From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2001:470:1f0b:db:abcd:42:0:1]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3CE071A0D9B for ; Wed, 21 Jan 2015 20:56:37 +1100 (AEDT) Date: Wed, 21 Jan 2015 10:56:14 +0100 (CET) From: Thomas Gleixner To: Preeti U Murthy Subject: Re: [PATCH] idle/tick-broadcast: Exit cpu idle poll loop when cleared from tick_broadcast_force_mask In-Reply-To: <54BE3B0E.2050502@linux.vnet.ibm.com> Message-ID: References: <20150119052754.20256.54721.stgit@preeti.in.ibm.com> <54BE3B0E.2050502@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: peterz@infradead.org, linuxppc-dev@lists.ozlabs.org, mingo@kernel.org, linux-kernel@vger.kernel.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, 20 Jan 2015, Preeti U Murthy wrote: > On 01/20/2015 04:51 PM, Thomas Gleixner wrote: > > On Mon, 19 Jan 2015, Preeti U Murthy wrote: > >> An idle cpu enters cpu_idle_poll() if it is set in the tick_broadcast_force_mask. > >> This is so that it does not incur the overhead of entering idle states when it is expected > >> to be woken up anytime then through a broadcast IPI. The condition that forces an exit out > >> of the idle polling is the check on setting of the TIF_NEED_RESCHED flag for the idle thread. > >> > >> When the broadcast IPI does arrive, it is not guarenteed that the handler sets the > >> TIF_NEED_RESCHED flag. Hence although the cpu is cleared in the tick_broadcast_force_mask, > >> it continues to loop in cpu_idle_poll unnecessarily wasting power. Hence exit the idle > >> poll loop if the tick_broadcast_force_mask gets cleared and enter idle states. > >> > >> Of course if the cpu has entered cpu_idle_poll() on being asked to poll explicitly, > >> it continues to poll till it is asked to reschedule. > >> > >> Signed-off-by: Preeti U Murthy > >> --- > >> > >> kernel/sched/idle.c | 3 ++- > >> 1 file changed, 2 insertions(+), 1 deletion(-) > >> > >> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > >> index c47fce7..aaf1c1d 100644 > >> --- a/kernel/sched/idle.c > >> +++ b/kernel/sched/idle.c > >> @@ -47,7 +47,8 @@ static inline int cpu_idle_poll(void) > >> rcu_idle_enter(); > >> trace_cpu_idle_rcuidle(0, smp_processor_id()); > >> local_irq_enable(); > >> - while (!tif_need_resched()) > >> + while (!tif_need_resched() && > >> + (cpu_idle_force_poll || tick_check_broadcast_expired())) > > > > You explain the tick_check_broadcast_expired() bit, but what about the > > cpu_idle_force_poll part? > > The last few lines which say "Of course if the cpu has entered > cpu_idle_poll() on being asked to poll explicitly, it continues to poll > till it is asked to reschedule" explains the cpu_idle_force_poll part. Well, I read it more than once and did not figure it out. The paragraph describes some behaviour. Now I know it's the behaviour before the patch. So maybe something like this: cpu_idle_poll() is entered when cpu_idle_force_poll is set or tick_check_broadcast_expired() returns true. The exit condition from cpu_idle_poll() is tif_need_resched(). But this does not take into account that cpu_idle_force_poll and tick_check_broadcast_expired() can change without setting the resched flag. So a cpu can be caught in cpu_idle_poll() needlessly and thereby wasting power. Add an explicit check for cpu_idle_force_poll and tick_check_broadcast_expired() to the exit condition of cpu_idle_poll() to avoid this. This explains the technical issue without confusing people with IPIs and other completely irrelevant information. Hmm? Thanks, tglx