On Sun, Jul 27, 2014 at 7:49 PM, Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote: > ----- Original Message ----- >> From: "Pranith Kumar" <bobby.pr...@gmail.com> >> To: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>, "Josh Triplett" >> <j...@joshtriplett.org>, "Steven Rostedt" >> <rost...@goodmis.org>, "Mathieu Desnoyers" <mathieu.desnoy...@efficios.com>, >> "Lai Jiangshan" <la...@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.pr...@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. */ > >> + */ >> + 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/