[RFC PATCH 0/5] Generic IPI sending tracepoint

Valentin Schneider
2022-10-07T15:42:51Z

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(-)

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2.31.1