netdev - Re: [PATCH v4 bpf-next 12/14] bpf: Introduce bpf_mem_free_rcu() similar to kfree

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Date: Thu, 6 Jul 2023 19:10:51 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Hou Tao <houtao@...weicloud.com>
Cc: Tejun Heo <tj@...nel.org>, rcu@...r.kernel.org, 
	Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Kernel Team <kernel-team@...com>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, David Vernet <void@...ifault.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v4 bpf-next 12/14] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

On Thu, Jul 6, 2023 at 6:45 PM Hou Tao <houtao@...weicloud.com> wrote:
>
>
>
> On 7/6/2023 11:34 AM, Alexei Starovoitov wrote:
> > From: Alexei Starovoitov <ast@...nel.org>
> >
> > Introduce bpf_mem_[cache_]free_rcu() similar to kfree_rcu().
> > Unlike bpf_mem_[cache_]free() that links objects for immediate reuse into
> > per-cpu free list the _rcu() flavor waits for RCU grace period and then moves
> > objects into free_by_rcu_ttrace list where they are waiting for RCU
> > task trace grace period to be freed into slab.
> >
> > The life cycle of objects:
> > alloc: dequeue free_llist
> > free: enqeueu free_llist
> > free_rcu: enqueue free_by_rcu -> waiting_for_gp
> > free_llist above high watermark -> free_by_rcu_ttrace
> > after RCU GP waiting_for_gp -> free_by_rcu_ttrace
> > free_by_rcu_ttrace -> waiting_for_gp_ttrace -> slab
> >
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>
> Acked-by: Hou Tao <houtao1@...wei.com>

Thank you very much for code reviews and feedback.

btw I still believe that ABA is a non issue and prefer to keep the code as-is,
but for the sake of experiment I've converted it to spin_lock
(see attached patch which I think uglifies the code)
and performance across bench htab-mem and map_perf_test
seems to be about the same.
Which was a bit surprising to me.
Could you please benchmark it on your system?

Download attachment "0001-bpf-Address-hypothetical-ABA-issue-with-llist_del_fi.patch" of type "application/octet-stream" (6679 bytes)