linux-kernel - Re: [PATCH 1/7] perf: Generic hotplug support for a PMU with a scope

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Message-ID: <dacb08c6-dc38-4292-93cd-12cdfba3972f@roeck-us.net>
Date: Thu, 19 Sep 2024 08:43:16 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: kan.liang@...ux.intel.com
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
	namhyung@...nel.org, irogers@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] perf: Generic hotplug support for a PMU with a scope

On Fri, Aug 02, 2024 at 08:16:37AM -0700, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> The perf subsystem assumes that the counters of a PMU are per-CPU. So
> the user space tool reads a counter from each CPU in the system wide
> mode. However, many PMUs don't have a per-CPU counter. The counter is
> effective for a scope, e.g., a die or a socket. To address this, a
> cpumask is exposed by the kernel driver to restrict to one CPU to stand
> for a specific scope. In case the given CPU is removed,
> the hotplug support has to be implemented for each such driver.
> 
> The codes to support the cpumask and hotplug are very similar.
> - Expose a cpumask into sysfs
> - Pickup another CPU in the same scope if the given CPU is removed.
> - Invoke the perf_pmu_migrate_context() to migrate to a new CPU.
> - In event init, always set the CPU in the cpumask to event->cpu
> 
> Similar duplicated codes are implemented for each such PMU driver. It
> would be good to introduce a generic infrastructure to avoid such
> duplication.
> 
> 5 popular scopes are implemented here, core, die, cluster, pkg, and
> the system-wide. The scope can be set when a PMU is registered. If so, a
> "cpumask" is automatically exposed for the PMU.
> 
> The "cpumask" is from the perf_online_<scope>_mask, which is to track
> the active CPU for each scope. They are set when the first CPU of the
> scope is online via the generic perf hotplug support. When a
> corresponding CPU is removed, the perf_online_<scope>_mask is updated
> accordingly and the PMU will be moved to a new CPU from the same scope
> if possible.
> 
> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>

This patch results in build failures in the mainline kernel.

Building arm:allmodconfig ... failed
Building arm64:allmodconfig ... failed
Building loongarch:allmodconfig ... failed
Building mips:allmodconfig ... failed
Building parisc:allmodconfig ... failed

[ and more ]

--------------
Error log:
kernel/events/core.c: In function 'perf_event_setup_cpumask':
kernel/events/core.c:14012:13: error: the comparison will always evaluate as 'true' for the address of 'thread_sibling' will never be NULL