linux-kernel - Re: [PATCH v2 2/3] serial: core: fix sysrq overhead regression

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Message-ID: <aa34e2f3-5b19-bf70-8a80-703aca5eeaae@gmail.com>
Date:   Fri, 12 Jun 2020 16:42:45 +0100
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] serial: core: fix sysrq overhead regression

On 6/12/20 4:29 PM, Johan Hovold wrote:
> On Wed, Jun 10, 2020 at 05:24:57PM +0100, Dmitry Safonov wrote:
>> Hi Johan,
>>
>> On 6/10/20 4:22 PM, Johan Hovold wrote:
>>> Commit 8e20fc391711 ("serial_core: Move sysrq functions from header
>>> file") converted the inline sysrq helpers to exported functions which
>>> are now called for every received character, interrupt and break signal
>>> also on systems without CONFIG_MAGIC_SYSRQ_SERIAL instead of being
>>> optimised away by the compiler.
>>
>> The part with ifdeffing looks good to me.
>>
>>> Inlining these helpers again also avoids the function call overhead when
>>> CONFIG_MAGIC_SYSRQ_SERIAL is enabled (e.g. when the port is not used as
>>> a console).
>>
>> But this one, coul you add measures? (it will also help to understand if
>> it's a stable material).
> 
> Interrupt processing takes 2-3% longer without the inlining with
> 8250_omap on a beagleboard for example.

I think the number justifies moving them back to header.

> 
>> If one function call actually matters here, than should
>> uart_insert_char() also go into header?
> 
> Good question, it actually was originally intended to be inlined as all
> other per-character processing. Separate discussion though.

Fair enough

> The point is that we don't want a rarely used debugging feature to incur
> unnecessary additional overhead that can easily be avoided.

Well, it wasn't related to the debug feature, rather I wanted to cleanup
the header from a bit over-grown functions those have realization
details. And couldn't foresee that a function call would matter for some
setup.

Thanks,
           Dmitry