Re: [PATCH] memcg: decouple memcg_hotplug_cpu_dead from stock_lock

[Vlastimil Babka <vbabka@suse.cz>]

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 :) 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.