Messages in this thread | | | Date | Mon, 24 Jul 2023 16:53:28 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH] cpufreq: schedutil: next_freq need update when cpufreq_limits changed |
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On 07/24/23 11:36, Xuewen Yan wrote: > On Sat, Jul 22, 2023 at 7:02 AM Qais Yousef <qyousef@layalina.io> wrote: > > > > On 07/19/23 21:05, Xuewen Yan wrote: > > > When cpufreq's policy is single, there is a scenario that will > > > cause sg_policy's next_freq to be unable to update. > > > > > > When the cpu's util is always max, the cpufreq will be max, > > > and then if we change the policy's scaling_max_freq to be a > > > lower freq, indeed, the sg_policy's next_freq need change to > > > be the lower freq, however, because the cpu_is_busy, the next_freq > > > would keep the max_freq. > > > > > > For example: > > > The cpu7 is single cpu: > > > > > > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # while true;do done& > > > [1] 4737 > > > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # taskset -p 80 4737 > > > pid 4737's current affinity mask: ff > > > pid 4737's new affinity mask: 80 > > > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # cat scaling_max_freq > > > 2301000 > > > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # cat scaling_cur_freq > > > 2301000 > > > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # echo 2171000 > scaling_max_freq > > > unisoc:/sys/devices/system/cpu/cpufreq/policy7 # cat scaling_max_freq > > > 2171000 > > > > > > At this time, the sg_policy's next_freq would keep 2301000. > > > > > > To prevent the case happen, add the judgment of the need_freq_update flag. > > > > > > Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com> > > > Co-developed-by: Guohua Yan <guohua.yan@unisoc.com> > > > Signed-off-by: Guohua Yan <guohua.yan@unisoc.com> > > > --- > > > kernel/sched/cpufreq_schedutil.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > > > index 4492608b7d7f..458d359f5991 100644 > > > --- a/kernel/sched/cpufreq_schedutil.c > > > +++ b/kernel/sched/cpufreq_schedutil.c > > > @@ -350,7 +350,8 @@ static void sugov_update_single_freq(struct update_util_data *hook, u64 time, > > > * Except when the rq is capped by uclamp_max. > > > */ > > > if (!uclamp_rq_is_capped(cpu_rq(sg_cpu->cpu)) && > > > - sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) { > > > + sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq && > > > + !sg_policy->need_freq_update) { > > > > What about sugov_update_single_perf()? It seems to have the same problem, no? > > There is no problem in sugov_update_single_perf, because the next_freq > is updated by drivers, maybe the next_freq is not used when using > sugov_update_single_perf..
Ah I see; we just use prev_util but the request will go through and the driver should observe the new limit regardless of what util value we pass to it. Got ya.
> > But for the last_freq_update_time, I think there are some problems > when using sugov_update_single_perf: > Now, there is no judgment condition for the update of the > last_freq_update_time. That means the last_freq_update_time is always > updated in sugov_update_single_perf. > And in sugov_should_update_freq: it would judge the > freq_update_delay_ns. As a result, If we use the > sugov_update_single_perf, the cpu frequency would only be periodically > updated according to freq_update_delay_ns. > Maybe we should judge the cpufreq_driver_adjust_perf's return value, > if the freq is not updated, the last_freq_update_time also does not > have to update. > > Just like: > --- > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > index 458d359f5991..10f18b054f01 100644 > --- a/kernel/sched/cpufreq_schedutil.c > +++ b/kernel/sched/cpufreq_schedutil.c > @@ -381,6 +381,7 @@ static void sugov_update_single_perf(struct > update_util_data *hook, u64 time, > struct sugov_cpu *sg_cpu = container_of(hook, struct > sugov_cpu, update_util); > unsigned long prev_util = sg_cpu->util; > unsigned long max_cap; > + bool freq_updated; > > /* > * Fall back to the "frequency" path if frequency invariance is not > @@ -407,10 +408,11 @@ static void sugov_update_single_perf(struct > update_util_data *hook, u64 time, > sugov_cpu_is_busy(sg_cpu) && sg_cpu->util < prev_util) > sg_cpu->util = prev_util; > > - cpufreq_driver_adjust_perf(sg_cpu->cpu, map_util_perf(sg_cpu->bw_dl), > + freq_updated = cpufreq_driver_adjust_perf(sg_cpu->cpu, > map_util_perf(sg_cpu->bw_dl), > map_util_perf(sg_cpu->util), max_cap); > > - sg_cpu->sg_policy->last_freq_update_time = time; > + if (freq_updated) > + sg_cpu->sg_policy->last_freq_update_time = time; > }
Sound reasonable in principle, but it could lead to overhead; for example when the system is busy and maxed out, the last_freq_update_time will never be updated and will end up continuously calling to the driver to change frequency without any rate limit AFAICS. Which might not be an acceptable overhead, I don't know. Logically this is wasted cycles preventing the tasks from doing useful work. I think we need to look at such corner cases and treat them appropriately to not call the driver if we go with this approach.
Cheers
-- Qais Yousef
> > > BR > Thanks! > > --- > xuewen > > > > LGTM otherwise. > > > > > > Cheers > > > > -- > > Qais Yousef > > > > > next_f = sg_policy->next_freq; > > > > > > /* Restore cached freq as next_freq has changed */ > > > -- > > > 2.25.1 > > >
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