Messages in this thread | | | Date | Mon, 10 Oct 2022 11:49:52 +0100 | Subject | Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in store_scaling_max_freq() | From | Lukasz Luba <> |
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+CC Daniel
On 10/10/22 11:22, Vincent Guittot wrote: > On Mon, 10 Oct 2022 at 12:12, Lukasz Luba <lukasz.luba@arm.com> wrote: >> >> >> >> On 10/10/22 10:32, Vincent Guittot wrote: >>> On Mon, 10 Oct 2022 at 11:30, Lukasz Luba <lukasz.luba@arm.com> wrote: >>>> >>>> >>>> >>>> On 10/10/22 10:15, Vincent Guittot wrote: >>>>> On Mon, 10 Oct 2022 at 11:02, Lukasz Luba <lukasz.luba@arm.com> 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@arm.com> >>>>>>>> --- >>>>>>>> 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. >>>> >>>> I tend to agree, but the EM capacity would be only used in part of EAS >>>> code. The whole fair.c view to the capacity_of() (RT + DL + irq + >>>> thermal_pressure) would be still wrong in other parts, e.g. >>>> select_idle_sibling() and load balance. >>>> >>>> When we get this powerhint we might be already in overutilied state >>>> so EAS is disabled. IMO other mechanisms in the task scheduler >>>> should be also aware of that capacity reduction. >>> >>> That's why I also mentioned the capacity_orig >> >> Well, I think this is a bit more complex. Thermal framework governor >> reduces the perf IDs from top in the freq asc table and keeps that >> in the statistics in sysfs. It also updates the thermal pressure signal. >> When we rebuild the capacity of CPUs and make the capacity_orig smaller, >> the capacity_of would still have the thermal framework reduced capacity >> in there. We would end up with too small CPU capacity due to this >> subtraction in capacity_of. > > That's why using user space interface should not be used to do dynamic scaling. > I still think that user space interface is not the right interface > >> >> Ideally, I would see a mechanism which is aware of this performance >> reduction reason: >> 1. thermal capping >> 2. power capping (from DTPM) >> 3. max freq reduction by user space > > Yes for thermal and power capping but no for user space > >> >> That common place would figure and maintain the context for the >> requested capacity reduction. >> >> BTW, those Android user space max freq requests are not that long, >> mostly due to camera capturing (you can see a few in this file, >> e.g. [1]). > > Why are they doing this ? > This doesn't seem to be the correct interface to use. It seems to do > some power budget and they should use the right interface for this
Yes, I agree. I have sent explanation with this to Peter's emails. Daniel tries to give them a better interface: DTPM, but also would suffer the same issue of capacity reduction for this short time.
We have a few discussions about it, also Daniel presented on a few LPC those issues.
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