From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758545Ab3D2Qmt (ORCPT ); Mon, 29 Apr 2013 12:42:49 -0400 Received: from mail-ob0-f180.google.com ([209.85.214.180]:34336 "EHLO mail-ob0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757031Ab3D2Qmq (ORCPT ); Mon, 29 Apr 2013 12:42:46 -0400 MIME-Version: 1.0 In-Reply-To: <20130429161917.GB19814@mtj.dyndns.org> References: <480f5d6da920e2f2b868f311228c81635727799e.1366803121.git.viresh.kumar@linaro.org> <20130424161208.GA16991@mtj.dyndns.org> <20130426191118.GD1433@mtj.dyndns.org> <20130429161917.GB19814@mtj.dyndns.org> Date: Mon, 29 Apr 2013 22:12:46 +0530 Message-ID: Subject: Re: [PATCH V5 1/5] workqueues: Introduce new flag WQ_POWER_EFFICIENT for power oriented workqueues From: Viresh Kumar To: Tejun Heo Cc: Amit Kucheria , davem@davemloft.net, linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Robin Randhawa , Charles Garcia-Tobin , Steve Bannister , Peter Zijlstra , Steven Rostedt , Arvind Chauhan , Patch Tracking , airlied@redhat.com, mingo@redhat.com, Jens Axboe , Liviu Dudau , Lists linaro-kernel Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29 April 2013 21:49, Tejun Heo wrote: > On Mon, Apr 29, 2013 at 12:06:28PM +0530, Viresh Kumar wrote: > Yeap, !WQ_UNBOUND workqueues == per-cpu workqueues. Sigh!! You were talking about thread per cpu here... Sorry for missing it earlier :( >> At this time local cpu may be busy or idle (Atleast according to scheduler). >> We don't want a idle cpu (From schedulers perspective) to be used for >> running this work's handler due to two reasons. >> - idle cpu may be in WFI or deeper idle states and so we can avoid waking >> it up. > > I have no idea what WFI is but the physical CPU is already awake at > that time. It can't be idle - it's running queue_work(). It could be > running in lower freq tho, which each code piece doesn't really have > much control over. Stupid point. WFI: Wait for interrupt (low power mode of cpu). >> - We will make idle cpu look busy and so other kernel stuff may be scheduled >> on it now. But we could have kept it idle for a long time. > > Hmmm... yeah, about the same thing I wrote, it's not really about not > waking up the CPU right now physically but avoiding forcing the > scheduler scheduling a pinned task on an otherwise quiescent CPU. > This effectively allows the scheduler to migrate such work items > towards a CPU which the scheduler considers to be better (in power or > whatever) leading to noticeable powersave. Correct. >> And what timer are you talking about? I am not talking about deffered work only, >> but normal work too. > > Deferred work item == timer + work item. Ya, i knew that :) >> I might have wrongly phrased some part of my patch (maybe used workqueue >> instead of work), will fix that up. > > I think it'd be necessary to distinguish the physical CPU being idle > and the scheduler considers it to be idle (no task to schedule on it) > and explain how increasing the latter can lead to powersave. As it's > currently written, it seemingly, to me anyway, suggests that the > proposed change somehow avoids waking up actually idle CPU, which > isn't the case as queue_work() *always* schedules on the local CPU. > The local CPU can't be idle by definition. Yes you are correct. I will fix it.