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

[Liang, Kan]

On 2024-09-19 11:43 a.m., Guenter Roeck wrote:
> On Fri, Aug 02, 2024 at 08:16:37AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
>> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxxxx>
> 
> 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
> 

The patch has been posted, but haven't been merged yet.

https://lore.kernel.org/lkml/20240912145025.1574448-1-kan.liang@xxxxxxxxxxxxxxx/

Thanks,
Kan