netdev - Re: [PATCH v3 bpf-next 08/13] bpf: Add a hint to allocated objects.

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Date:   Wed, 28 Jun 2023 14:42:16 +0800
From:   Hou Tao <houtao@...weicloud.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     tj@...nel.org, rcu@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com, daniel@...earbox.net,
        andrii@...nel.org, void@...ifault.com, paulmck@...nel.org
Subject: Re: [PATCH v3 bpf-next 08/13] bpf: Add a hint to allocated objects.

Hi,

On 6/28/2023 9:56 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
>
> To address OOM issue when one cpu is allocating and another cpu is freeing add
> a target bpf_mem_cache hint to allocated objects and when local cpu free_llist
> overflows free to that bpf_mem_cache. The hint addresses the OOM while
> maintaing the same performance for common case when alloc/free are done on the
> same cpu.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: Hou Tao <houtao1@...wei.com>

But have a minor comment for do_call_rcu_ttrace() below.
> ---
>  kernel/bpf/memalloc.c | 46 ++++++++++++++++++++++++++-----------------
>  1 file changed, 28 insertions(+), 18 deletions(-)
SNIP
>  static void do_call_rcu_ttrace(struct bpf_mem_cache *c)
> @@ -295,7 +289,7 @@ static void do_call_rcu_ttrace(struct bpf_mem_cache *c)
>  		return;
>  
>  	WARN_ON_ONCE(!llist_empty(&c->waiting_for_gp_ttrace));
> -	llist_for_each_safe(llnode, t, __llist_del_all(&c->free_by_rcu_ttrace))
> +	llist_for_each_safe(llnode, t, llist_del_all(&c->free_by_rcu_ttrace))
>  		/* There is no concurrent __llist_add(waiting_for_gp_ttrace) access.
>  		 * It doesn't race with llist_del_all either.
>  		 * But there could be two concurrent llist_del_all(waiting_for_gp_ttrace):
> @@ -312,16 +306,22 @@ static void do_call_rcu_ttrace(struct bpf_mem_cache *c)
>  	 * If RCU Tasks Trace grace period implies RCU grace period, free
>  	 * these elements directly, else use call_rcu() to wait for normal
>  	 * progs to finish and finally do free_one() on each element.
> +	 *
> +	 * call_rcu_tasks_trace() enqueues to a global queue, so it's ok
> +	 * that current cpu bpf_mem_cache != target bpf_mem_cache.
>  	 */
>  	call_rcu_tasks_trace(&c->rcu_ttrace, __free_rcu_tasks_trace);
"a global queue" in the comment is not accurate. call_rcu_tasks_trace()
will switch between to per-CPU queue when the global queue is too busy
and rcupdate.rcu_task_enqueue_lim in boot cmdline also can be used to
control whether or not a per-CPU queue is used.