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

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Message-ID: <20200612152921.GP19480@localhost>
Date:   Fri, 12 Jun 2020 17:29:21 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Dmitry Safonov <0x7f454c46@...il.com>
Cc:     Johan Hovold <johan@...nel.org>,
        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 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.

> 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.

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

Johan