linux-kernel - Re: [RFC PATCH v5 2/2] trace,smp: Add tracepoints for scheduling remotelly called functions

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Date:   Thu, 15 Jun 2023 17:42:46 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Leonardo Bras <leobras@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Leonardo Bras <leobras@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Yury Norov <yury.norov@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v5 2/2] trace,smp: Add tracepoints for scheduling
 remotelly called functions

On 15/06/23 03:59, Leonardo Bras wrote:
> Add a tracepoint for when a CSD is queued to a remote CPU's
> call_single_queue. This allows finding exactly which CPU queued a given CSD
> when looking at a csd_function_{entry,exit} event, and also enables us to
> accurately measure IPI delivery time with e.g. a synthetic event:
>
>   $ echo 'hist:keys=cpu,csd.hex:ts=common_timestamp.usecs' >\
>       /sys/kernel/tracing/events/smp/csd_queue_cpu/trigger
>   $ echo 'csd_latency unsigned int dst_cpu; unsigned long csd; u64 time' >\
>       /sys/kernel/tracing/synthetic_events
>   $ echo \
>   'hist:keys=common_cpu,csd.hex:'\
>   'time=common_timestamp.usecs-$ts:'\
>   'onmatch(smp.csd_queue_cpu).trace(csd_latency,common_cpu,csd,$time)' >\
>       /sys/kernel/tracing/events/smp/csd_function_entry/trigger
>
>   $ trace-cmd record -e 'synthetic:csd_latency' hackbench
>   $ trace-cmd report
>   <...>-467   [001]    21.824263: csd_queue_cpu:        cpu=0 callsite=try_to_wake_up+0x2ea func=sched_ttwu_pending csd=0xffff8880076148b8
>   <...>-467   [001]    21.824280: ipi_send_cpu:         cpu=0 callsite=try_to_wake_up+0x2ea callback=generic_smp_call_function_single_interrupt+0x0
>   <...>-489   [000]    21.824299: csd_function_entry:   func=sched_ttwu_pending csd=0xffff8880076148b8
>   <...>-489   [000]    21.824320: csd_latency:          dst_cpu=0, csd=18446612682193848504, time=36
>

Nit: these commands now need a s/smp/csd/.

I played with those a little more and found out how to record the src CPU,
so feel free to update the changelog with this:

  $ echo 'hist:keys=cpu,csd.hex:ts=common_timestamp.usecs:src=common_cpu' >\
       /sys/kernel/tracing/events/csd/csd_queue_cpu/trigger
  $ echo 'csd_latency unsigned int src_cpu; '\
       'unsigned int dst_cpu; '\
       'unsigned long csd; u64 time' >\
       /sys/kernel/tracing/synthetic_events

  $ echo 'hist:keys=common_cpu,csd.hex:
  time=common_timestamp.usecs-$ts:
  onmatch(csd.csd_queue_cpu).trace(csd_latency,$src,common_cpu,csd,$time)' >\
       /sys/kernel/tracing/events/csd/csd_function_entry/trigger

  $ trace-cmd record -e 'synthetic:csd_latency' hackbench
  $ trace-cmd report
  <idle>-0     [001]   115.236810: csd_latency:          src_cpu=7, dst_cpu=1, csd=18446612682588476192, time=134
  <idle>-0     [000]   115.240676: csd_latency:          src_cpu=7, dst_cpu=0, csd=18446612682588214048, time=103
  <idle>-0     [009]   115.241320: csd_latency:          src_cpu=7, dst_cpu=9, csd=18446612682143963384, time=83
  <idle>-0     [007]   115.242817: csd_latency:          src_cpu=8, dst_cpu=7, csd=18446612682150759032, time=93
  <idle>-0     [005]   115.247802: csd_latency:          src_cpu=7, dst_cpu=5, csd=18446612682144441144, time=114
  <idle>-0     [005]   115.271775: csd_latency:          src_cpu=7, dst_cpu=5, csd=18446612682144441144, time=151
  <idle>-0     [000]   115.279620: csd_latency:          src_cpu=7, dst_cpu=0, csd=18446612682588214048, time=87
  <idle>-0     [000]   115.281727: csd_latency:          src_cpu=7, dst_cpu=0, csd=18446612682588214048, time=101

> Suggested-by: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: Leonardo Bras <leobras@...hat.com>

Other than that:

Tested-and-reviewed-by: Valentin Schneider <vschneid@...hat.com>