[v4,0/7] Generic IPI sending tracepoint

  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).

This is instead introducing a new tracepoint which exports the relevant context
(callsite, and requested callback for when the callsite isn't helpful), and is
usable by all architectures as it sits in generic code. 

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, which is why the new
tracepoint also has a @callback argument.

Patches
=======

o Patches 1-5 spread out the tracepoint across relevant sites.
  Patch 5 ends up sprinkling lots of #include <trace/events/ipi.h> which I'm not
  the biggest fan of, but is the least horrible solution I've been able to come
  up with so far.
  
o Patch 7 is trying to be smart about tracing the callback associated with the
  IPI.

This results in having IPI trace events for:

o smp_call_function*()
o smp_send_reschedule()
o irq_work_queue*()
o standalone uses of __smp_call_single_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 apart from IPI_TIMER (cf. tick_broadcast()), those IPIs are both unfrequent
and accompanied with identifiable interference (stopper or cpuhp threads being
scheduled). I've added a point in my todolist to handle those in a later series
for the sake of completeness.

Links
=====

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

Revisions
=========

v3 -> v4
++++++++

o Rebased against 6.2-rc4
  Re-ran my coccinelle scripts for the treewide change; only loongarch needed
  changes
o Dropped cpumask trace event field patch (now in 6.2-rc1)
o Applied RB and Ack tags
  Ingo, I wasn't sure if you meant to Ack the whole series or just the patch you
  replied to, so since I didn't want to unlawfully forge any tag I only added
  the one.
o Did a small pass on comments and changelogs

v2 -> v3
++++++++

o Dropped the generic export of smp_send_reschedule(), turned it into a macro
  and a bunch of imports
o Dropped the send_call_function_single_ipi() macro madness, split it into sched
  and smp bits using some of Peter's suggestions

v1 -> v2
++++++++

o Ditched single-CPU tracepoint
o Changed tracepoint signature to include callback
o Changed tracepoint callsite field to void *; the parameter is still UL to save
  up on casts due to using _RET_IP_.
o Fixed linking failures due to not exporting smp_send_reschedule()

git range-diff v3 vs v4
=========================

1:  6820c1880d97d < -:  ------------- tracing: Add __cpumask to denote a trace event field that is a cpumask_t
2:  ef594e168af0d ! 1:  8f1309849c859 trace: Add trace_ipi_send_cpumask()
    @@ Commit message
         its "reason" argument being an uninformative string (on arm64 all you get
         is "Function call interrupts" for SMP calls).
     
    -    Add a variant of it that exports a target CPU, a callsite and a callback.
    +    Add a variant of it that exports a target cpumask, a callsite and a callback.
     
         Signed-off-by: Valentin Schneider <vschneid@redhat.com>
         Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
3:  17ccdc591aec9 ! 2:  3e0f952a905ce sched, smp: Trace IPIs sent via send_call_function_single_ipi()
    @@ Commit message
     
         Signed-off-by: Valentin Schneider <vschneid@redhat.com>
         Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
    +    Acked-by: Ingo Molnar <mingo@kernel.org>
     
      ## arch/arm/kernel/smp.c ##
     @@
4:  9253a0b5abf59 = 3:  6345aa71c64be smp: Trace IPIs sent via arch_send_call_function_ipi_mask()
5:  a5d13519caa7e = 4:  d2ebdfedcb5f1 irq_work: Trace self-IPIs sent via arch_irq_work_raise()
6:  d3e59fe921eae ! 5:  0167b33c7be0c treewide: Trace IPIs sent via smp_send_reschedule()
    @@ Commit message
         Signed-off-by: Valentin Schneider <vschneid@redhat.com>
         [csky bits]
         Acked-by: Guo Ren <guoren@kernel.org>
    +    [riscv bits]
    +    Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
     
      ## arch/alpha/kernel/smp.c ##
     @@ arch/alpha/kernel/smp.c: handle_ipi(struct pt_regs *regs)
    @@ arch/ia64/kernel/smp.c: kdump_smp_send_init(void)
      /*
       * Called with preemption disabled.
     
    - ## arch/loongarch/include/asm/smp.h ##
    -@@ arch/loongarch/include/asm/smp.h: extern void show_ipi_list(struct seq_file *p, int prec);
    + ## arch/loongarch/kernel/smp.c ##
    +@@ arch/loongarch/kernel/smp.c: void loongson_send_ipi_mask(const struct cpumask *mask, unsigned int action)
       * it goes straight through and wastes no time serializing
       * anything. Worst case is that we lose a reschedule ...
       */
    --static inline void smp_send_reschedule(int cpu)
    -+static inline void arch_smp_send_reschedule(int cpu)
    +-void smp_send_reschedule(int cpu)
    ++void arch_smp_send_reschedule(int cpu)
      {
      	loongson_send_ipi_single(cpu, SMP_RESCHEDULE);
      }
    +-EXPORT_SYMBOL_GPL(smp_send_reschedule);
    ++EXPORT_SYMBOL_GPL(arch_smp_send_reschedule);
    + 
    + irqreturn_t loongson_ipi_interrupt(int irq, void *dev)
    + {
     
      ## arch/mips/include/asm/smp.h ##
     @@ arch/mips/include/asm/smp.h: extern void calculate_cpu_foreign_map(void);
    @@ arch/powerpc/platforms/powernv/subcore.c
      
     
      ## arch/riscv/kernel/smp.c ##
    -@@ arch/riscv/kernel/smp.c: void smp_send_stop(void)
    - 			   cpumask_pr_args(cpu_online_mask));
    +@@ arch/riscv/kernel/smp.c: bool smp_crash_stop_failed(void)
      }
    + #endif
      
     -void smp_send_reschedule(int cpu)
     +void arch_smp_send_reschedule(int cpu)
    @@ include/linux/smp.h: extern void smp_send_stop(void);
     -extern void smp_send_reschedule(int cpu);
     -
     +extern void arch_smp_send_reschedule(int cpu);
    ++/*
    ++ * scheduler_ipi() is inline so can't be passed as callback reason, but the
    ++ * callsite IP should be sufficient for root-causing IPIs sent from here.
    ++ */
     +#define smp_send_reschedule(cpu) ({				  \
    -+	/* XXX scheduler_ipi is inline :/ */                      \
     +	trace_ipi_send_cpumask(cpumask_of(cpu), _RET_IP_, NULL);  \
     +	arch_smp_send_reschedule(cpu);				  \
     +})
    @@ include/linux/smp.h: extern void smp_send_stop(void);
     
      ## virt/kvm/kvm_main.c ##
     @@
    - #include "kvm_mm.h"
    - #include "vfio.h"
    + 
    + #include <linux/kvm_dirty_ring.h>
      
     +#include <trace/events/ipi.h>
    - #define CREATE_TRACE_POINTS
    - #include <trace/events/kvm.h>
    ++
    + /* Worst case buffer size needed for holding an integer. */
    + #define ITOA_MAX_LEN 12
      
7:  a4027c288ce0c = 6:  fa0de903cf99a smp: reword smp call IPI comment
8:  124946403688e = 7:  7ecb1c29c7f1f sched, smp: Trace smp callback causing an IPI

Valentin Schneider (7):
  trace: Add trace_ipi_send_cpumask()
  sched, smp: Trace IPIs sent via send_call_function_single_ipi()
  smp: Trace IPIs sent via arch_send_call_function_ipi_mask()
  irq_work: Trace self-IPIs sent via arch_irq_work_raise()
  treewide: Trace IPIs sent via smp_send_reschedule()
  smp: reword smp call IPI comment
  sched, smp: Trace smp callback causing an IPI

 arch/alpha/kernel/smp.c                  |  2 +-
 arch/arc/kernel/smp.c                    |  2 +-
 arch/arm/kernel/smp.c                    |  5 +-
 arch/arm/mach-actions/platsmp.c          |  2 +
 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/kernel/smp.c              |  4 +-
 arch/mips/include/asm/smp.h              |  2 +-
 arch/mips/kernel/rtlx-cmp.c              |  2 +
 arch/openrisc/kernel/smp.c               |  2 +-
 arch/parisc/kernel/smp.c                 |  4 +-
 arch/powerpc/kernel/smp.c                |  6 +-
 arch/powerpc/kvm/book3s_hv.c             |  3 +
 arch/powerpc/platforms/powernv/subcore.c |  2 +
 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/x86/kvm/svm/svm.c                   |  4 ++
 arch/x86/kvm/x86.c                       |  2 +
 arch/xtensa/kernel/smp.c                 |  2 +-
 include/linux/smp.h                      | 11 +++-
 include/trace/events/ipi.h               | 22 +++++++
 kernel/irq_work.c                        | 14 ++++-
 kernel/sched/core.c                      | 19 ++++--
 kernel/sched/smp.h                       |  2 +-
 kernel/smp.c                             | 78 +++++++++++++++++++-----
 virt/kvm/kvm_main.c                      |  2 +
 32 files changed, 164 insertions(+), 53 deletions(-)

--
2.31.1
  

Hey folks,

On 19/01/23 14:36, Valentin Schneider wrote:
> Patches
> =======
>
> o Patches 1-5 spread out the tracepoint across relevant sites.
>   Patch 5 ends up sprinkling lots of #include <trace/events/ipi.h> which I'm not
>   the biggest fan of, but is the least horrible solution I've been able to come
>   up with so far.
>
> o Patch 7 is trying to be smart about tracing the callback associated with the
>   IPI.
>
> This results in having IPI trace events for:
>
> o smp_call_function*()
> o smp_send_reschedule()
> o irq_work_queue*()
> o standalone uses of __smp_call_single_queue()
>

This still rebases cleanly on top of the latest tip/sched/core, any
objections to parking it there?