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@redhat.com>
Signed-off-by: Leonardo Bras <redacted>

Other than that:

Tested-and-reviewed-by: Valentin Schneider [off-list ref]