Re: [PATCH v8 04/23] slab: add sheaf support for batching kfree_rcu() operations

[Vlastimil Babka <vbabka@suse.cz>]

On 9/17/25 13:32, Harry Yoo wrote:
> On Wed, Sep 17, 2025 at 11:55:10AM +0200, Vlastimil Babka wrote:
>> On 9/17/25 10:30, Harry Yoo wrote:
>> > On Wed, Sep 10, 2025 at 10:01:06AM +0200, Vlastimil Babka wrote:
>> >> +				sfw->skip = true;
>> >> +				continue;
>> >> +			}
>> >>
>> >> +			INIT_WORK(&sfw->work, flush_rcu_sheaf);
>> >> +			sfw->skip = false;
>> >> +			sfw->s = s;
>> >> +			queue_work_on(cpu, flushwq, &sfw->work);
>> >> +			flushed = true;
>> >> +		}
>> >> +
>> >> +		for_each_online_cpu(cpu) {
>> >> +			sfw = &per_cpu(slub_flush, cpu);
>> >> +			if (sfw->skip)
>> >> +				continue;
>> >> +			flush_work(&sfw->work);
>> >> +		}
>> >> +
>> >> +		mutex_unlock(&flush_lock);
>> >> +	}
>> >> +
>> >> +	mutex_unlock(&slab_mutex);
>> >> +	cpus_read_unlock();
>> >> +
>> >> +	if (flushed)
>> >> +		rcu_barrier();
>> > 
>> > I think we need to call rcu_barrier() even if flushed == false?
>> > 
>> > Maybe a kvfree_rcu()'d object was already waiting for the rcu callback to
>> > be processed before flush_all_rcu_sheaves() is called, and
>> > in flush_all_rcu_sheaves() we skipped all (cache, cpu) pairs,
>> > so flushed == false but the rcu callback isn't processed yet
>> > by the end of the function?
>> > 
>> > That sounds like a very unlikely to happen in a realistic scenario,
>> > but still possible...
>> 
>> Yes also good point, will flush unconditionally.
>> 
>> Maybe in __kfree_rcu_sheaf() I should also move the call_rcu(...) before
>> local_unlock(). So we don't end up seeing a NULL pcs->rcu_free in
>> flush_all_rcu_sheaves() because __kfree_rcu_sheaf() already set it to NULL,
>> but didn't yet do the call_rcu() as it got preempted after local_unlock().
> 
> Makes sense to me.
> 
>> But then rcu_barrier() itself probably won't mean we make sure such cpus
>> finished the local_locked section, if we didn't queue work on them. So maybe
>> we need synchronize_rcu()?
> 
> Ah, it works because preemption disabled section works as a RCU
> read-side critical section?

AFAIK yes? Or maybe not on RT where local_lock is taking a mutex? So we
should denote the RCU critical section explicitly too?

> But then are we allowed to do release the local_lock to allocate empty
> sheaves in __kfree_rcu_sheaf()?

I think so, as we do that when we found no rcu_sheaf in the first place.