[RFC PATCH 0/5] Generic IPI sending tracepoint

[Valentin Schneider]

Background
==========

Detecting IPI *reception* is relatively easy, e.g. using
trace_irq_handler_{entry,exit} or even just function-trace
flush_smp_call_function_queue() for SMP calls.  

Figuring out their *origin*, is trickier as there is no generic tracepoint tied
to e.g. smp_call_function():

o AFAIA x86 has no tracepoint tied to sending IPIs, only receiving them
  (cf. trace_call_function{_single}_entry()).
o arm/arm64 do have trace_ipi_raise(), which gives us the target cpus but also a
  mostly useless string (smp_calls will all be "Function call interrupts").
o Other architectures don't seem to have any IPI-sending related tracepoint.  

I believe one reason those tracepoints used by arm/arm64 ended up as they were
is because these archs used to handle IPIs differently from regular interrupts
(the IRQ driver would directly invoke an IPI-handling routine), which meant they 
never showed up in trace_irq_handler_{entry, exit}. The trace_ipi_{entry,exit}
tracepoints gave a way to trace IPI reception but those have become redundant as
of: 

      56afcd3dbd19 ("ARM: Allow IPIs to be handled as normal interrupts")
      d3afc7f12987 ("arm64: Allow IPIs to be handled as normal interrupts")

which gave IPIs a "proper" handler function used through
generic_handle_domain_irq(), which makes them show up via
trace_irq_handler_{entry, exit}.

Changing stuff up
=================

Per the above, it would make sense to reshuffle trace_ipi_raise() and move it
into generic code. This also came up during Daniel's talk on Osnoise at the CPU
isolation MC of LPC 2022 [1]. 

Now, to be useful, such a tracepoint needs to export:
o targeted CPU(s)
o calling context

The only way to get the calling context with trace_ipi_raise() is to trigger a
stack dump, e.g. $(trace-cmd -e ipi* -T echo 42).

As for the targeted CPUs, the existing tracepoint does export them, albeit in
cpumask form, which is quite inconvenient from a tooling perspective. For
instance, as far as I'm aware, it's not possible to do event filtering on a
cpumask via trace-cmd.

Because of the above points, this is introducing a new tracepoint.

Patches
=======

This results in having trace events for:

o smp_call_function*()
o smp_send_reschedule()
o irq_work_queue*()

This is incomplete, just looking at arm64 there's more IPI types that aren't covered:

  IPI_CPU_STOP,
  IPI_CPU_CRASH_STOP,
  IPI_TIMER,
  IPI_WAKEUP,

... But it feels like a good starting point.

Another thing worth mentioning is that depending on the callsite, the _RET_IP_
fed to the tracepoint is not always useful - generic_exec_single() doesn't tell
you much about the actual callback being sent via IPI, so there might be value
in exploding the single tracepoint into at least one variant for smp_calls.

Links
=====

[1]: https://youtu.be/5gT57y4OzBM?t=14234

Valentin Schneider (5):
  trace: Add trace_ipi_send_{cpu, cpumask}
  sched, smp: Trace send_call_function_single_ipi()
  smp: Add a multi-CPU variant to send_call_function_single_ipi()
  irq_work: Trace calls to arch_irq_work_raise()
  treewide: Rename and trace arch-definitions of smp_send_reschedule()

 arch/alpha/kernel/smp.c          |  2 +-
 arch/arc/kernel/smp.c            |  2 +-
 arch/arm/kernel/smp.c            |  5 +----
 arch/arm64/kernel/smp.c          |  3 +--
 arch/csky/kernel/smp.c           |  2 +-
 arch/hexagon/kernel/smp.c        |  2 +-
 arch/ia64/kernel/smp.c           |  4 ++--
 arch/loongarch/include/asm/smp.h |  2 +-
 arch/mips/include/asm/smp.h      |  2 +-
 arch/openrisc/kernel/smp.c       |  2 +-
 arch/parisc/kernel/smp.c         |  4 ++--
 arch/powerpc/kernel/smp.c        |  4 ++--
 arch/riscv/kernel/smp.c          |  4 ++--
 arch/s390/kernel/smp.c           |  2 +-
 arch/sh/kernel/smp.c             |  2 +-
 arch/sparc/kernel/smp_32.c       |  2 +-
 arch/sparc/kernel/smp_64.c       |  2 +-
 arch/x86/include/asm/smp.h       |  2 +-
 arch/xtensa/kernel/smp.c         |  2 +-
 include/linux/smp.h              |  1 +
 include/trace/events/ipi.h       | 27 +++++++++++++++++++++++++++
 kernel/irq_work.c                | 12 +++++++++++-
 kernel/sched/core.c              |  7 +++++--
 kernel/smp.c                     | 18 +++++++++++++++++-
 24 files changed, 84 insertions(+), 31 deletions(-)

--
2.31.1