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

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Date:   Sat, 30 Apr 2022 05:36:50 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Sebastian Ene <sebastianene@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        maz@...nel.org, will@...nel.org, qperret@...gle.com
Subject: Re: [PATCH v4 2/2] misc: Add a mechanism to detect stalls on guest
 vCPUs

On 4/29/22 23:18, Greg Kroah-Hartman wrote:
> On Fri, Apr 29, 2022 at 09:51:51AM -0700, Guenter Roeck wrote:
>> On 4/29/22 01:48, 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@...gle.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.
>>>> +	  To compile this driver as a module, choose M here: the
>>>> +	  module will be called vm-wdt.
>>>
>>> You forgot to name the module properly here based on the Makefile change
>>> you made.
>>>
>>> And again, as this is called a "watchdog", it seems crazy that it is not
>>> in drivers/watchdog/
>>>
>>
>> I disagree. It is not a watchdog driver in the traditional sense (it does
>> not use, want to use, or need to use the watchdog driver API or ABI).
>> Its functionality is similar to the functionality of kernel/watchdog.c,
>> which doesn't belong into drivers/watchdog either.
> 
> Ah, ok, that makes more sense, the user/kernel api is not the same.
> Someone should put that in the changelog next time :)
> 

Renaming it to "VCPU stall detector" or similar should fix the confusion.

Guenter