Re: [PATCH v4 4/4] workqueue: Unbind workers before sending them to exit()

[Valentin Schneider <vschneid@redhat.com>]

On 05/10/22 09:08, Hillf Danton wrote:
> On 4 Oct 2022 16:05:21 +0100 Valentin Schneider <vschneid@redhat.com>
>> It has been reported that isolated CPUs can suffer from interference due to
>> per-CPU kworkers waking up just to die.
>>
>> A surge of workqueue activity during initial setup of a latency-sensitive
>> application (refresh_vm_stats() being one of the culprits) can cause extra
>> per-CPU kworkers to be spawned. Then, said latency-sensitive task can be
>> running merrily on an isolated CPU only to be interrupted sometime later by
>> a kworker marked for death (cf. IDLE_WORKER_TIMEOUT, 5 minutes after last
>> kworker activity).
>>
> Is tick stopped on the isolated CPU? If tick can hit it then it can accept
> more than exiting kworker.

From what I've seen in the scenarios where that happens, yes. The
pool->idle_timer gets queued from an isolated CPU and ends up on a
housekeeping CPU (cf. get_target_base()).


> Another option is exclude isolated CPUs from
> active CPUs because workqueue has other works to do than isolating CPUs.
>

With nohz_full on the cmdline, wq_unbound_cpumask already excludes isolated
CPU, but that doesn't apply to per-CPU kworkers. Or did you mean some other
mechanism?

>> Prevent this by affining kworkers to the wq_unbound_cpumask (which doesn't
>> contain isolated CPUs, cf. HK_TYPE_WQ) before waking them up after marking
>> them with WORKER_DIE.
>>