linux-kernel - Re: [PATCH 1/2] Add TRACE_EVENTs in pwc_isoc

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Date:   Wed, 20 Jun 2018 11:05:51 +0300
From:   "Matwey V. Kornilov" <matwey@....msu.ru>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     hverkuil@...all.nl, mchehab@...nel.org, mingo@...hat.com,
        Mike Isely <isely@...ox.com>,
        Bhumika Goyal <bhumirks@...il.com>,
        Colin King <colin.king@...onical.com>,
        linux-media@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] Add TRACE_EVENTs in pwc_isoc_handler()

2018-06-19 19:33 GMT+03:00 Steven Rostedt <rostedt@...dmis.org>:
> On Tue, 19 Jun 2018 19:23:04 +0300
> "Matwey V. Kornilov" <matwey@....msu.ru> wrote:
>
>> Hi Steven,
>>
>> Thank you for valuable comments.
>>
>> This is for measuring performance of URB completion callback inside PWC driver.
>> What do you think about moving events to __usb_hcd_giveback_urb() in
>> order to make this more generic? Like the following:
>>
>>         local_irq_save(flags);
>> // trace urb complete enter
>>         urb->complete(urb);
>> // trace urb complete exit
>>         local_irq_restore(flags);
>>
>>
>
> If that can work for you, I'm fine with that. Trace events may be
> cheap, but they do come with some cost. I'd like to have all trace
> events be as valuable as possible, and limit the "special case" ones.

What is the cost for events? I suppose one conditional check when
trace is disabled? There is already similar debugging stuff related to
usbmon in __usb_hcd_giveback_urb(), so I don't think that another
conditional check will hurt performance dramatically there. When
discussing second patch in this series I see that the issue that it is
intended to resolve may be common to other USB media drivers.


>
> -- Steve
>



-- 
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382