From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752991AbbAUKiv (ORCPT ); Wed, 21 Jan 2015 05:38:51 -0500 Received: from e8.ny.us.ibm.com ([32.97.182.138]:43603 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752008AbbAUKip (ORCPT ); Wed, 21 Jan 2015 05:38:45 -0500 Message-ID: <54BF81A0.8030705@linux.vnet.ibm.com> Date: Wed, 21 Jan 2015 16:08:24 +0530 From: Preeti U Murthy User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Thomas Gleixner CC: peterz@infradead.org, linuxppc-dev@lists.ozlabs.org, mingo@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] idle/tick-broadcast: Exit cpu idle poll loop when cleared from tick_broadcast_force_mask References: <20150119052754.20256.54721.stgit@preeti.in.ibm.com> <54BE3B0E.2050502@linux.vnet.ibm.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15012110-0029-0000-0000-000001BF95E0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/21/2015 03:26 PM, Thomas Gleixner wrote: > 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? Yep, much simpler, thanks! I will send out the next version with this changelog. Regards Preeti U Murthy > > Thanks, > > tglx > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev >