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From: Valentin Schneider <vschneid@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI
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Date: Wed, 22 Mar 2023 18:22:28 +0000
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On 22/03/23 18:22, Peter Zijlstra wrote:
> On Wed, Mar 22, 2023 at 05:01:13PM +0000, Valentin Schneider wrote:
>
>> > So I was thinking something like this:
>
>> Hm, this does get rid of the func being passed down the helpers, but this
>> means the trace events are now stateful, i.e. I need the first and last
>> events in a CSD stack to figure out which one actually caused the IPI.
>
> Isn't much of tracing stateful? I mean, why am I always writing awk
> programs to parse trace output?
>
> The one that is directly followed by
> generic_smp_call_function_single_interrupt() (horrible name that), is
> the one that tripped the IPI.
>

Right.

>> It also requires whoever is looking at the trace to be aware of which IPIs
>> are attached to a CSD, and which ones aren't. ATM that's only the resched
>> IPI, but per the cover letter there's more to come (e.g. tick_broadcast()
>> for arm64/riscv and a few others). For instance:
>> 
>>        hackbench-157   [001]    10.894320: ipi_send_cpu:         cpu=3 callsite=check_preempt_curr+0x37 callback=0x0
>
> Arguably we should be setting callback to scheduler_ipi(), except
> ofcourse, that's not an actual function...
>
> Maybe we can do "extern inline" for the actual users and provide a dummy
> function for the symbol when tracing.
>

Huh, I wasn't aware that was an option, I'll look into that. I did scribble
down a comment next to smp_send_reschedule(), but having a decodable
function name would be better!

>>        hackbench-157   [001]    10.895068: ipi_send_cpu:         cpu=3 callsite=try_to_wake_up+0x29e callback=sched_ttwu_pending+0x0
>>        hackbench-157   [001]    10.895068: ipi_send_cpu:         cpu=3 callsite=try_to_wake_up+0x29e callback=generic_smp_call_function_single_interrupt+0x0
>> 
>> That first one sent a RESCHEDULE IPI, the second one a CALL_FUNCTION one,
>> but you really have to know what you're looking at...
>
> But you have to know that anyway, you can't do tracing and not know wtf
> you're doing. Or rather, if you do, I don't give a crap and you can keep
> the pieces :-)
>
> Grepping the callback should be pretty quick resolution at to what trips
> it, no?
>
> (also, if you *realllllly* can't manage, we can always add yet another
> argument that gives a type thingy)
>

Ah, I was a bit unclear here - I don't care too much about the IPI type
being used, but rather being able to figure out on IRQ entry where that IPI
came from - thinking some more about now, I don't think logging *all* CSDs
causes an issue there, as you'd look at the earliest-not-seen-yet event
targeting this CPU anyway.

That'll be made easy once I get to having cpumask filters for ftrace, so
I can just issue something like:

  trace-cmd record -e 'ipi_send_cpu' -f "cpu == 3" -e 'ipi_send_cpumask' -f "cpus \in {3}" -T hackbench 

(it's somewhere on the todolist...)

TL;DR: I *think* I've convinced myself logging all of them isn't an issue -
I'm going to play with this on something "smarter" than just hackbench
under QEMU just to drill it in.