From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932868AbbJ3Uq5 (ORCPT ); Fri, 30 Oct 2015 16:46:57 -0400 Received: from mail-lf0-f43.google.com ([209.85.215.43]:35744 "EHLO mail-lf0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758935AbbJ3Uqz (ORCPT ); Fri, 30 Oct 2015 16:46:55 -0400 MIME-Version: 1.0 In-Reply-To: References: From: Ashwin Chaugule Date: Fri, 30 Oct 2015 16:46:33 -0400 Message-ID: Subject: Re: [PATCH 5/6] cpufreq: governor: replace per-cpu delayed work with timers To: Viresh Kumar Cc: Rafael Wysocki , Linaro Kernel Mailman List , "linux-pm@vger.kernel.org" , "Rafael J. Wysocki" , open list Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Viresh, On 29 October 2015 at 08:27, Viresh Kumar wrote: > cpufreq governors evaluate load at sampling rate and based on that they > update frequency for a group of CPUs belonging to the same cpufreq > policy. > > This is required to be done in a single thread for all policy->cpus, but > because we don't want to wakeup idle CPUs to do just that, we use > deferrable work for this. If we would have used a single delayed > deferrable work for the entire policy, there were chances that the CPU > required to run the handler can be in idle and we might end up not > changing the frequency for the entire group with load variations. > > And so we were forced to keep per-cpu works, and only the one that > expires first need to do the real work and others are rescheduled for > next sampling time. > > We have been using the more complex solution until now, where we used a > delayed deferrable work for this, which is a combination of a timer and > a work. > > This could be made lightweight by keeping per-cpu deferred timers with a > single work item, which is scheduled by the first timer that expires. Single shared work item - would perhaps make it a bit more clear. > > This patch does just that and here are important changes: > - The timer handler will run in irq context and so we need to use a > spin_lock instead of the timer_mutex. And so a separate timer_lock is > created. This also makes the use of the mutex and lock quite clear, as > we know what exactly they are protecting. > - A new field 'skip_work' is added to track when the timer handlers can > queue a work. More comments present in code. > > Suggested-by: Rafael J. Wysocki > Signed-off-by: Viresh Kumar > --- > drivers/cpufreq/cpufreq_governor.c | 139 +++++++++++++++++++++---------------- > drivers/cpufreq/cpufreq_governor.h | 20 ++++-- > drivers/cpufreq/cpufreq_ondemand.c | 8 +-- > 3 files changed, 98 insertions(+), 69 deletions(-) > > diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c > index 999e1f6addf9..a3f9bc9b98e9 100644 > --- a/drivers/cpufreq/cpufreq_governor.c > +++ b/drivers/cpufreq/cpufreq_governor.c [..] > > +void gov_cancel_work(struct cpu_common_dbs_info *shared) > +{ > + unsigned long flags; > + > + /* > + * No work will be queued from timer handlers after skip_work is > + * updated. And so we can safely cancel the work first and then the > + * timers. > + */ > + spin_lock_irqsave(&shared->timer_lock, flags); > + shared->skip_work++; > + spin_unlock_irqrestore(&shared->timer_lock, flags); > + > + cancel_work_sync(&shared->work); > + > + gov_cancel_timers(shared->policy); > + > + shared->skip_work = 0; Why doesnt this require the spin_lock protection? > +} > + > /* Will return if we need to evaluate cpu load again or not */ > static bool need_load_eval(struct cpu_common_dbs_info *shared, > unsigned int sampling_rate) > @@ -217,29 +222,21 @@ static bool need_load_eval(struct cpu_common_dbs_info *shared, > return true; > } > > -static void dbs_timer(struct work_struct *work) > +static void dbs_work_handler(struct work_struct *work) > { > - struct cpu_dbs_info *cdbs = container_of(work, struct cpu_dbs_info, > - dwork.work); > - struct cpu_common_dbs_info *shared = cdbs->shared; > + struct cpu_common_dbs_info *shared = container_of(work, struct > + cpu_common_dbs_info, work); > struct cpufreq_policy *policy; > struct dbs_data *dbs_data; > unsigned int sampling_rate, delay; > - bool modify_all = true; > - > - mutex_lock(&shared->timer_mutex); > + bool eval_load; > > policy = shared->policy; > - > - /* > - * Governor might already be disabled and there is no point continuing > - * with the work-handler. > - */ > - if (!policy) > - goto unlock; > - > dbs_data = policy->governor_data; > > + /* Kill all timers */ > + gov_cancel_timers(policy); > + > if (dbs_data->cdata->governor == GOV_CONSERVATIVE) { > struct cs_dbs_tuners *cs_tuners = dbs_data->tuners; > > @@ -250,14 +247,44 @@ static void dbs_timer(struct work_struct *work) > sampling_rate = od_tuners->sampling_rate; > } > > - if (!need_load_eval(cdbs->shared, sampling_rate)) > - modify_all = false; > + eval_load = need_load_eval(shared, sampling_rate); > > - delay = dbs_data->cdata->gov_dbs_timer(policy, modify_all); > - gov_queue_work(dbs_data, policy, delay, modify_all); > + /* > + * Make sure cpufreq_governor_limits() isn't evaluating load in > + * parallel. > + */ > + mutex_lock(&shared->timer_mutex); > + delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load); > + mutex_unlock(&shared->timer_mutex); > + > + shared->skip_work--; Ditto. > + gov_add_timers(policy, delay); > +} > + > +static void dbs_timer_handler(unsigned long data) > +{ > + struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data; > + struct cpu_common_dbs_info *shared = cdbs->shared; > + struct cpufreq_policy *policy; > + unsigned long flags; > + > + spin_lock_irqsave(&shared->timer_lock, flags); > + policy = shared->policy; > + > + /* > + * Timer handler isn't allowed to queue work at the moment, because: > + * - Another timer handler has done that > + * - We are stopping the governor > + * - Or we are updating the sampling rate of ondemand governor > + */ > + if (shared->skip_work) > + goto unlock; > + > + shared->skip_work++; > + queue_work(system_wq, &shared->work); > So, IIUC, in the event that this function gets called back to back and the first Work hasn't dequeued yet, then this queue_work() will not really enqueue, since queue_work_on() will return False? If so, then does it mean we're skipping more recent CPU freq requests? Should we cancel past Work if it hasn't been serviced? Regards, Ashwin.