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Date:   Mon, 8 Jun 2020 10:44:24 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>,
        Mel Gorman <mgorman@suse.de>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
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        Valentin Schneider <valentin.schneider@arm.com>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        linux-doc@vger.kernel.org,
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        linux-fs <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT
 default boost value
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On Mon, 8 Jun 2020 13:31:03 +0100
Qais Yousef <qais.yousef@arm.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.

> to capture similar runs for uclamp, the numbers didn't add up compared to
> running the test without ftrace generating the graph. If juno is suffering from
> bad branching costs in this path, then I suspect ftrace will amplify this as
> AFAIU it'll cause extra jumps on entry and exit.
> 
> 
> 
>       sched-pipe-6532  [001]  9407.276302: funcgraph_entry:                   |  deactivate_task() {
>       sched-pipe-6532  [001]  9407.276302: funcgraph_entry:                   |    dequeue_task_fair() {
>       sched-pipe-6532  [001]  9407.276303: funcgraph_entry:                   |      update_curr() {
>       sched-pipe-6532  [001]  9407.276304: funcgraph_entry:        0.780 us   |        update_min_vruntime();
>       sched-pipe-6532  [001]  9407.276306: funcgraph_entry:                   |        cpuacct_charge() {
>       sched-pipe-6532  [001]  9407.276306: funcgraph_entry:        0.820 us   |          __rcu_read_lock();
>       sched-pipe-6532  [001]  9407.276308: funcgraph_entry:        0.740 us   |          __rcu_read_unlock();

The above is more accurate than...

>       sched-pipe-6532  [001]  9407.276309: funcgraph_exit:         3.980 us   |        }

this one. Because this one has nested tracing within it.

-- Steve


>       sched-pipe-6532  [001]  9407.276310: funcgraph_entry:        0.720 us   |        __rcu_read_lock();
>       sched-pipe-6532  [001]  9407.276312: funcgraph_entry:        0.720 us   |        __rcu_read_unlock();
>       sched-pipe-6532  [001]  9407.276313: funcgraph_exit:         9.840 us   |      }
>       sched-pipe-6532  [001]  9407.276314: funcgraph_entry:                   |      __update_load_avg_se() {
>       sched-pipe-6532  [001]  9407.276315: funcgraph_entry:        0.720 us   |        __accumulate_pelt_segments();
>       sched-pipe-6532  [001]  9407.276316: funcgraph_exit:         2.260 us   |      }
>       sched-pipe-6532  [001]  9407.276317: funcgraph_entry:                   |      __update_load_avg_cfs_rq() {
>       sched-pipe-6532  [001]  9407.276318: funcgraph_entry:        0.860 us   |        __accumulate_pelt_segments();
>       sched-pipe-6532  [001]  9407.276319: funcgraph_exit:         2.340 us   |      }