netdev - Re: [PATCH v9 net-next 15/15] net: Move per-CPU flush-lists to bpf_net_context on PREEMPT

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Date: Mon, 24 Jun 2024 09:24:40 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Jakub Kicinski <kuba@...nel.org>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Bristot de Oliveira <bristot@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Paolo Abeni <pabeni@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	Björn Töpel <bjorn@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Jonathan Lemon <jonathan.lemon@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
	Magnus Karlsson <magnus.karlsson@...el.com>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Toke Høiland-Jørgensen <toke@...hat.com>,
	Yonghong Song <yonghong.song@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH v9 net-next 15/15] net: Move per-CPU flush-lists to
 bpf_net_context on PREEMPT_RT.

On 2024-06-21 19:05:58 [-0700], Jakub Kicinski wrote:
> On Thu, 20 Jun 2024 15:22:05 +0200 Sebastian Andrzej Siewior wrote:
> >  void __cpu_map_flush(void)
> >  {
> > -	struct list_head *flush_list = this_cpu_ptr(&cpu_map_flush_list);
> > +	struct list_head *flush_list = bpf_net_ctx_get_cpu_map_flush_list();
> >  	struct xdp_bulk_queue *bq, *tmp;
> >  
> >  	list_for_each_entry_safe(bq, tmp, flush_list, flush_node) {
> 
> Most of the time we'll init the flush list just to walk its (empty)
> self. It feels really tempting to check the init flag inside
> xdp_do_flush() already. Since the various sub-flush handles may not get
> inlined - we could save ourselves not only the pointless init, but
> also the function calls. So the code would potentially be faster than
> before the changes?

Yeah. We have this lazy init now and the debug check forces the init. So
not only xdp_do_check_flushed() will initialize all three lists but also
xdp_do_flush() if only one was used by the caller.
Sure this can be optimized based on the init flag of the lists.

> Can be a follow up, obviously.
Will add it to my list.

Sebastian