake_up() whenever we can by > >> using > >> rcu_gp_kthread_wake() function. > >> > >> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com> > >> --- > >> kernel/rcu/tree.c | 10 ++++++++-- > >> 1 file changed, 8 insertions(+), 2 deletions(-) > >> > >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > >> index b63517c..36911ee 100644 > >> --- a/kernel/rcu/tree.c > >> +++ b/kernel/rcu/tree.c > >> @@ -1938,7 +1938,10 @@ static void rcu_report_qs_rsp(struct rcu_state > >> *rsp, > >> unsigned long flags) > >> { > >> WARN_ON_ONCE(!rcu_gp_in_progress(rsp)); > >> raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags); > >> - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. > >> */ > >> + /* ->gp_flags is properly protected by locks, so a memory barrier > >> + * is not necessary here > > > > Two point: > > > > 1- The format of this comment is odd, and should be: > > > > /* > > * Text... > > */ > > OK, I will update it according to this format. > > > > > 2- Since when can a memory barrier be replaced by a lock ? More explanation > > appears to be needed on what this barrier matches exactly. > > On re-reading I realize that this comment is very vague and introduces > more doubts than it clears. > > The context here is that in rcu_gp_kthread_wake() we are accessing > ->gp_flags to determine whether we need to wake up the gp kthreads. We > don't need a barrier here since we are accessing it using > ACCESS_ONCE() and all other accesses are properly protected by using > ACCESS_ONCE() and taking the root rcu_node lock. > > So how about this: > > /* > * ->gp_flags is being accessed using ACCESS_ONCE() because of > * which a memory barrier is not required here. > */ >
A memory barrier is typically not there to interact with a single variable and a single memory access. I'm concerned that this memory barrier might be ordering other things besides gp_flags.
Thanks,
Mathieu
> > > >> + */ > >> + rcu_gp_kthread_wake(rsp); > >> } > >> > >> /* > >> @@ -2516,7 +2519,10 @@ static void force_quiescent_state(struct rcu_state > >> *rsp) > >> ACCESS_ONCE(rsp->gp_flags) = > >> ACCESS_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS; > >> raw_spin_unlock_irqrestore(&rnp_old->lock, flags); > >> - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. > >> */ > >> + /* ->gp_flags is properly protected by locks, so a memory barrier > >> + * is not necessary here > >> + */ > >> + rcu_gp_kthread_wake(rsp); > >> } > >> > >> /* > >> -- > >> 1.9.1 > >> > >> > > > > -- > > Mathieu Desnoyers > > EfficiOS Inc. > > http://www.efficios.com > > > > -- > Pranith >
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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