From mboxrd@z Thu Jan 1 00:00:00 1970 From: Preeti U Murthy Subject: Re: [PATCH 1/2] PPC: powernv: remove redundant cpuidle_idle_call() Date: Mon, 10 Feb 2014 09:15:04 +0530 Message-ID: <52F84B40.8090403@linux.vnet.ibm.com> References: <1391696188-14540-1-git-send-email-nicolas.pitre@linaro.org> <52F3BCFE.3010703@linux.vnet.ibm.com> <52F46EB3.5080403@linux.vnet.ibm.com> <52F4C666.4050308@linux.vnet.ibm.com> <20140207124140.GB9987@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from e28smtp06.in.ibm.com ([122.248.162.6]:39105 "EHLO e28smtp06.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752631AbaBJDsm (ORCPT ); Sun, 9 Feb 2014 22:48:42 -0500 Received: from /spool/local by e28smtp06.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 10 Feb 2014 09:18:38 +0530 In-Reply-To: <20140207124140.GB9987@twins.programming.kicks-ass.net> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Peter Zijlstra , Nicolas Pitre Cc: Lists linaro-kernel , "linux-pm@vger.kernel.org" , Daniel Lezcano , "Rafael J. Wysocki" , LKML , Ingo Molnar , Thomas Gleixner , linuxppc-dev@lists.ozlabs.org Hi Peter, On 02/07/2014 06:11 PM, Peter Zijlstra wrote: > On Fri, Feb 07, 2014 at 05:11:26PM +0530, Preeti U Murthy wrote: >> But observe the idle state "snooze" on powerpc. The power that this idle >> state saves is through the lowering of the thread priority of the CPU. >> After it lowers the thread priority, it is done. It cannot >> "wait_for_interrupts". It will exit my_idle(). It is now upto the >> generic idle loop to increase the thread priority if the need_resched >> flag is set. Only an interrupt routine can increase the thread priority. >> Else we will need to do it explicitly. And in such states which have a >> polling nature, the cpu will not receive a reschedule IPI. >> >> That is why in the snooze_loop() we poll on need_resched. If it is set >> we up the priority of the thread using HMT_MEDIUM() and then exit the >> my_idle() loop. In case of interrupts, the priority gets automatically >> increased. > > You can poll without setting TS_POLLING/TIF_POLLING_NRFLAGS just fine > and get the IPI if that is what you want. > > Depending on how horribly unprovisioned the thread gets at the lowest > priority, that might actually be faster than polling and raising the > prio whenever it does get ran. So I am assuming you mean something like the below: my_idle() { local_irq_enable(); /* Remove the setting of the polling flag */ HMT_low(); return index; } And then exit into the generic idle loop. But the issue I see here is that the TS_POLLING/TIF_POLLING_NRFLAGS gets set immediately. So, if on testing need_resched() immediately after this returns that the TIF_NEED_RESCHED flag is set, the thread will exit at low priority right? We could raise the priority of the thread in arch_cpu_idle_exit() soon after setting the polling flag but that would mean for cases where the TIF_NEED_RESCHED flag is not set we unnecessarily raise the priority of the thread. Thanks Regards Preeti U Murthy >