linux-kernel - Re: [PATCH v8 04/23] slab: add sheaf support for batching kfree

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Message-ID: <aMqyn6nenR8V_p1y@hyeyoo>
Date: Wed, 17 Sep 2025 22:07:43 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Uladzislau Rezki <urezki@...il.com>,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        maple-tree@...ts.infradead.org,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v8 04/23] slab: add sheaf support for batching
 kfree_rcu() operations

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?

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