linux-kernel - Will smp_call_function_single(cpu, ...) broadcast IPI to all other cpus?

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Message-ID: <CAJNi4rMfRmWoYdsyH6ibNKN8DSCL_DO8Wa08mWbe8t7vH21Dpw@mail.gmail.com>
Date: Thu, 27 Jun 2024 17:26:05 +0800
From: richard clark <richard.xnu.clark@...il.com>
To: Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>, will@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	"Russell King (Oracle)" <linux@...linux.org.uk>, Steven Rostedt <rostedt@...dmis.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Will smp_call_function_single(cpu, ...) broadcast IPI to all other cpus?

Hi,
There's a smp_call_function_single(10, ipi_cb_func, &data_info, 1)
called by the init function of my kmod, then I ipi traces this
smp_call_function_single(...) with below cmdline:

# echo 0 > /sys/kernel/debug/tracing/tracing_on; echo >
/sys/kernel/debug/tracing/trace; echo 'reason=="Function call
interrupts"' > /sys/kernel/debug/tracing/events/ipi/filter; echo 1 >
/sys/kernel/debug/tracing/events/ipi/ipi_raise/enable; echo 1 >
/sys/kernel/debug/tracing/events/ipi/ipi_entry/enable; echo 1 >
/sys/kernel/debug/tracing/tracing_on; taskset -c 0 insmod
/kmods/ipi_test.ko; echo 0 > /sys/kernel/debug/tracing/tracing_on;
echo 0 > /sys/kernel/debug/tracing/events/ipi/ipi_raise/enable; echo 0
> /sys/kernel/debug/tracing/events/ipi/ipi_entry/enable; echo >
/sys/kernel/debug/tracing/events/ipi/filter; cat
/sys/kernel/debug/tracing/trace

The trace output as below:

# tracer: nop
#
# entries-in-buffer/entries-written: 28/28   #P:12
#
#                                _-------=> irqs-off/BH-disabled
#                               / _------=> need-resched
#                              | / _-----=> need-resched-lazy
#                              || / _----=> hardirq/softirq
#                              ||| / _---=> preempt-depth
#                              |||| / _--=> preempt-lazy-depth
#                              ||||| / _-=> migrate-disable
#                              |||||| /     delay
#           TASK-PID     CPU#  |||||||  TIMESTAMP  FUNCTION
#              | |         |   |||||||      |         |

          <idle>-0       [009] dn.h2..   555.400822: ipi_raise:
target_mask=00000000,00000002 (Function call interrupts)
          <idle>-0       [001] d..h1..   555.400832: ipi_entry:
(Function call interrupts)
          insmod-1644    [000] ....1..   555.401628: ipi_raise:
target_mask=00000000,00000ffe (Function call interrupts)
          <idle>-0       [001] d..h1..   555.401630: ipi_entry:
(Function call interrupts)
          <idle>-0       [009] d..h1..   555.401631: ipi_entry:
(Function call interrupts)
          <idle>-0       [011] d..h1..   555.401633: ipi_entry:
(Function call interrupts)
          <idle>-0       [003] d..h1..   555.401633: ipi_entry:
(Function call interrupts)
          <idle>-0       [008] d..h1..   555.401633: ipi_entry:
(Function call interrupts)
          <idle>-0       [002] d..h1..   555.401634: ipi_entry:
(Function call interrupts)
          <idle>-0       [010] d..h1..   555.401634: ipi_entry:
(Function call interrupts)
          <idle>-0       [004] d..h1..   555.401635: ipi_entry:
(Function call interrupts)
          <idle>-0       [005] d..h1..   555.401635: ipi_entry:
(Function call interrupts)
          <idle>-0       [006] d..h1..   555.401635: ipi_entry:
(Function call interrupts)
          <idle>-0       [007] d..h1..   555.401635: ipi_entry:
(Function call interrupts)
          insmod-1644    [000] ....1..   555.401643: ipi_raise:
target_mask=00000000,00000ffe (Function call interrupts)
          <idle>-0       [001] d..h1..   555.401644: ipi_entry:
(Function call interrupts)
          <idle>-0       [002] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [003] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [004] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [006] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [005] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [007] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [008] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [009] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [010] d..h1..   555.401645: ipi_entry:
(Function call interrupts)
          <idle>-0       [011] d..h1..   555.401646: ipi_entry:
(Function call interrupts)
          insmod-1644    [000] ....1..   555.401908: ipi_raise:
target_mask=00000000,00000400 (Function call interrupts)
          <idle>-0       [010] d..h1..   555.401911: ipi_entry:
(Function call interrupts)

I am curious why there're two IPI broadcasts to all the other
cpus(1-11) before the real smp_call_function_single(10) trace
event(the last two lines of the above trace output), any comments
about that?