From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755095AbaEIOlY (ORCPT ); Fri, 9 May 2014 10:41:24 -0400 Received: from fw-tnat.austin.arm.com ([217.140.110.23]:32870 "EHLO collaborate-mta1.arm.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750987AbaEIOlX (ORCPT ); Fri, 9 May 2014 10:41:23 -0400 Date: Fri, 9 May 2014 15:40:34 +0100 From: Catalin Marinas To: Peter Zijlstra Cc: James Hogan , "mingo@kernel.org" , Thomas Gleixner , "luto@amacapital.net" , "nicolas.pitre@linaro.org" , "daniel.lezcano@linaro.org" , "umgwanakikbuti@gmail.com" , LKML , Will Deacon , ARM Kernel List Subject: Re: [RFC][PATCH 6/8] sched,idle: Avoid spurious wakeup IPIs Message-ID: <20140509144034.GF7950@arm.com> References: <20140411134243.160989490@infradead.org> <20140411135218.478299389@infradead.org> <20140509141520.GV2844@laptop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140509141520.GV2844@laptop.programming.kicks-ass.net> 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 Hi Peter, On Fri, May 09, 2014 at 03:15:20PM +0100, Peter Zijlstra wrote: > On Fri, May 09, 2014 at 02:37:27PM +0100, James Hogan wrote: > > On 11 April 2014 14:42, Peter Zijlstra wrote: > > > + return !(fetch_or(&ti->flags, _TIF_NEED_RESCHED) & _TIF_POLLING_NRFLAG); > > > > This breaks the build on metag, and I suspect arm64 too: > > Yep, I just got a patch for arm64. [...] > Any SMP arch that has a polling idle function of any kind (including the > default cpu_idle_poll()). > > That said, even if that's true, not having TIF_POLLING_NRFLAG isn't > fatal, just sub-optimal in that we'll send an unconditional IPI to wake > the CPU even though its polling TIF_NEED_RESCHED and doesn't need > anything other than that write to wake up. > > Most archs have (x86) hlt or (arm) wfi like idle instructions, and if > that is your only possible idle function, you'll require the interrupt > to wake up and there's really no point to having the POLLING bit. I wonder why we still need TIF_POLLING_NRFLAG for arm64. It was on arm until commit 16a8016372c42c7628eb (sanitize tsk_is_polling()). On arm64 we use wfi for idle or a firmware call but in both cases the assumption is that we need an interrupt for waking up. So I think we should remove this macro for arm64. -- Catalin