linux-kernel - Re: [PATCH] hrtimer: Update softirq_expires_next correctly after __hrtimer_get_next

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Date:   Tue, 13 Apr 2021 09:55:08 -0700
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, mikael.beckius@...driver.com,
        tglx@...utronix.de, Lorenzo Colitti <lorenzo@...gle.com>,
        Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH] hrtimer: Update softirq_expires_next correctly after __hrtimer_get_next_event()

This patch (or at least the version of it that showed up in 5.10.24
LTS when combined with Android Common Kernel and some arm phone
platform drivers) causes a roughly 60% regression (from ~5.3-6 gbps
down to ~2.2gbps) on running pktgen when egressing via ncm gadget on a
SuperSpeed+ 10gbps USB3 connection.

The regression is not there in 5.10.23, and is present in 5.10.24 and
5.10.26.  Reverting just this one patch is confirmed to restore
performance (both on 5.10.24 and 5.10.26).

We don't know the cause, as we know nothing about hrtimers... but we
lightly suspect that the ncm->task_timer in f_ncm.c is perhaps not
firing as often as it should...

Unfortunately I have no idea how to replicate this on commonly
available hardware (or with a pure stable or 5.11/5.12 Linus tree)
since it requires a gadget capable usb3.1 10gbps controller (which I
only have access to in combination with a 5.10-based arm+dwc3 soc).

(the regression is visible with just usb3.0, but it's smaller due to
usb3.0 topping out at just under 4gbps, though it still drops to
2.2gbps -- and this still doesn't help since usb3 gadget capable
controllers are nearly as rare)

- Maciej & Lorenzo