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Message-ID: <CAJ5Y-ebyMg1KUSCJ9+5UVN=6TNAMufGP9S4PqQWS0vnfX6XXOg@mail.gmail.com>
Date: Fri, 30 Oct 2015 16:46:33 -0400
From: Ashwin Chaugule <ashwin.chaugule@...aro.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Rafael Wysocki <rjw@...ysocki.net>,
Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] cpufreq: governor: replace per-cpu delayed work with timers
Hi Viresh,
On 29 October 2015 at 08:27, Viresh Kumar <viresh.kumar@...aro.org> 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 <rafael.j.wysocki@...el.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> 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.
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