From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: linaro-kernel@lists.linaro.org, linux-pm@vger.kernel.org,
ashwin.chaugule@linaro.org,
"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers
Date: Fri, 04 Dec 2015 02:18:41 +0100 [thread overview]
Message-ID: <10439879.00aCyM9quW@vostro.rjw.lan> (raw)
In-Reply-To: <691a3524afaa8d08d61987b1c2913948b4e59f01.1449115453.git.viresh.kumar@linaro.org>
On Thursday, December 03, 2015 09:37:53 AM 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.
>
> 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@intel.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> Reviewed-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
I've tentatively queued this one up, but I still have a couple of questions.
> ---
> 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(-)
[cut]
> @@ -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--;
Is there any reason for incrementing and decrementing this instead of setting
it to either 0 or 1 (or maybe either 'true' or 'false' for that matter)?
If my reading of the patch is correct, it can only be either 0 or 1 anyway, right?
> + 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;
Why do we need policy here?
> +
> + /*
> + * 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);
>
> unlock:
What about writing the above as
if (!shared->work_in_progress) {
shared->work_in_progress = true;
queue_work(system_wq, &shared->work);
}
and then you won't need the unlock label.
> - mutex_unlock(&shared->timer_mutex);
> + spin_unlock_irqrestore(&shared->timer_lock, flags);
> }
>
> static void set_sampling_rate(struct dbs_data *dbs_data,
Thanks,
Rafael
next prev parent reply other threads:[~2015-12-04 0:48 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1449115453.git.viresh.kumar@linaro.org>
2015-12-03 4:07 ` [PATCH V2 1/6] cpufreq: ondemand: Update sampling rate only for concerned policies Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 2/6] cpufreq: ondemand: Work is guaranteed to be pending Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 3/6] cpufreq: governor: Pass policy as argument to ->gov_dbs_timer() Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 4/6] cpufreq: governor: initialize/destroy timer_mutex with 'shared' Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers Viresh Kumar
2015-12-04 1:18 ` Rafael J. Wysocki [this message]
2015-12-04 6:11 ` Viresh Kumar
2015-12-05 2:14 ` Rafael J. Wysocki
2015-12-05 4:10 ` Viresh Kumar
2015-12-07 1:28 ` Rafael J. Wysocki
2015-12-07 7:50 ` Viresh Kumar
2015-12-07 22:43 ` Rafael J. Wysocki
2015-12-07 23:17 ` Rafael J. Wysocki
2015-12-08 0:39 ` [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization Rafael J. Wysocki
2015-12-08 6:59 ` Viresh Kumar
2015-12-08 13:30 ` Rafael J. Wysocki
2015-12-08 13:36 ` Viresh Kumar
2015-12-08 14:19 ` Rafael J. Wysocki
2015-12-08 13:55 ` Viresh Kumar
2015-12-08 14:30 ` Rafael J. Wysocki
2015-12-08 14:56 ` Viresh Kumar
2015-12-08 16:42 ` Rafael J. Wysocki
2015-12-08 16:34 ` Viresh Kumar
2015-12-08 6:46 ` [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers Viresh Kumar
2015-12-08 6:56 ` Viresh Kumar
2015-12-08 13:18 ` Rafael J. Wysocki
2015-12-08 13:30 ` Viresh Kumar
2015-12-08 14:04 ` Rafael J. Wysocki
2015-12-04 6:13 ` [PATCH V3 " Viresh Kumar
2015-12-09 2:04 ` [PATCH V4 " Viresh Kumar
2015-12-09 22:06 ` Rafael J. Wysocki
2015-12-10 2:36 ` Viresh Kumar
2015-12-10 22:17 ` Rafael J. Wysocki
2015-12-11 1:42 ` Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 6/6] cpufreq: ondemand: update update_sampling_rate() to make it more efficient Viresh Kumar
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