From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lb0-f193.google.com ([209.85.217.193]:33386 "EHLO mail-lb0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751627AbcEZLmi (ORCPT ); Thu, 26 May 2016 07:42:38 -0400 Subject: Re: [PATCH] soc/tegra: pmc: Fix "scheduling while atomic" To: Jon Hunter References: <1460900051-3065-1-git-send-email-digetx@gmail.com> <572B47DE.1090804@nvidia.com> <5745C02A.20308@nvidia.com> <5746B6F7.9040101@nvidia.com> Cc: Stephen Warren , Thierry Reding , Alexandre Courbot , Peter De Schrijver , Prashant Gaikwad , linux-clk@vger.kernel.org, linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org From: Dmitry Osipenko Message-ID: Date: Thu, 26 May 2016 14:42:33 +0300 MIME-Version: 1.0 In-Reply-To: <5746B6F7.9040101@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Sender: linux-clk-owner@vger.kernel.org List-ID: On 26.05.2016 11:42, Jon Hunter wrote: > > On 25/05/16 19:51, Dmitry Osipenko wrote: >> On 25.05.2016 18:09, Jon Hunter wrote: > > ... > >>> If you are able to reproduce this on v3.18, then it would be good if you >>> could trace the CCF calls around this WARNING to see what is causing the >>> contention. >> >> I managed to reproduce it with some CCF "tracing". >> Full kmsg log is here: https://bpaste.net/show/d8ab7b7534b7 >> >> Looks like CPU freq governor thread yields during clk_set_rate() and >> then CPU idle kicks in, taking the same mutex. > > On the surface that sounds odd to me, but without understanding the > details, I guess I don't know if this is a valid thing to be doing or > even how that actually works! > The reason of that happening should be that I'm using clk PRE/POST rate change notifiers in my DVFS driver that takes other mutexes and they could be locked, causing schedule. I haven't mentioned it before, sorry. From drivers/clk/clk.c: static struct task_struct *prepare_owner; ... /*** locking ***/ static void clk_prepare_lock(void) { if (!mutex_trylock(&prepare_lock)) { if (prepare_owner == current) { prepare_refcnt++; return; } mutex_lock(&prepare_lock); } You can see that it would lock the mutex if prepare_owner != current, in my case it's idle thread != interactive gov. thread. >> However, cpufreq_interactive governor is android specific governor and >> isn't in upstream kernel yet. Quick googling shows that recent >> "upstreaming" patch uses same cpufreq_interactive_speedchange_task: >> https://lkml.org/lkml/2016/5/20/41 > > Do you know if this version they are upstreaming could also yield during > the clk_set_rate()? > I think it should be assumed that any clk_set_rate() potentially could. Please correct me if I'm wrong. >> I'm not aware of other possibility to reproduce this issue, it needs >> some CCF interaction from a separate task. So the current upstream >> kernel shouldn't be affected, I guess. > > What still does not make sense to me is why any frequency changes have > not completed before we attempt to enter the LP2 state? > Why not? I don't see any CPUIDLE <-> CPUFREQ interlocking. Do you think it could be harmful somehow? > OK, well may be we will hold off on this change for the moment. -- Dmitry