Re: [PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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