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

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Date: Thu, 27 Jun 2024 11:38:58 +0100
From: Marc Zyngier <maz@...nel.org>
To: richard clark <richard.xnu.clark@...il.com>
Cc: Mark Rutland <mark.rutland@....com>,
	will@...nel.org,
	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: Re: Will smp_call_function_single(cpu, ...) broadcast IPI to all other cpus?

On Thu, 27 Jun 2024 10:26:05 +0100,
richard clark <richard.xnu.clark@...il.com> wrote:
> 
> 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?

You may want to enable stack trace recording and find out for yourself
where these ipi_raise() calls are coming from.

	M.

-- 
Without deviation from the norm, progress is not possible.