linux-kernel - Re: [PATCH] usb: typec: tcpm: Move to high priority workqueue for processing events

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Message-ID: <040b4cde-91d4-7776-60b5-8f55b6e75831@roeck-us.net>
Date:   Mon, 13 Jul 2020 23:57:59 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     reg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Badhri Jagan Sridharan <badhri@...gle.com>
Cc:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: typec: tcpm: Move to high priority workqueue for
 processing events

On 7/13/20 11:05 PM, reg Kroah-Hartman wrote:
> On Mon, Jul 13, 2020 at 01:43:00PM -0700, Badhri Jagan Sridharan wrote:
>> "tReceiverResponse 15 ms Section 6.6.2
>> The receiver of a Message requiring a response Shall respond
>> within tReceiverResponse in order to ensure that the
>> sender’s SenderResponseTimer does not expire."
>>
>> When the cpu complex is busy running other lower priority
>> work items, TCPM's work queue sometimes does not get scheduled
>> on time to meet the above requirement from the spec.
>> Elevating the TCPM's work queue to higher priority allows
>> TCPM to meet tReceiverResponse in a busy system.
>>
>> Signed-off-by: Badhri Jagan Sridharan <badhri@...gle.com>
>> ---
>>  drivers/usb/typec/tcpm/tcpm.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
>> index 82b19ebd7838e0..088b6f1fa1ff89 100644
>> --- a/drivers/usb/typec/tcpm/tcpm.c
>> +++ b/drivers/usb/typec/tcpm/tcpm.c
>> @@ -4747,7 +4747,7 @@ struct tcpm_port *tcpm_register_port(struct device *dev, struct tcpc_dev *tcpc)
>>  	mutex_init(&port->lock);
>>  	mutex_init(&port->swap_lock);
>>  
>> -	port->wq = create_singlethread_workqueue(dev_name(dev));
>> +	port->wq = alloc_ordered_workqueue("%s", WQ_HIGHPRI, dev_name(dev));
> 
> How are you "guaranteeing" that this is really going to change anything
> on a highly loaded machine?
> 
> Yes, it might make things better, but if you have a hard deadline like
> this, you need to do things a bit differently to always ensure that you
> meet it.  I do not think this change is that fix, do you?
> 

Good point. The worker in drivers/watchdog/watchdog_dev.c might be
useful as a starting point. There may be better examples - this is
just one I know of which had a similar problem. See commits
38a1222ae4f3 and 1ff688209e2e.

Guenter