From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752664AbbJWWUp (ORCPT ); Fri, 23 Oct 2015 18:20:45 -0400 Received: from mga14.intel.com ([192.55.52.115]:26304 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750869AbbJWWUn (ORCPT ); Fri, 23 Oct 2015 18:20:43 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.20,188,1444719600"; d="scan'208";a="834186739" Date: Fri, 23 Oct 2015 15:10:15 -0700 From: Yunhong Jiang To: Viresh Kumar Cc: Frederic Weisbecker , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH] timer: Lazily wakup nohz CPU when adding new timer. Message-ID: <20151023221015.GA20818@jnakajim-build> References: <1443466096-31252-1-git-send-email-yunhong.jiang@linux.intel.com> <20151020224751.GB31289@jnakajim-build> <20151021104631.GB7784@ubuntu> <20151022214003.GA22993@jnakajim-build> <20151023021951.GP3897@ubuntu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151023021951.GP3897@ubuntu> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 23, 2015 at 07:49:51AM +0530, Viresh Kumar wrote: > On 22-10-15, 14:40, Yunhong Jiang wrote: > > A naive question is, why it's sure a tick will happen when the tickless > > processor is in idle? > > How do you get this impression? I don't think anyone has said that. Viresh, thanks for your reply for my question. I got this impression from Frederic's comments on http://marc.info/?l=linux-kernel&m=139048415303210&w=2, "So you simply rely on the next tick to see the new timer. This should work with CONFIG_NO_HZ_IDLE but not with CONFIG_NO_HZ_FULL since the target may be running without the tick". Per my understanding of this comment, it means we can rely on the next tick for CONFIG_NO_HZ_IDLE, which means it's sure a tick will happen for CONFIG_NO_HZ_IDLE, am I right? > > We are talking about deferrable timers, which by design are only > required if the target CPU is not-idle. If it is idle, then the timer > isn't required to be serviced until the CPU wakes up. And the CPU can > take whatever time it wants to wake up again. Hmm, per http://lxr.free-electrons.com/source/include/linux/timer.h#L51, the deferreable timer will be serviced when the CPU eventually wakes up "with a subsequent non-deferrable timer". If there is no non-deferrable timer, based on Frederic's comments, we in fact depends on next tick. My confusion is, why we are sure there is next tick on CONFIG_NO_HZ_IDLE idle processor to wake it up. If there is no tick, and no other timer, will the timer get no chance to be waken up at all? I don't think "deferred for ever" is deferreable. Thanks -jyh > > > Is it because scheduler load balance is sure to send a > > tick to the processor in future? > > No. We aren't expecting the CPU to wake up any time soon. Just ignore > the deferrable timer. > > -- > viresh