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

From: Guenter Roeck <linux@roeck-us.net>
To: Sebastian Ene <sebastianene@google.com>,
	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>
Subject: Re: [PATCH v4 2/2] misc: Add a mechanism to detect stalls on guest vCPUs
Date: Fri, 29 Apr 2022 10:02:24 -0700	[thread overview]
Message-ID: <dbbe0db1-9a0d-4f14-f8e9-81fae4422d31@roeck-us.net> (raw)
In-Reply-To: <YmuvQvuPF/mn5S4C@google.com>

On 4/29/22 02:26, Sebastian Ene wrote:
> 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"

Agreed, that would be a much better name.

Guenter

> 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