linux-kernel - Re: [PATCH net-next v3] net: dqs: add NIC stall detector based on BQL

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Date: Tue, 13 Feb 2024 14:57:49 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Breno Leitao <leitao@...ian.org>
Cc: kuba@...nel.org, davem@...emloft.net, pabeni@...hat.com, 
	Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	weiwan@...gle.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	horms@...nel.org, Jonathan Corbet <corbet@....net>, Randy Dunlap <rdunlap@...radead.org>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Johannes Berg <johannes.berg@...el.com>, 
	Thomas Weißschuh <linux@...ssschuh.net>, 
	"open list:TRACING" <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v3] net: dqs: add NIC stall detector based on BQL

On Fri, Feb 2, 2024 at 5:55 PM Breno Leitao <leitao@...ian.org> wrote:
>
> From: Jakub Kicinski <kuba@...nel.org>
>
> softnet_data->time_squeeze is sometimes used as a proxy for
> host overload or indication of scheduling problems. In practice
> this statistic is very noisy and has hard to grasp units -
> e.g. is 10 squeezes a second to be expected, or high?
>
> Delaying network (NAPI) processing leads to drops on NIC queues
> but also RTT bloat, impacting pacing and CA decisions.
> Stalls are a little hard to detect on the Rx side, because
> there may simply have not been any packets received in given
> period of time. Packet timestamps help a little bit, but
> again we don't know if packets are stale because we're
> not keeping up or because someone (*cough* cgroups)
> disabled IRQs for a long time.

Please note that adding other sysfs entries is expensive for workloads
creating/deleting netdev and netns often.

I _think_ we should find a way for not creating
/sys/class/net/<interface>/queues/tx-{Q}/byte_queue_limits  directory
and files
for non BQL enabled devices (like loopback !)