netdev - Re: [PATCH 6/8] net: tcp: tsq: Convert from tasklet to BH workqueue

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Message-ID: <CANn89iKDsJPY=QQrTHK0Jw=s=A_G_GzcOA0WsqXaytWAVV3R4Q@mail.gmail.com>
Date: Fri, 16 Feb 2024 09:23:00 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Tejun Heo <tj@...nel.org>
Cc: torvalds@...ux-foundation.org, mpatocka@...hat.com, 
	linux-kernel@...r.kernel.org, dm-devel@...ts.linux.dev, msnitzer@...hat.com, 
	ignat@...udflare.com, damien.lemoal@....com, bob.liu@...cle.com, 
	houtao1@...wei.com, peterz@...radead.org, mingo@...nel.org, 
	netdev@...r.kernel.org, allen.lkml@...il.com, kernel-team@...a.com, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, David Wei <davidhwei@...a.com>
Subject: Re: [PATCH 6/8] net: tcp: tsq: Convert from tasklet to BH workqueue

On Fri, Feb 16, 2024 at 6:31 AM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Mon, Jan 29, 2024 at 11:11:53PM -1000, Tejun Heo wrote:
> > The only generic interface to execute asynchronously in the BH context is
> > tasklet; however, it's marked deprecated and has some design flaws. To
> > replace tasklets, BH workqueue support was recently added. A BH workqueue
> > behaves similarly to regular workqueues except that the queued work items
> > are executed in the BH context.
> >
> > This patch converts TCP Small Queues implementation from tasklet to BH
> > workqueue.
> >
> > Semantically, this is an equivalent conversion and there shouldn't be any
> > user-visible behavior changes. While workqueue's queueing and execution
> > paths are a bit heavier than tasklet's, unless the work item is being queued
> > every packet, the difference hopefully shouldn't matter.
> >
> > My experience with the networking stack is very limited and this patch
> > definitely needs attention from someone who actually understands networking.
>
> On Jakub's recommendation, I asked David Wei to perform production memcache
> benchmark on the backported conversion patch. There was no discernible
> difference before and after. Given that this is likely as hot as it gets for
> the path on a real workloal, the conversions shouldn't hopefully be
> noticeable in terms of performance impact.
>
> Jakub, I'd really appreciate if you could ack. David, would it be okay if I
> add your Tested-by?

I presume memcache benchmark is using small RPC ?

TSQ matters for high BDP, and is very time sensitive.

Things like slow TX completions (firing from napi poll, BH context)
can hurt TSQ.

If we add on top of these slow TX completions, an additional work
queue overhead, I really am not sure...

I would recommend tests with pfifo_fast qdisc (not FQ which has a
special override for TSQ limits)

Eventually we could add in TCP a measure of the time lost because of
TSQ, regardless of the kick implementation (tasklet or workqueue).
Measuring the delay between when a tcp socket got tcp_wfree approval
to deliver more packets, and time it finally delivered these packets
could be implemented with a bpftrace program.