netdev - Re: [PATCH 0/2] net: mvpp2: Survive CPU hotplug events

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Date:   Wed, 16 Feb 2022 13:29:51 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Marcin Wojtas <mw@...ihalf.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Garry <john.garry@...wei.com>, kernel-team@...roid.com
Subject: Re: [PATCH 0/2] net: mvpp2: Survive CPU hotplug events

On Wed, 16 Feb 2022 13:19:30 +0000,
Marcin Wojtas <mw@...ihalf.com> wrote:
> 
> Hi Marc,
> 
> śr., 16 lut 2022 o 10:08 Marc Zyngier <maz@...nel.org> napisał(a):
> >
> > I recently realised that playing with CPU hotplug on a system equiped
> > with a set of MVPP2 devices (Marvell 8040) was fraught with danger and
> > would result in a rapid lockup or panic.
> >
> > As it turns out, the per-CPU nature of the MVPP2 interrupts are
> > getting in the way. A good solution for this seems to rely on the
> > kernel's managed interrupt approach, where the core kernel will not
> > move interrupts around as the CPUs for down, but will simply disable
> > the corresponding interrupt.
> >
> > Converting the driver to this requires a bit of refactoring in the IRQ
> > subsystem to expose the required primitive, as well as a bit of
> > surgery in the driver itself.
> >
> > Note that although the system now survives such event, the driver
> > seems to assume that all queues are always active and doesn't inform
> > the device that a CPU has gone away. Someout who actually understand
> > this driver should have a look at it.
> >
> > Patches on top of 5.17-rc3, lightly tested on a McBin.
> >
> 
> Thank you for the patches. Can you, please, share the commands you
> used? I'd like to test it more.

Offline CPU3:
# echo 0 > /sys/devices/system/cpu/cpu3/online

Online CPU3:
# echo 1 > /sys/devices/system/cpu/cpu3/online

Put that in a loop, using different CPUs.

On my HW, turning off CPU0 leads to odd behaviours (I wouldn't be
surprised if the firmware was broken in that respect, and also the
fact that the device keeps trying to send stuff to that CPU...).

	M.

-- 
Without deviation from the norm, progress is not possible.