netdev - Question on tw_timer TIMER

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Date: Wed, 6 Sep 2023 13:58:52 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	Valentin Schneider <vschneid@...hat.com>
Subject: Question on tw_timer TIMER_PINNED

Hi Eric,

I'm bothering you with a question about timewait_sock tw_timer, as I
believe you are one of the last persons touching it sometime ago. Please
feel free to redirect if I failed to git blame it correctly.

At my end, latency spikes (entering the kernel) have been reported when
running latency sensitive applications in the field (essentially a
polling userspace application that doesn't want any interruption at
all). I think I've been able to track down one of such interruptions to
the servicing of tw_timer_handler. This system isolates application CPUs
dynamically, so what I think it happens is that at some point tw_timer
is armed on a CPU, and it is PINNED to that CPU, meanwhile (before the
60s timeout) such CPU is 'isolated' and the latency sensitive app
started on it. After 60s the timer fires and interrupts the app
generating a spike.

I'm not very familiar with this part of the kernel and from staring
at code for a while I had mixed feeling about the need to keep tw_timer
as TIMER_PINNED. Could you please shed some light on it? Is it a strict
functional requirement or maybe a nice to have performance (locality I'd
guess) improvement? Could we in principle make it !PINNED (so that it
can be moved/queued away and prevent interruptions)?

Thanks a lot in advance!
Juri