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

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Sep 17, 2025 at 03:21:31PM +0200, Vlastimil Babka wrote:
> On 9/17/25 15:07, Harry Yoo wrote:
> > On Wed, Sep 17, 2025 at 02:05:49PM +0200, Vlastimil Babka wrote:
> >> 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.
> > 
> > Wait, I'm confused.
> > 
> > I think the caller of kvfree_rcu_barrier() should make sure that it's invoked
> > only after a kvfree_rcu(X, rhs) call has returned, if the caller expects
> > the object X to be freed before kvfree_rcu_barrier() returns?
> 
> Hmm, the caller of kvfree_rcu(X, rhs) might have returned without filling up
> the rcu_sheaf fully and thus without submitting it to call_rcu(), then
> migrated to another cpu. Then it calls kvfree_rcu_barrier() while another
> unrelated kvfree_rcu(X, rhs) call on the previous cpu is for the same
> kmem_cache (kvfree_rcu_barrier() is not only for cache destruction), fills
> up the rcu_sheaf fully and is about to call_rcu() on it. And since that
> sheaf also contains the object X, we should make sure that is flushed.

I was going to say "but we queue and wait for the flushing work to
complete, so the sheaf containing object X should be flushed?"

But nah, that's true only if we see pcs->rcu_free != NULL in
flush_all_rcu_sheaves().

You are right...

Hmm, maybe it's simpler to fix this by never skipping queueing the work
even when pcs->rcu_sheaf == NULL?

> > IOW if flush_all_rcu_sheaves() is called while __kfree_rcu_sheaf(s, X) was
> > running on another CPU, we don't have to guarantee that
> > flush_all_rcu_sheaves() returns after the object X is freed?
> > 
> >> >> 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()?
> > 
> > So... we don't need a synchronize_rcu() then?
> > 
> > Or my brain started malfunctioning again :D

-- 
Cheers,
Harry / Hyeonggon