Re: [PATCH] cpufreq: Change default transition delay to 2ms

[Qais Yousef <qyousef@layalina.io>]

On 02/14/24 10:19, Pierre Gondois wrote:
> Hello,
> 
> On 2/12/24 16:53, Rafael J. Wysocki wrote:
> > On Mon, Feb 5, 2024 at 8:45 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > > 
> > > On 05-02-24, 02:25, Qais Yousef wrote:
> > > > 10ms is too high for today's hardware, even low end ones. This default
> > > > end up being used a lot on Arm machines at least. Pine64, mac mini and
> > > > pixel 6 all end up with 10ms rate_limit_us when using schedutil, and
> > > > it's too high for all of them.
> > > > 
> > > > Change the default to 2ms which should be 'pessimistic' enough for worst
> > > > case scenario, but not too high for platforms with fast DVFS hardware.
> > > > 
> > > > Signed-off-by: Qais Yousef <qyousef@layalina.io>
> > > > ---
> > > >   drivers/cpufreq/cpufreq.c | 4 ++--
> > > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > > > index 44db4f59c4cc..8207f7294cb6 100644
> > > > --- a/drivers/cpufreq/cpufreq.c
> > > > +++ b/drivers/cpufreq/cpufreq.c
> > > > @@ -582,11 +582,11 @@ unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy)
> > > >                 * for platforms where transition_latency is in milliseconds, it
> > > >                 * ends up giving unrealistic values.
> > > >                 *
> > > > -              * Cap the default transition delay to 10 ms, which seems to be
> > > > +              * Cap the default transition delay to 2 ms, which seems to be
> > > >                 * a reasonable amount of time after which we should reevaluate
> > > >                 * the frequency.
> > > >                 */
> > > > -             return min(latency * LATENCY_MULTIPLIER, (unsigned int)10000);
> > > > +             return min(latency * LATENCY_MULTIPLIER, (unsigned int)(2*MSEC_PER_SEC));
> > > 
> > > Please add spaces around '*'.
> > > 
> > > Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> > 
> > I've adjusted the whitespace as suggested above and applied the patch
> > as 5.9 material.
> > 
> > Thanks!
> > 
> 
> To add some numbers, on a Juno-r2, with latency measured between the frequency
> request on the kernel side and the SCP actually making the frequency update.
> 
> The SCP is the firmware responsible of making the frequency updates. It receives
> the kernel requests and coordinate them/make the actual changes. The SCP also has
> a mechanism called 'fast channel' (FC) where the kernel writes the requested
> frequency to a memory area shared with the SCP. Every 4ms, the SCP polls/reads
> these memory area and make the required modifications.
> 
> Latency values (in ms)
> Workload:
> Idle system, during ~30s
> +---------------------------------------+
> |       |   Without FC  |      With FC  |
> +-------+---------------+---------------+
> | count |       1663    |        1102   |
> | mean  |          2.92 |          2.10 |
> | std   |          1.90 |          1.58 |
> | min   |          0.21 |          0.00 |
> | 25%   |          1.64 |          0.91 |
> | 50%   |          2.57 |          1.68 |
> | 75%   |          3.66 |          2.97 |
> | max   |         14.37 |         13.50 |
> +-------+---------------+---------------+
> 
> Latency values (in ms)
> Workload:
> One 1% task per CPU, period = 32ms. This allows to wake up the CPU
> every 32ms and send more requests/give more work to the SCP. Indeed
> the SCP is also responsible of idle state transitions.
> Test duration ~=30s.
> +---------------------------------------+
> |       |   Without FC  |      With FC  |
> +-------+---------------+---------------+
> | count |       1629    |       1446    |
> | mean  |          3.23 |          2.31 |
> | std   |          2.40 |          1.73 |
> | min   |          0.05 |          0.02 |
> | 25%   |          1.91 |          0.98 |
> | 50%   |          2.65 |          2.00 |
> | 75%   |          3.65 |          3.23 |
> | max   |         20.56 |         16.73 |
> +-------+---------------+---------------+
> 
> ---
> 
> The latency increases when fast channels are not used and when there is an actual
> workload. On average it is always > 2ms. Juno's release date seems to be 2014,
> so the platform is quite old, but it should also have benefited from regular
> firmware updates.

Thanks for sharing the numbers

> 
> Regards,
> Pierre