Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750750AbWIYJjp (ORCPT ); Mon, 25 Sep 2006 05:39:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750798AbWIYJjp (ORCPT ); Mon, 25 Sep 2006 05:39:45 -0400 Received: from ug-out-1314.google.com ([66.249.92.173]:14826 "EHLO ug-out-1314.google.com") by vger.kernel.org with ESMTP id S1750750AbWIYJjo (ORCPT ); Mon, 25 Sep 2006 05:39:44 -0400 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=beta; d=gmail.com; h=received:message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=C8NgRBKsNc7nHq+dJYdlYF+k/UckZhWjhdcgqq74A04R4cGWc44s6NO9dR6TvqDTDb5mEZvQ+zlWNtBL47+4B/UXBWu7ukwu+7jI4unbKe7is2cYitxIlCIutT/XJCGQkR1SoTovWwVyJ10lvSHOvZZJXxRut+5KFJigbTeNRaE= Message-ID: <489ecd0c0609250239v1e23010fu15e707ef7d2162a8@mail.gmail.com> Date: Mon, 25 Sep 2006 17:39:42 +0800 From: "Luke Yang" To: "Arnd Bergmann" Subject: Re: [PATCH 1/4] Blackfin: arch patch for 2.6.18 Cc: Aubrey , linux-kernel@vger.kernel.org, "Andrew Morton" In-Reply-To: <200609251126.17494.arnd@arndb.de> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <489ecd0c0609202032l1c5540f7t980244e30d134ca0@mail.gmail.com> <200609250854.04470.arnd@arndb.de> <6d6a94c50609250049l75b2f070q81583b90d8fcfaec@mail.gmail.com> <200609251126.17494.arnd@arndb.de> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2157 Lines: 52 On 9/25/06, Arnd Bergmann wrote: > On Monday 25 September 2006 09:49, Aubrey wrote: > > Oh, sorry for my unclear description, any of the peripherals can be configured > > to wake up the core from its idled state to process the interrupt on > > Blackfin. I should say __if no other interrupts__ here is the timer > > interrupt waking up the processor every one jiffy. > > And that works if interrupts are disabled as it should? > > > I don't understand if interrupt occurs between need_resched() and the > > idle instruction, what will become the racy object? Can you comment it > > detailed? thanks. > > It's the same problem as why sleep_on() is wrong and wait_event() is > right, you can find lots of documentation about that. > > Think about a process calling nanosleep() to wait for a timeout. > It eventually ends up going to sleep and the kernel schedules > the idle task. > > The idle task checks need_resched(), which returns false, so it > will call the idle instruction. Before it gets there, the timer > tick happens, which calls the timer softirq. That softirq notices > that the user process should continue running and calls wake_up(), > which makes the condition for need_resched() true. > > Since we're handling that softirq that interrupted the idle task, > that task continues what it was last doing and calls the idle instruction. > This is the point where it goes wrong, because your user task should > run, but the CPU is sleeping until the next interrupt happens. Got it. You are right. We'll check to see what we can do on Blackfin. Thank you! > > Note that you should in the future disable timer ticks during the > idle function (CONFIG_NO_IDLE_HZ or similar) to save more power, but > in that case the CPU may be in idle indefinitely after missing the > one interrupt that should have woken it up. > > Arnd <>< > -- Best regards, Luke Yang luke.adi@gmail.com - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/