Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Date:   Thu, 30 Nov 2023 12:23:13 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     "Ashley, William" <wash@amazon.com>
Cc:     "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-perf-users@vger.kernel.org" <linux-perf-users@vger.kernel.org>,
        will@kernel.org
Subject: Re: armv8pmu: Pending overflow interrupt is discarded when perf
 event is disabled
Message-ID: <ZWh-sbrHJu2b-BU6@FVFF77S0Q05N>
References: <950001BD-490C-4BAC-8EEA-CDB9F7C4ADFC@amazon.com>
 <EBAF38AB-2BE5-425F-8A52-DDCB0B390309@amazon.com>
 <ZWdoNWps4izj5WJy@FVFF77S0Q05N.cambridge.arm.com>
 <ZWh2pnR_Z5-CKuZb@FVFF77S0Q05N>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZWh2pnR_Z5-CKuZb@FVFF77S0Q05N>
Precedence: bulk

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@gravadlaks:~]% ./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.