Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in store_scaling_max_freq()

[Vincent Guittot]

On Mon, 10 Oct 2022 at 11:02, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
>
>
>
> On 10/10/22 06:39, Viresh Kumar wrote:
> > Would be good to always CC Scheduler maintainers for such a patch.
>
> Agree, I'll do that.
>
> >
> > On 30-09-22, 10:48, Lukasz Luba wrote:
> >> When the new max frequency value is stored, the task scheduler must
> >> know about it. The scheduler uses the CPUs capacity information in the
> >> task placement. Use the existing mechanism which provides information
> >> about reduced CPU capacity to the scheduler due to thermal capping.
> >>
> >> Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> >> ---
> >>   drivers/cpufreq/cpufreq.c | 18 +++++++++++++++++-
> >>   1 file changed, 17 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> >> index 1f8b93f42c76..205d9ea9c023 100644
> >> --- a/drivers/cpufreq/cpufreq.c
> >> +++ b/drivers/cpufreq/cpufreq.c
> >> @@ -27,6 +27,7 @@
> >>   #include <linux/slab.h>
> >>   #include <linux/suspend.h>
> >>   #include <linux/syscore_ops.h>
> >> +#include <linux/thermal.h>
> >>   #include <linux/tick.h>
> >>   #include <linux/units.h>
> >>   #include <trace/events/power.h>
> >> @@ -718,6 +719,8 @@ static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
> >>   static ssize_t store_scaling_max_freq
> >>   (struct cpufreq_policy *policy, const char *buf, size_t count)
> >>   {
> >> +    unsigned int frequency;
> >> +    struct cpumask *cpus;
> >>      unsigned long val;
> >>      int ret;
> >>
> >> @@ -726,7 +729,20 @@ static ssize_t store_scaling_max_freq
> >>              return -EINVAL;
> >>
> >>      ret = freq_qos_update_request(policy->max_freq_req, val);
> >> -    return ret >= 0 ? count : ret;
> >> +    if (ret >= 0) {
> >> +            /*
> >> +             * Make sure that the task scheduler sees these CPUs
> >> +             * capacity reduction. Use the thermal pressure mechanism
> >> +             * to propagate this information to the scheduler.
> >> +             */
> >> +            cpus = policy->related_cpus;
> >
> > No need of this, just use related_cpus directly.
> >
> >> +            frequency = __resolve_freq(policy, val, CPUFREQ_RELATION_HE);
> >> +            arch_update_thermal_pressure(cpus, frequency);
> >
> > I wonder if using the thermal-pressure API here is the right thing to
> > do. It is a change coming from User, which may or may not be
> > thermal-related.
>
> Yes, I thought the same. Thermal-pressure name might be not the
> best for covering this use case. I have been thinking about this
> thermal pressure mechanism for a while, since there are other
> use cases like PowerCap DTPM which also reduces CPU capacity
> because of power policy from user-space. We don't notify
> the scheduler about it. There might be also an issue with virtual
> guest OS and how that kernel 'sees' the capacity of CPUs.
> We might try to use this 'thermal-pressure' in the guest kernel
> to notify about available CPU capacity (just a proposal, not
> even an RFC, since we are missing requirements, but issues where
> discussed on LPC 2022 on ChromeOS+Android_guest)

The User space setting scaling_max_freq is a long scale event and it
should be considered as a new running environnement instead of a
transient event. I would suggest updating the EM is and capacity orig
of the system in this case. Similarly, we rebuild sched_domain with a
cpu hotplug. scaling_max_freq interface should not be used to do any
kind of dynamic scaling.

>
> Android middleware has 'powerhits' (IIRC since ~4-5 versions now)
> but our capacity in task scheduler is not aware of those reductions.
>
> IMO thermal-pressure mechanism is good, but the naming convention
> just might be a bit more 'generic' to cover those two users.
>
> Some proposals of better naming:
> 1. Performance capping
> 2. Capacity capping
> 3. Performance reduction
>
> What do you think about changing the name of this and cover
> those two users: PowerCap DTPM and this user-space cpufreq?