From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
Viresh Kumar <viresh.kumar@linaro.org>,
Rafael Wysocki <rjw@rjwysocki.net>,
Linux PM <linux-pm@vger.kernel.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
cristian.marussi@arm.com, Sudeep Holla <sudeep.holla@arm.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record_transition()
Date: Thu, 24 Sep 2020 12:24:56 +0200 [thread overview]
Message-ID: <CAJZ5v0iFjzqTKTPFF5hB5C0TYSQn2rxL_6099gqUwoTARKRnZA@mail.gmail.com> (raw)
In-Reply-To: <a4c5a6b9-10f8-34f8-f01d-8b373214d173@arm.com>
On Thu, Sep 24, 2020 at 11:25 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
> Hi Rafael,
>
> On 9/23/20 2:48 PM, Rafael J. Wysocki wrote:
> > On Wed, Sep 16, 2020 at 8:45 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> >>
> >> In order to prepare for lock-less stats update, add support to defer any
> >> updates to it until cpufreq_stats_record_transition() is called.
> >
> > This is a bit devoid of details.
> >
> > I guess you mean reset in particular, but that's not clear from the above.
> >
> > Also, it would be useful to describe the design somewhat.
> >
> >> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> >> ---
> >> drivers/cpufreq/cpufreq_stats.c | 75 ++++++++++++++++++++++++---------
> >> 1 file changed, 56 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> >> index 94d959a8e954..3e7eee29ee86 100644
> >> --- a/drivers/cpufreq/cpufreq_stats.c
> >> +++ b/drivers/cpufreq/cpufreq_stats.c
> >> @@ -22,17 +22,22 @@ struct cpufreq_stats {
> >> spinlock_t lock;
> >> unsigned int *freq_table;
> >> unsigned int *trans_table;
> >> +
> >> + /* Deferred reset */
> >> + unsigned int reset_pending;
> >> + unsigned long long reset_time;
> >> };
> >>
> >> -static void cpufreq_stats_update(struct cpufreq_stats *stats)
> >> +static void cpufreq_stats_update(struct cpufreq_stats *stats,
> >> + unsigned long long time)
> >> {
> >> unsigned long long cur_time = get_jiffies_64();
> >>
> >> - stats->time_in_state[stats->last_index] += cur_time - stats->last_time;
> >> + stats->time_in_state[stats->last_index] += cur_time - time;
> >> stats->last_time = cur_time;
> >> }
> >>
> >> -static void cpufreq_stats_clear_table(struct cpufreq_stats *stats)
> >> +static void cpufreq_stats_reset_table(struct cpufreq_stats *stats)
> >> {
> >> unsigned int count = stats->max_state;
> >>
> >> @@ -41,42 +46,67 @@ static void cpufreq_stats_clear_table(struct cpufreq_stats *stats)
> >> memset(stats->trans_table, 0, count * count * sizeof(int));
> >> stats->last_time = get_jiffies_64();
> >> stats->total_trans = 0;
> >> +
> >> + /* Adjust for the time elapsed since reset was requested */
> >> + WRITE_ONCE(stats->reset_pending, 0);
> >
> > What if this runs in parallel with store_reset()?
> >
> > The latter may update reset_pending to 1 before the below runs.
> > Conversely, this may clear reset_pending right after store_reset() has
> > set it to 1, but before it manages to set reset_time. Is that not a
> > problem?
>
> I wonder if we could just drop the reset feature. Is there a tool
> which uses this file? The 'reset' sysfs would probably have to stay
> forever, but an empty implementation is not an option?
Well, having an empty sysfs attr would be a bit ugly, but the
implementation of it could be simplified.
> The documentation states:
> 'This can be useful for evaluating system behaviour under different
> governors without the need for a reboot.'
> With the scenario of fast-switch this resetting complicates the
> implementation and the justification of having it just for experiments
> avoiding reboot is IMO weak. The real production code would have to pay
> extra cycles every time. Also, we would probably not experiment with
> cpufreq different governors, since the SchedUtil is considered the best
> option.
It would still be good to have a way to test it against the other
available options, though.
next prev parent reply other threads:[~2020-09-24 10:25 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-16 6:45 [PATCH V2 0/4] cpufreq: Record stats with fast-switching Viresh Kumar
2020-09-16 6:45 ` [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record_transition() Viresh Kumar
2020-09-23 13:48 ` Rafael J. Wysocki
2020-09-24 9:25 ` Lukasz Luba
2020-09-24 10:24 ` Rafael J. Wysocki [this message]
2020-09-24 11:00 ` Lukasz Luba
2020-09-24 11:07 ` Rafael J. Wysocki
2020-09-24 12:39 ` Viresh Kumar
2020-09-24 16:10 ` Lukasz Luba
2020-09-25 6:09 ` Viresh Kumar
2020-09-25 8:10 ` Lukasz Luba
2020-09-24 13:15 ` Viresh Kumar
2020-09-25 10:04 ` Rafael J. Wysocki
2020-09-25 10:58 ` Viresh Kumar
2020-09-25 11:09 ` Rafael J. Wysocki
2020-09-25 11:26 ` Viresh Kumar
2020-09-25 8:21 ` Lukasz Luba
2020-09-25 10:46 ` Viresh Kumar
2020-09-16 6:45 ` [PATCH V2 2/4] cpufreq: stats: Remove locking Viresh Kumar
2020-09-16 6:45 ` [PATCH V2 3/4] cpufreq: stats: Enable stats for fast-switch as well Viresh Kumar
2020-09-23 15:14 ` Rafael J. Wysocki
2020-09-23 15:17 ` Rafael J. Wysocki
2020-09-16 6:45 ` [PATCH V2 4/4] cpufreq: Move traces and update to policy->cur to cpufreq core Viresh Kumar
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