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Date:   Fri, 15 Dec 2017 21:23:07 +0100
From:   Mike Galbraith <>
To:     Patrick Bellasi <>
        Ingo Molnar <>,
        Peter Zijlstra <>,
        "Rafael J . Wysocki" <>,
        Viresh Kumar <>,
        Vincent Guittot <>,
        Paul Turner <>,
        Dietmar Eggemann <>,
        Morten Rasmussen <>,
        Juri Lelli <>,
        Todd Kjos <>,
        Joel Fernandes <>
Subject: Re: [PATCH v2 0/4] Utilization estimation (util_est) for FAIR tasks

On Fri, 2017-12-15 at 16:13 +0000, Patrick Bellasi wrote:
> Hi Mike,
> On 13-Dec 18:56, Mike Galbraith wrote:
> > On Tue, 2017-12-05 at 17:10 +0000, Patrick Bellasi wrote:
> > > This is a respin of:
> > >
> > > which has been rebased on v4.15-rc2 to have util_est now working on top
> > > of the recent PeterZ's:
> > >    [PATCH -v2 00/18] sched/fair: A bit of a cgroup/PELT overhaul
> > > 
> > > The aim of this series is to improve some PELT behaviors to make it a
> > > better fit for the scheduling of tasks common in embedded mobile
> > > use-cases, without affecting other classes of workloads.
> > 
> > I thought perhaps this patch set would improve the below behavior, but
> > alas it does not.  That's 3 instances of firefox playing youtube clips
> > being shoved into a corner by hogs sitting on 7 of 8 runqueues.  PELT
> > serializes the threaded desktop, making that threading kinda pointless,
> > and CFS not all that fair.
> Perhaps I don't completely get your use-case.
> Are the cpuhog thread pinned to a CPU or just happens to be always
> running on the same CPU?

Nothing is pinned.

> I guess you would expect the three Firefox instances to be spread on
> different CPUs. But whether this is possible depends also on the
> specific tasks composition generated by Firefox, isn't it?

It depends on load balancing.  We're letting firefox threads stack up
to 6 deep while single hogs dominate the box.

> Being a video playback pipeline I would not be surprised to see that
> most of the time we actually have only 1 or 2 tasks RUNNABLE, while
> the others are sleeping... and if an HW decoder is involved, even if
> you have three instances running you likely get only one pipeline
> active at each time...
> If that's the case, why should CFS move Fairfox tasks around?

No, while they are indeed ~fairly synchronous, there is overlap.  If
there were not, there would be no wait time being accumulated. The load
wants to consume roughly one full core worth, but to achieve that, it
needs access to more than one runqueue, which we are not facilitating.

> Is this always happening... or sometimes Firefox tasks gets a chance
> to run on CPUs other then CPU2?

There is some escape going on, but not enough for the load to get its
fair share.  I have it sort of fixed up locally, but while patch keeps
changing, it's not getting any prettier, nor is it particularly
interested in letting me keep some performance gains I want, so...

> How do you get these stats?

perf sched record/perf sched lat.  I twiddled it to output accumulated
wait times as well for convenience, stock only shows max.  See below.
 If you play with perf sched, you'll notice some.. oddities about it.

> It's definitively an interesting use-case however I think it's out of
> the scope of util_est.

Yeah.  If I had been less busy and read the whole thing, I wouldn't
have taken it out for a spin.

> Regarding the specific statement "CFS not all that fair", I would say
> that the fairness of CFS is defined and has to be evaluated within a
> single CPU and on a temporal (not clock cycles) base.

No, that doesn't really fly.  In fact, in the group scheduling code, we
actively pursue box wide fairness.  PELT is going a bit too far ATM.

Point: if you think it's OK to serialize these firefox threads, would
you still think so if those were kernel threads instead?  Serializing
your kernel is a clear fail, but unpinned kthreads can be stacked up
just as effectively as those browser threads are, eat needless wakeup
latency and pass it on.

> AFAIK, vruntime is progressed based on elapsed time, thus you can have
> two tasks which gets the same slice time but consume it at different
> frequencies. In this case also we are not that fair, isn't it?

Time slices don't really exist as a concrete quantum in CFS.  There's
vruntime equalization, and that's it.

> Thus, at the end it all boils down to some (as much as possible)
> low-overhead heuristics. Thus, a proper description of a
> reproducible use-case can help on improving them.

Nah, heuristics are fickle beasts, they WILL knife you in the back,
it's just a question of how often, and how deep.

> Can we model your use-case using a simple rt-app configuration?

No idea.

> This would likely help to have a simple and reproducible testing
> scenario to better understand where the issue eventually is...
> maybe by looking at an execution trace.

It should be reproducible by anyone, just fire up NR_CPUS-1 pure hogs,
point firefox at youtube, open three clips in tabs, watch tasks stack.

Root cause IMHO is PELT having grown too aggressive.  SIS was made more
aggressive to compensate, but when you slam that door you get the full
PELT impact, and it stings, as does too aggressive bouncing when you
leave the escape hatch open.  Sticky wicket that.  Both of those want a
gentle wrap upside the head, as they're both acting a bit nutty.


 tools/perf/builtin-sched.c |   34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -212,6 +212,7 @@ struct perf_sched {
 	u64		 run_avg;
 	u64		 all_runtime;
 	u64		 all_count;
+	u64		 all_lat;
 	u64		 cpu_last_switched[MAX_CPUS];
 	struct rb_root	 atom_root, sorted_atom_root, merged_atom_root;
 	struct list_head sort_list, cmp_pid;
@@ -1286,6 +1287,7 @@ static void output_lat_thread(struct per
 	sched->all_runtime += work_list->total_runtime;
 	sched->all_count   += work_list->nb_atoms;
+	sched->all_lat += work_list->total_lat;
 	if (work_list->num_merged > 1)
 		ret = printf("  %s:(%d) ", thread__comm_str(work_list->thread), work_list->num_merged);
@@ -1298,10 +1300,11 @@ static void output_lat_thread(struct per
 	avg = work_list->total_lat / work_list->nb_atoms;
 	timestamp__scnprintf_usec(work_list->max_lat_at, max_lat_at, sizeof(max_lat_at));
-	printf("|%11.3f ms |%9" PRIu64 " | avg:%9.3f ms | max:%9.3f ms | max at: %13s s\n",
+	printf("|%11.3f ms |%9" PRIu64 " | avg:%9.3f ms | max:%9.3f ms | sum:%9.3f ms | max at: %13s s\n",
 	      (double)work_list->total_runtime / NSEC_PER_MSEC,
 		 work_list->nb_atoms, (double)avg / NSEC_PER_MSEC,
 		 (double)work_list->max_lat / NSEC_PER_MSEC,
+		 (double)work_list->total_lat / NSEC_PER_MSEC,
@@ -1347,6 +1350,16 @@ static int max_cmp(struct work_atoms *l,
 	return 0;
+static int sum_cmp(struct work_atoms *l, struct work_atoms *r)
+	if (l->total_lat < r->total_lat)
+		return -1;
+	if (l->total_lat > r->total_lat)
+		return 1;
+	return 0;
 static int switch_cmp(struct work_atoms *l, struct work_atoms *r)
 	if (l->nb_atoms < r->nb_atoms)
@@ -1378,6 +1391,10 @@ static int sort_dimension__add(const cha
 		.name = "max",
 		.cmp  = max_cmp,
+	static struct sort_dimension sum_sort_dimension = {
+		.name = "sum",
+		.cmp  = sum_cmp,
+	};
 	static struct sort_dimension pid_sort_dimension = {
 		.name = "pid",
 		.cmp  = pid_cmp,
@@ -1394,6 +1411,7 @@ static int sort_dimension__add(const cha
+		&sum_sort_dimension,
@@ -3090,9 +3108,9 @@ static int perf_sched__lat(struct perf_s
-	printf("\n -----------------------------------------------------------------------------------------------------------------\n");
-	printf("  Task                  |   Runtime ms  | Switches | Average delay ms | Maximum delay ms | Maximum delay at       |\n");
-	printf(" -----------------------------------------------------------------------------------------------------------------\n");
+	printf("\n ------------------------------------------------------------------------------------------------------------------------------------\n");
+	printf("  Task                  |   Runtime ms  | Switches | Average delay ms | Maximum delay ms | Sum     delay ms | Maximum delay at       |\n");
+	printf(" ------------------------------------------------------------------------------------------------------------------------------------\n");
 	next = rb_first(&sched->sorted_atom_root);
@@ -3105,11 +3123,11 @@ static int perf_sched__lat(struct perf_s
-	printf(" -----------------------------------------------------------------------------------------------------------------\n");
-	printf("  TOTAL:                |%11.3f ms |%9" PRIu64 " |\n",
-		(double)sched->all_runtime / NSEC_PER_MSEC, sched->all_count);
+	printf(" ------------------------------------------------------------------------------------------------------------\n");
+	printf("  TOTAL:                |%11.3f ms |%9" PRIu64 " |                                     |%14.3f ms |\n",
+		(double)sched->all_runtime / NSEC_PER_MSEC, sched->all_count, (double)sched->all_lat / NSEC_PER_MSEC);
-	printf(" ---------------------------------------------------\n");
+	printf(" ------------------------------------------------------------------------------------------------------------\n");

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