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Date: Wed, 10 Apr 2024 16:34:19 +0100
From: Will Deacon <will@kernel.org>
To: Yicong Yang <yangyicong@huawei.com>
Cc: mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, jonathan.cameron@huawei.com,
	prime.zeng@hisilicon.com, linuxarm@huawei.com,
	yangyicong@hisilicon.com
Subject: Re: [PATCH 1/2] perf: arm_pmu: Only show online CPUs in device's
 "cpus" attribute
Message-ID: <20240410153419.GA25171@willie-the-truck>
References: <20240410095833.63934-1-yangyicong@huawei.com>
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On Wed, Apr 10, 2024 at 05:58:32PM +0800, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@hisilicon.com>
> 
> When there're CPUs offline after system booting, perf will failed:
> [root@localhost ~]# /home/yang/perf stat -a -e armv8_pmuv3_0/cycles/
> Error:
> The sys_perf_event_open() syscall returned with 19 (No such device) for event (cpu-clock).
> /bin/dmesg | grep -i perf may provide additional information.
> 
> This is due to PMU's "cpus" is not updated and still contains offline
> CPUs and perf will try to open perf event on the offlined CPUs.
> 
> Make "cpus" attribute only shows online CPUs and introduced a new
> "supported_cpus" where users can get the range of the CPUs this
> PMU supported monitoring.
> 
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> ---
>  drivers/perf/arm_pmu.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)

Hmm. Is the complexity in the driver really worth it here? CPUs can be
onlined and offlined after the perf_event_open() syscall has been
executed, so this feels like something userspace should be aware of and
handle on a best-effort basis anyway.

Does x86 get away with this because CPU0 is never offlined?

Will