Re: [RFC PATCH 6/6] mm: Drain LRUs upon resume to userspace on nohz_full CPUs

[Vlastimil Babka]

On 6/25/24 4:20 PM, Michal Hocko wrote:
> On Tue 25-06-24 15:52:44, Frederic Weisbecker wrote:
>> LRUs can be drained through several ways. One of them may add disturbances
>> to isolated workloads while queuing a work at any time to any target,
>> whether running in nohz_full mode or not.
>> 
>> Prevent from that on isolated tasks with draining LRUs upon resuming to
>> userspace using the isolated task work framework.
>> 
>> It's worth noting that this is inherently racy against
>> lru_add_drain_all() remotely queueing the per CPU drain work and
>> therefore it prevents from the undesired disturbance only
>> *most of the time*.
> 
> Can we simply not schedule flushing on remote CPUs and leave that to the
> "return to the userspace" path?
> 
> I do not think we rely on LRU cache flushing for correctness purposes anywhere.

I guess drain via lru_cache_disable() should be honored, but also rare.

> Please also CC linux MM ML once the core infrastructure is agreed on.