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Message-ID: <CAJhHMCAzt-E09QBWn2D-hbWv7omxtky_oKjxt1Y9O_JG221enw@mail.gmail.com> Date: Thu, 14 Aug 2014 18:00:04 -0400 From: Pranith Kumar <bobby.prani@...il.com> To: Paul McKenney <paulmck@...ux.vnet.ibm.com> Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, Lai Jiangshan <laijs@...fujitsu.com>, Dipankar Sarma <dipankar@...ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Josh Triplett <josh@...htriplett.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, David Howells <dhowells@...hat.com>, Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com, Frédéric Weisbecker <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com> Subject: Re: [PATCH v5 tip/core/rcu 09/16] rcu: Improve RCU-tasks energy efficiency On Thu, Aug 14, 2014 at 5:55 PM, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote: > On Thu, Aug 14, 2014 at 05:42:06PM -0400, Pranith Kumar wrote: >> On Mon, Aug 11, 2014 at 6:48 PM, Paul E. McKenney >> <paulmck@...ux.vnet.ibm.com> wrote: >> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> >> > >> > The current RCU-tasks implementation uses strict polling to detect >> > callback arrivals. This works quite well, but is not so good for >> > energy efficiency. This commit therefore replaces the strict polling >> > with a wait queue. >> > >> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> >> > --- >> > kernel/rcu/update.c | 14 ++++++++++++-- >> > 1 file changed, 12 insertions(+), 2 deletions(-) >> > >> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c >> > index f1535404a79e..1256a900cd01 100644 >> > --- a/kernel/rcu/update.c >> > +++ b/kernel/rcu/update.c >> > @@ -368,6 +368,7 @@ early_initcall(check_cpu_stall_init); >> > /* Global list of callbacks and associated lock. */ >> > static struct rcu_head *rcu_tasks_cbs_head; >> > static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; >> > +static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq); >> > static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock); >> > >> > /* Track exiting tasks in order to allow them to be waited for. */ >> > @@ -381,13 +382,17 @@ module_param(rcu_task_stall_timeout, int, 0644); >> > void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp)) >> > { >> > unsigned long flags; >> > + bool needwake; >> > >> > rhp->next = NULL; >> > rhp->func = func; >> > raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); >> > + needwake = !rcu_tasks_cbs_head; >> > *rcu_tasks_cbs_tail = rhp; >> > rcu_tasks_cbs_tail = &rhp->next; >> > raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); >> > + if (needwake) >> > + wake_up(&rcu_tasks_cbs_wq); >> > } >> > EXPORT_SYMBOL_GPL(call_rcu_tasks); >> >> I think you want >> >> needwake = !!rcu_tasks_cbs_head; >> >> otherwise it will wake up when rcu_tasks_cbs_head is null, no? > > Well, that is exactly what we want. Note that we do the test -before- > the enqueue. This means that we do the wakeup if the list -was- > empty before the enqueue, which is exactly the case where the task > might be asleep without having already been sent a wakeup. > > Assuming that wakeups are reliably delivered, of course. But if they > are not reliably delivered, that is a bug that needs to be fixed. > Ohk, I did not notice the modification through rcu_tasks_cbs_tail! All is well. -- Pranith -- 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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