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Date: Thu, 11 Jun 2020 11:13:49 +0100
From: Qais Yousef <qais.yousef@....com>
To: Steven Rostedt <rostedt@...dmis.org>,
Vincent Guittot <vincent.guittot@...aro.org>
Cc: Mel Gorman <mgorman@...e.de>,
Patrick Bellasi <patrick.bellasi@...bug.net>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Randy Dunlap <rdunlap@...radead.org>,
Jonathan Corbet <corbet@....net>,
Juri Lelli <juri.lelli@...hat.com>,
Ben Segall <bsegall@...gle.com>,
Luis Chamberlain <mcgrof@...nel.org>,
Kees Cook <keescook@...omium.org>,
Iurii Zaikin <yzaikin@...gle.com>,
Quentin Perret <qperret@...gle.com>,
Valentin Schneider <valentin.schneider@....com>,
Pavan Kondeti <pkondeti@...eaurora.org>,
linux-doc@...r.kernel.org,
linux-kernel <linux-kernel@...r.kernel.org>,
linux-fs <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default
boost value
On 06/08/20 10:44, Steven Rostedt wrote:
> On Mon, 8 Jun 2020 13:31:03 +0100
> Qais Yousef <qais.yousef@....com> wrote:
>
> > I admit I don't know how much of these numbers is ftrace overhead. When trying
>
> Note, if you want to get a better idea of how long a function runs, put it
> into set_ftrace_filter, and then trace it. That way you remove the overhead
> of the function graph tracer when its nesting within a function.
Thanks for the tip!
With CONFIG_FAIR_GROUP_SCHED I see (uclamp disabled)
sched-pipe-602 [001] 73.755392: funcgraph_entry: 2.080 us | activate_task();
sched-pipe-602 [001] 73.755399: funcgraph_entry: 2.000 us | deactivate_task();
sched-pipe-601 [001] 73.755407: funcgraph_entry: 2.220 us | activate_task();
sched-pipe-601 [001] 73.755414: funcgraph_entry: 2.020 us | deactivate_task();
sched-pipe-602 [001] 73.755422: funcgraph_entry: 2.160 us | activate_task();
sched-pipe-602 [001] 73.755429: funcgraph_entry: 1.920 us | deactivate_task();
sched-pipe-601 [001] 73.755437: funcgraph_entry: 2.260 us | activate_task();
sched-pipe-601 [001] 73.755444: funcgraph_entry: 2.080 us | deactivate_task();
sched-pipe-602 [001] 73.755452: funcgraph_entry: 2.160 us | activate_task();
sched-pipe-602 [001] 73.755459: funcgraph_entry: 2.080 us | deactivate_task();
sched-pipe-601 [001] 73.755468: funcgraph_entry: 2.200 us | activate_task();
sched-pipe-601 [001] 73.755521: funcgraph_entry: 3.160 us | activate_task();
update_cfs_group() overhead
sched-pipe-622 [001] 156.790478: funcgraph_entry: 0.820 us | update_cfs_group();
sched-pipe-622 [001] 156.790483: funcgraph_entry: 0.840 us | update_cfs_group();
sched-pipe-622 [001] 156.790485: funcgraph_entry: 0.820 us | update_cfs_group();
sched-pipe-622 [001] 156.790487: funcgraph_entry: 0.820 us | update_cfs_group();
sched-pipe-622 [001] 156.790488: funcgraph_entry: 0.800 us | update_cfs_group();
sched-pipe-622 [001] 156.790508: funcgraph_entry: 1.040 us | update_cfs_group();
sched-pipe-622 [001] 156.790510: funcgraph_entry: 0.920 us | update_cfs_group();
sched-pipe-622 [001] 156.790511: funcgraph_entry: 1.040 us | update_cfs_group();
sched-pipe-622 [001] 156.790513: funcgraph_entry: 0.840 us | update_cfs_group();
sched-pipe-623 [001] 156.790540: funcgraph_entry: 1.160 us | update_cfs_group();
sched-pipe-623 [001] 156.790543: funcgraph_entry: 1.020 us | update_cfs_group();
sched-pipe-623 [001] 156.790544: funcgraph_entry: 0.880 us | update_cfs_group();
sched-pipe-623 [001] 156.790546: funcgraph_entry: 0.840 us | update_cfs_group();
sched-pipe-621 [001] 156.790905: funcgraph_entry: 1.780 us | update_cfs_group();
sched-pipe-621 [001] 156.790908: funcgraph_entry: 1.060 us | update_cfs_group();
sched-pipe-621 [001] 156.790910: funcgraph_entry: 0.880 us | update_cfs_group();
sched-pipe-621 [001] 156.790912: funcgraph_entry: 0.880 us | update_cfs_group();
sched-pipe-621 [001] 156.790916: funcgraph_entry: 0.800 us | update_cfs_group();
sched-pipe-621 [001] 156.790917: funcgraph_entry: 0.820 us | update_cfs_group();
sched-pipe-621 [001] 156.790919: funcgraph_entry: 0.840 us | update_cfs_group();
sched-pipe-621 [001] 156.790921: funcgraph_entry: 0.880 us | update_cfs_group();
sched-pipe-621 [001] 156.790932: funcgraph_entry: 0.960 us | update_cfs_group();
sched-pipe-621 [001] 156.790934: funcgraph_entry: 0.960 us | update_cfs_group();
sched-pipe-621 [001] 156.790936: funcgraph_entry: 1.080 us | update_cfs_group();
sched-pipe-621 [001] 156.790937: funcgraph_entry: 0.840 us | update_cfs_group();
Without CONFIG_FAIR_GROUP_SCHED and without CONFIG_UCLAMP_TASK
sched-pipe-604 [001] 76.386078: funcgraph_entry: 1.380 us | activate_task();
sched-pipe-604 [001] 76.386084: funcgraph_entry: 1.360 us | deactivate_task();
sched-pipe-605 [001] 76.386091: funcgraph_entry: 1.400 us | activate_task();
sched-pipe-605 [001] 76.386096: funcgraph_entry: 1.260 us | deactivate_task();
sched-pipe-604 [001] 76.386104: funcgraph_entry: 1.500 us | activate_task();
sched-pipe-604 [001] 76.386109: funcgraph_entry: 1.280 us | deactivate_task();
sched-pipe-605 [001] 76.386117: funcgraph_entry: 1.380 us | activate_task();
sched-pipe-605 [001] 76.386122: funcgraph_entry: 1.300 us | deactivate_task();
sched-pipe-604 [001] 76.386130: funcgraph_entry: 1.380 us | activate_task();
sched-pipe-604 [001] 76.386135: funcgraph_entry: 1.260 us | deactivate_task();
sched-pipe-605 [001] 76.386142: funcgraph_entry: 1.400 us | activate_task();
sched-pipe-605 [001] 76.386148: funcgraph_entry: 1.340 us | deactivate_task();
So approximately 800ns are added by update_cfs_group() for enqueue and dequeue.
This overhead affects 2 tasks in the tests, so the total effect on the
generated usecs/ops
2 * enqueue_overhead + 2 * dequeue overhead = 4 * ~800ns = 3.2 us
Which explains the 3us drop I see when fair group config is enabled.
Applying similar analysis to uclamp
With uclamp enabled
sched-pipe-610 [001] 173.429431: funcgraph_entry: 1.580 us | activate_task();
sched-pipe-610 [001] 173.429437: funcgraph_entry: 1.440 us | deactivate_task();
sched-pipe-609 [001] 173.429444: funcgraph_entry: 1.580 us | activate_task();
sched-pipe-609 [001] 173.429450: funcgraph_entry: 1.440 us | deactivate_task();
sched-pipe-610 [001] 173.429458: funcgraph_entry: 1.700 us | activate_task();
sched-pipe-610 [001] 173.429464: funcgraph_entry: 1.460 us | deactivate_task();
sched-pipe-609 [001] 173.429471: funcgraph_entry: 1.540 us | activate_task();
sched-pipe-609 [001] 173.429477: funcgraph_entry: 1.460 us | deactivate_task();
sched-pipe-610 [001] 173.429485: funcgraph_entry: 1.560 us | activate_task();
sched-pipe-610 [001] 173.429491: funcgraph_entry: 1.500 us | deactivate_task();
sched-pipe-609 [001] 173.429498: funcgraph_entry: 1.600 us | activate_task();
sched-pipe-609 [001] 173.429504: funcgraph_entry: 1.460 us | deactivate_task();
Which adds approximately 200ns at enqueue and dequeue.
2 * enqueue_overhead + 2 * dequeue overhead = 4 * ~200ns = 0.8 us
Which would explain the ~1us drop I've seen with uclamp when running sched
bench. Apologies for the very course averaging of the numbers from my side.
As a reminder the results I reported before:
*** uclamp disabled/fair group enabled ***
# Executed 50000 pipe operations between two threads
Total time: 0.958 [sec]
19.177100 usecs/op
52145 ops/sec
*** uclamp disabled/fair group disabled ***
# Executed 50000 pipe operations between two threads
Total time: 0.808 [sec]
16.176200 usecs/op
61819 ops/sec
*** uclamp enabled/fair group disabled ***
# Executed 50000 pipe operations between two threads
Total time: 0.856 [sec]
17.125740 usecs/op
58391 ops/sec
Based on my observation with code shuffling it seems a lot of this 200ns comes
from terrible I$ performance on the particular platform I am testing on.
When I run on x86 machine, if I interpreted perf annotation correctly I see D$
misses on accessing rq->uclamp_rq.bucket[] and p->uclamp[]. But I'll share this
result on a separate email in-reply to Mel.
Thanks
--
Qais Yousef
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