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Date: Fri, 25 Jul 2014 07:44:58 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Pranith Kumar <bobby.prani@...il.com> Cc: Josh Triplett <josh@...htriplett.org>, Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan <laijs@...fujitsu.com>, "open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org> Subject: Re: [RFC PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up kthreads On Fri, Jul 25, 2014 at 01:06:58AM -0400, Pranith Kumar wrote: > 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(). > > In rcu_report_qs_rsp(), I added a pr_info() call testing if any of the above > conditions is true, in which case we can avoid calling wake_up(). It turns out > that quite a few actually are. Most of the cases where we can avoid is condition 2 > above and condition 1 also occurs quite often. Condition 3 never happens. > > I could not test the wake_up() in force_quiescent_state() as that is not > triggered trivially, but I am assuming we can replace wake_up() there too. > > Hence this commit tries to avoid calling wake_up() whenever we can by using > rcu_gp_kthread_wake() function. This one does sound much more plausible than the earlier one. I have a few more questions that I will ask in your follow-up message. > One concern is the comment which states that we need a memory barrier at that > location which is being implied by the wake_up(). Should we put an smp_mb() and > just not rely on the barrier provided by wake_up()? Thoughts? Let's see... The memory barriers are unnecessary for your case 1 and case 3. That leaves your case 2, which is all about ->gp_flags. It is quite possible that this case is now fully covered by locking, so that the comment is obsolete. But why don't you check? Thanx, Paul > Signed-off-by: Pranith Kumar <bobby.prani@...il.com> > --- > kernel/rcu/tree.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 72e0b1f..d0e0d6e 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1938,7 +1938,8 @@ 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. */ > + /* Memory barrier implied by wake_up() path. */ > + rcu_gp_kthread_wake(rsp); > } > > /* > @@ -2516,7 +2517,8 @@ 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. */ > + /* Memory barrier implied by wake_up() path. */ > + rcu_gp_kthread_wake(rsp); > } > > /* > -- > 2.0.1 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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