Re: [PATCH v5 05/11] alpha/perf: implement ProfileMe sampling

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] alpha/perf: pending PCTR1 IRQ misrouted in alpha_perf_event_irq_handler()
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commit f0c418fe426066f6a450b4931c312c3fa2526930
Author: Matt Turner <mattst88@gmail.com>

alpha/perf: implement ProfileMe sampling

This commit implements ProfileMe sampling mode on EV67 and later Alpha
processors, selectable via the profileme config bit or precise_ip. It
attributes samples to the instruction address latched in PMPC and handles
counter parking and rearming across mode switches.

> diff --git a/arch/alpha/kernel/perf_event.c b/arch/alpha/kernel/perf_event.c
> index ecec78c355cd3..546e30d46c61a 100644
> --- a/arch/alpha/kernel/perf_event.c
> +++ b/arch/alpha/kernel/perf_event.c

[ ... ]

> @@ -869,6 +1118,10 @@ static void alpha_perf_event_irq_handler(unsigned long la_ptr,
>  
>  	idx = la_ptr;
>  
> +	/* A ProfileMe event owns both counters but lives at index 0. */
> +	if (cpuc->config & EV67_PCTR_MODE_PROFILEME)
> +		idx = 0;

[Severity: Medium]
What happens here in alpha_perf_event_irq_handler() when an Aggregate mode
event on PCTR1 (such as cycles, bcache-misses, or mbox-replay, which
ev67_mapping[] assigns to idx 1) overflows just as perf multiplexes it out
and schedules a ProfileMe event in?

When PALcode delivers that pending PCTR1 interrupt, la_ptr is 1 while
cpuc->config has EV67_PCTR_MODE_PROFILEME set. By unconditionally
overwriting idx = 0, alpha_perf_event_irq_handler() matches the active
ProfileMe event at cpuc->current_idx[0] == 0 and calls
alpha_perf_event_profileme():

    alpha_perf_event_irq_handler()
      alpha_perf_event_profileme()
        alpha_perf_event_update(event, hwc, 0, alpha_pmu->pmc_max_period[0]+1);

Since PCTR0 has not actually wrapped, doesn't this add a bogus period of
pmc_max_period[0] + 1 (1,048,576) to event->count, subtract 1,048,576 from
hwc->period_left, and emit a false sample using a stale PMPC value?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261103-alpha-profileme-v5-0-bbe3937fbf3a@gmail.com?part=5