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

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Message-ID: <xhsmhmssehp6t.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Mon, 05 Feb 2024 17:05:14 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Mark Rutland <mark.rutland@....com>, Steven Rostedt <rostedt@...dmis.org>
Cc: richard clark <richard.xnu.clark@...il.com>, nico@...xnic.net,
 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 05/02/24 14:39, Mark Rutland wrote:
> [adding Valentin]
>

Thanks!

> On Mon, Feb 05, 2024 at 08:06:09AM -0500, Steven Rostedt wrote:
>> On Mon, 5 Feb 2024 10:28:57 +0000
>> Mark Rutland <mark.rutland@....com> wrote:
>>
>> > > 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'
>>
>> 11 == 0xb = b1011
>>
>> So the above would only be true for CPUs 0,1 and 3 ;-)
>
> Sorry, I misunderstood the scalar logic and thought that we treated:
>
>       '$mask $OP $scalar', e.g. 'target_cpus & 11'
>
> .. as a special case meaning a cpumask with that scalar bit set, i.e.
>
>       '$mask $OP CPUS{$scalar}', e.g. 'target_cpus & CPUS{11}'
>
> .. but evidently I was wrong.
>
>> I think you meant: 'target_cpus & 0x800'
>>
>> I tried "1 << 11' but it appears to not allow shifts. I wonder if we should add that?
>
> Hmm... shouldn't we make 'CPUS{11}' work for that?
>

It /should/ already be the case, the user input with the curly braces is
parsed as a cpulist. So CPUS{11} really does mean CPU11, not a hex value to
be interpreted as a cpumask.

However...

> From a quick test (below), that doesn't seem to work, though I think it
> probably should?
> Have I completely misunderstood how this is supposed to work, or is that a bug?
>

The CPUS{} thingie only works with an event field that is either declared as a
cpumask (__cpumask) or a scalar. That's not the case for ipi_raise, the
target_cpus event field is saved as a "raw" bitmask.

There /should/ have been a warning about the event filter though, but I
think it's not happening because I'm allowing more than just FILTER_CPUMASK
in parse_pred() to make it work for scalars. I'll go poke around some more.

Generally for this sort of IPI investigation I'd recommend using the newer
trace_ipi_send_cpu() and trace_ipi_send_cpumask() (for which CPUS{}
filtering works).
If it's only the function call interrupts you're interesting in, have a
look at trace_csd_queue_cpu().

> Mark.