linux-kernel - Re: armv8pmu: Pending overflow interrupt is discarded when perf event is disabled

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Date:   Thu, 30 Nov 2023 12:23:13 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     "Ashley, William" <wash@...zon.com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        will@...nel.org
Subject: Re: armv8pmu: Pending overflow interrupt is discarded when perf
 event is disabled

On Thu, Nov 30, 2023 at 11:48:54AM +0000, Mark Rutland wrote:
> On Wed, Nov 29, 2023 at 04:35:01PM +0000, Mark Rutland wrote:
> > Does RR set any of the perf_event_attr::exclude_* bits? If not, does RR
> > intentionally count events that occur within the kernel?
> 
> Looking at the test, I see it sets perf_event_attr::exclude_kernel to 1, but
> doesn't set perf_event_attr::exclude_host or perf_event_attr::exclude_hv. I
> think the poorly-defined exclude_* bits are part of the problem here.
> 
> Using your test as-is on my ThunderX2, I can reproduce the period being longer
> than expected by concurrently running the following in a shell:
> 
>   while true; do
>     for C in $(seq 0 63); do
>       taskset -c -p $C ${TEST_PID_HERE};
>     done; 
>   done > /dev/null
> 
> ... resulting in:
> 
> | [mark@...vadlaks:~]% ./counter-overflow                                     
> | Pid 20060 running with period 10000 tolerance 1000
> | Signal #1: last: 0, cur: 10292, max diff: 0
> | Signal #415330: delta of 19999 is outside 10000 +/- 1000
> | Signal #415330: last: 4153290187, cur: 4153310186, max diff: 10292
> | Signal #489879: delta of 19998 is outside 10000 +/- 1000
> | Signal #511842: delta of 20058 is outside 10000 +/- 1000
> | Signal #511842: last: 5118430130, cur: 5118450188, max diff: 19999
> 
> However, if I modify the test to also set perf_event_attr::exclude_host=1, I do
> not see any lost overflows after many minutes. On VHE hosts (like the
> ThunderX2), the host kernel gets counted when perf_event_attr::exclude_host=0,
> even if perf_event_attr::exclude_kernel=1 (which I agree is surprising), so I
> think what's happening is the counters are counting in the host kernel, which
> isn't what RR actually wants regardless.

> I'll continue to look at what we can do kernel-side, but I reckon it's worth
> having RR try the other exclude bits regardless, if that's possible? It would
> be interesting to know whether that helps you under a hypervisor.

Sorry, the above is wrong, and I do not recommend RR goes and changes its
exclude_* settings.

I had misread the logic in armv8pmu_set_event_filter(), but looking again
that's saner than I thought it was, and what was actually happening in my
testing is that exclude_host also filtered host EL0 (userspace), and so the
test received *no* overflow signals.

I'll get back to looking at how we can better capture the overflow when
removing an event.

Mark.