Re: [RFC PATCH v2 1/2] workqueue: Unbind workers before sending them to exit()

[Valentin Schneider <vschneid@redhat.com>]

On 28/07/22 07:31, Tejun Heo wrote:
> Hello,
>
> On Thu, Jul 28, 2022 at 06:24:17PM +0100, Valentin Schneider wrote:
>> > I don't understand why this would need MEM_RECLAIM when it isn't sitting in
>> > the memory reclaim path. Nothing in mm side can wait on this.
>>
>> Vaguely reading the doc I thought that'd be for anything that would
>> directly or indirectly help with reclaiming memory (not explicitly sitting
>> in some *mm reclaim* path), and I assumed freeing up a worker would count as
>> that - but that's the understanding of someone who doesn't know much about
>> all that :-)
>
> Oh, it's just needed for things that mm might end up waiting on. Here,
> there's no way for mm to know about or trigger this at all, so it doesn't
> need the flag.
>

Got it, thanks!

>> > There actually are spurious wakeups. We can't depend on there being no
>> > wakeups than ours.
>>
>> Myes, I suppose if a to-be-destroyed kworker spuriously wakes before having
>> been unbound then there's not much point in having the unbinding (harm has
>> been done and the kworker can do_exit(), though arguably we could reduce
>> the harm and still move it away), but let me see what I can do here.
>
> Yeah, it kinda sucks but is a kernel-wide thing and pretty rare, so for the
> most part, we can pretend that they don't exist but under specific
> conditions, there can be asynchronous wakeups coming from whereever, so we
> gotta be crash proof against those.
>

That's sensible, I'll look into Lai's suggestion and see if I can come up
with something not-too-horrible.