Re: [PATCH] memcg: decouple memcg_hotplug_cpu_dead from stock_lock

[Shakeel Butt]

On Fri, Apr 11, 2025 at 2:06 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 4/11/25 19:54, Shakeel Butt wrote:
> > (my migadu/linux.dev stopped working and I have to send through gmail,
> > sorry for any formatting issue)
> >
> > I don't see how local_irq_save() will break anything. We are working on
> > a stock of a dead remote cpu. We actually don't even need to disable irq
> > or need local cpu's local_lock. It is actually the calls to
> > __mod_memcg_lruvec_state() and __mod_memcg_state() in
> > __drain_obj_stock() which need irq-disabled on non-RT kernels and for
> > RT-kernels they already have preempt_disable_nested().
> >
> > Disabling irq even on RT seems excessive but this is not a performance
> > critical code, so I don't see an issue unless there is
> > local_lock_irqsave() alternative which does not disables irqs on RT
> > kernels.
>
> local_lock_irqsave() does not disable irqs on RT kernels :)

Sorry, I wanted to say local_irq_save() here instead of local_lock_irqsave().

> so keeping
> local_lock as is would do the irq disable on !RT and be more RT-friendly on
> RT. It's just wrong from the logical scope of the lock to perform it on a
> different cpu than the stock we modify. If one day we have some runtime
> checks for that, they would complain.

Basically I don't want to use stock_lock here. Maybe I should explore
adding a new local_lock for __mod_memcg_lruvec_state and
__mod_memcg_state.