Re: [PATCH v3 5/6] cpufreq: schedutil: relax rate-limiting while running RT/DL tasks

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 30-Nov 14:36, Juri Lelli wrote:
> Hi,
> 
> On 30/11/17 11:47, Patrick Bellasi wrote:
> > The policy in use for RT/DL tasks sets the maximum frequency when a task
> > in these classes calls for a cpufreq_update_util().  However, the
> > current implementation is still enforcing a frequency switch rate
> > limiting when these tasks are running.
> > This is potentially working against the goal to switch to the maximum OPP
> > when RT tasks are running. In certain unfortunate cases it can also happen
> > that a RT task almost completes its activation at a lower OPP.
> > 
> > This patch overrides on purpose the rate limiting configuration
> > to better serve RT/DL tasks. As long as a frequency scaling operation
> > is not in progress, a frequency switch is always authorized when
> > running in "rt_mode", i.e. the current task in a CPU belongs to the
> > RT/DL class.
> > 
> > Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
> > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > Cc: Viresh Kumar <viresh.kumar@linaro.org>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: linux-pm@vger.kernel.org
> > 
> > ---
> > Changes from v2:
> > - rebased on v4.15-rc1
> > 
> > Change-Id: I733d47b9e265cebb2e3e5e71a3cd468e9be002d1
> 
> Luckily this gets ignored... :)

Indeed, and it was intended... just to verify if ML folks can tolerate
the idea to have Change-Ids in the "notes section".

This would help some internal review workflow, which sometimes are
based... yes... ehm... on gerrit :-]

> > ---
> >  kernel/sched/cpufreq_schedutil.c | 19 ++++++++++++-------
> >  1 file changed, 12 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 40521d59630b..3eea8884e61b 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -74,7 +74,8 @@ static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
> >  
> >  /************************ Governor internals ***********************/
> >  
> > -static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> > +static bool sugov_should_update_freq(struct sugov_policy *sg_policy,
> > +				     u64 time, bool rt_mode)
> >  {
> >  	s64 delta_ns;
> >  
> > @@ -111,6 +112,10 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> >  		return true;
> >  	}
> >  
> > +	/* Always update if a RT/DL task is running */
> > +	if (rt_mode)
> > +		return true;
> > +
> >  	delta_ns = time - sg_policy->last_freq_update_time;
> >  	return delta_ns >= sg_policy->freq_update_delay_ns;
> >  }
> > @@ -268,11 +273,6 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >  	sugov_set_iowait_boost(sg_cpu, time, flags);
> >  	sg_cpu->last_update = time;
> >  
> > -	if (!sugov_should_update_freq(sg_policy, time))
> > -		return;
> > -
> > -	busy = sugov_cpu_is_busy(sg_cpu);
> > -
> >  	/*
> >  	 * While RT/DL tasks are running we do not want FAIR tasks to
> >  	 * overvrite this CPU's flags, still we can update utilization and
> > @@ -281,6 +281,11 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >  	rt_mode = task_has_dl_policy(current) ||
> >  		  task_has_rt_policy(current) ||
> >  		  (flags & SCHED_CPUFREQ_RT_DL);
> > +	if (!sugov_should_update_freq(sg_policy, time, rt_mode))
> > +		return;
> > +
> > +	busy = sugov_cpu_is_busy(sg_cpu);
> > +
> >  	if (rt_mode) {
> >  		next_f = policy->cpuinfo.max_freq;
> >  	} else {
> > @@ -379,7 +384,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time,
> >  	sugov_set_iowait_boost(sg_cpu, time, flags);
> >  	sg_cpu->last_update = time;
> >  
> > -	if (sugov_should_update_freq(sg_policy, time)) {
> > +	if (sugov_should_update_freq(sg_policy, time, rt_mode)) {
> >  		next_f = rt_mode
> >  			? sg_policy->policy->cpuinfo.max_freq
> >  			: sugov_next_freq_shared(sg_cpu, time);
> 
> Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

Tks ;-)
 
> I wonder if we would also need some way to trigger a back to back update
> as soon as a currently running one finishes and an RT/DL task asked for
> an update (without waiting for the next tick).

Good point, I think that would actually be an interesting
optimization. We already discussed that in the past, but refrained by
adding more on top of this already "substantial" set of changes.

Can we think about that once we decided about some patches of this
series?

> Best,
> 
> Juri

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi