Re: [PATCH v4 2/2] misc: Add a mechanism to detect stalls on guest vCPUs

From: Sebastian Ene <sebastianene@google.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org,
	Derek Kiernan <derek.kiernan@xilinx.com>,
	Dragan Cvetic <dragan.cvetic@xilinx.com>,
	Arnd Bergmann <arnd@arndb.de>,
	devicetree@vger.kernel.org, qperret@google.com, will@kernel.org,
	Rob Herring <robh+dt@kernel.org>,
	Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH v4 2/2] misc: Add a mechanism to detect stalls on guest vCPUs
Date: Fri, 29 Apr 2022 09:26:26 +0000	[thread overview]
Message-ID: <YmuvQvuPF/mn5S4C@google.com> (raw)
In-Reply-To: <YmunAm8ooJkNCx5D@kroah.com>

On Fri, Apr 29, 2022 at 10:51:14AM +0200, Greg Kroah-Hartman wrote:
> On Fri, Apr 29, 2022 at 08:30:33AM +0000, Sebastian Ene wrote:
> > This driver creates per-cpu hrtimers which are required to do the
> > periodic 'pet' operation. On a conventional watchdog-core driver, the
> > userspace is responsible for delivering the 'pet' events by writing to
> > the particular /dev/watchdogN node. In this case we require a strong
> > thread affinity to be able to account for lost time on a per vCPU.
> > 
> > This part of the driver is the 'frontend' which is reponsible for
> > delivering the periodic 'pet' events, configuring the virtual peripheral
> > and listening for cpu hotplug events. The other part of the driver
> > handles the peripheral emulation and this part accounts for lost time by
> > looking at the /proc/{}/task/{}/stat entries and is located here:
> > https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3548817
> > 
> > Signed-off-by: Sebastian Ene <sebastianene@google.com>
> > ---
> >  drivers/misc/Kconfig       |  12 +++
> >  drivers/misc/Makefile      |   1 +
> >  drivers/misc/vm-watchdog.c | 206 +++++++++++++++++++++++++++++++++++++
> >  3 files changed, 219 insertions(+)
> >  create mode 100644 drivers/misc/vm-watchdog.c
> > 
> > diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
> > index 2b9572a6d114..26c3a99e269c 100644
> > --- a/drivers/misc/Kconfig
> > +++ b/drivers/misc/Kconfig
> > @@ -493,6 +493,18 @@ config OPEN_DICE
> >  
> >  	  If unsure, say N.
> >  
> > +config VM_WATCHDOG
> > +	tristate "Virtual Machine Watchdog"
> > +	select LOCKUP_DETECTOR
> > +	help
> > +	  Detect CPU locks on the virtual machine. This driver relies on the
> > +	  hrtimers which are CPU-binded to do the 'pet' operation. When a vCPU
> > +	  has to do a 'pet', it exits the guest through MMIO write and the
> > +	  backend driver takes into account the lost ticks for this particular
> > +	  CPU.

Hi,

> 
> There's nothing to keep this tied to a virtual machine at all, right?
> You are just relying on some iomem address to be updated, so it should
> be a "generic_iomem_watchdog" driver as there's nothing specific to vms
> at all from what I can tell.
> 
> thanks,
> 
> greg k-h

That's right although I might think of using the term "generic lockup detector"
instead of watchdog. The only reason why I would keep "virtual machine"
word in, is that there is no actual hardware for this.

Thanks,
Seb