linux-kernel - Re: Question about the ipi_raise filter usage and output

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Message-ID: <ZcC4aWWKFU1NgrZM@FVFF77S0Q05N>
Date: Mon, 5 Feb 2024 10:28:57 +0000
From: Mark Rutland <mark.rutland@....com>
To: richard clark <richard.xnu.clark@...il.com>
Cc: nico@...xnic.net, Steven Rostedt <rostedt@...dmis.org>,
	mhiramat@...nel.org, linux-arm-kernel@...ts.infradead.org,
	linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Question about the ipi_raise filter usage and output

On Mon, Feb 05, 2024 at 05:57:29PM +0800, richard clark wrote:
> Hi guys,
> 
> With the ipi_raise event enabled and filtered with:
> echo 'reason == "Function call interrupts"' > filter, then the 'cat
> trace' output below messages:
> ...
> insmod-3355    [010] ....1.. 24479.230381: ipi_raise:
> target_mask=00000000,00000bff (Function call interrupts)
> ...
> The above output is triggered by my kernel module where it will smp
> cross call a remote function from cpu#10 to cpu#11, for the
> 'target_mask' value, what does the '00000000,00000bff' mean?

That's a cpumask bitmap: 0xbff is 0b1011_1111_1111, which is:

     ,- CPU 10
     |
    1011_1111_1111
    | '__________'
    |      |
    |      `- CPUs 9 to 0
    |
    `- CPU 11

Note that bitmap has CPUs 0-9 and CPU 11 set, but CPU 10 is not set.

I suspect your kernel module has generated the bitmap incorrectly; it looks
like you have a mask for CPUs 0-11 minus a mask for CPU 10?

For CPUs 10 and 11, that should be 0xc00 / 0b1100_0000_0000.

>  ~~~~~~~~~~~~~~
> 
> Another question is for the filter, I'd like to catch the IPI only
> happening on cpu#11 *AND* a remote function call, so how to write the
> 'target_cpus' in the filter expression?
> 
> I try to write below:
> echo 'target_cpus == 11 && reason == "Function call interrupts"' >
> events/ipi/ipi_raise/filter

The '=' checks if the target_cpus bitmap *only* contains CPU 11. If the cpumask
contains other CPUs, the filter will skip the call.

I believe you can use '&' to check whether a cpumask contains a CPU, e.g.

	'target_cpus & 11'

Thanks,
Mark.