netdev - Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache

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Date: Fri, 14 Jun 2024 21:33:45 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Uladzislau Rezki <urezki@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jakub Kicinski <kuba@...nel.org>,
	Julia Lawall <Julia.Lawall@...ia.fr>, linux-block@...r.kernel.org,
	kernel-janitors@...r.kernel.org, bridge@...ts.linux.dev,
	linux-trace-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	kvm@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Nicholas Piggin <npiggin@...il.com>, netdev@...r.kernel.org,
	wireguard@...ts.zx2c4.com, linux-kernel@...r.kernel.org,
	ecryptfs@...r.kernel.org, Neil Brown <neilb@...e.de>,
	Olga Kornievskaia <kolga@...app.com>, Dai Ngo <Dai.Ngo@...cle.com>,
	Tom Talpey <tom@...pey.com>, linux-nfs@...r.kernel.org,
	linux-can@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org
Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple
 kmem_cache_free callback

On Fri, Jun 14, 2024 at 02:35:33PM +0200, Uladzislau Rezki wrote:
> +	/* Should a destroy process be deferred? */
> +	if (s->flags & SLAB_DEFER_DESTROY) {
> +		list_move_tail(&s->list, &slab_caches_defer_destroy);
> +		schedule_delayed_work(&slab_caches_defer_destroy_work, HZ);
> +		goto out_unlock;
> +	}

Wouldn't it be smoother to have the actual kmem_cache_free() function
check to see if it's been marked for destruction and the refcount is
zero, rather than polling every one second? I mentioned this approach
in: https://lore.kernel.org/all/Zmo9-YGraiCj5-MI@zx2c4.com/ -

    I wonder if the right fix to this would be adding a `should_destroy`
    boolean to kmem_cache, which kmem_cache_destroy() sets to true. And
    then right after it checks `if (number_of_allocations == 0)
    actually_destroy()`, and likewise on each kmem_cache_free(), it
    could check `if (should_destroy && number_of_allocations == 0)
    actually_destroy()`. 

Jason