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

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Message-ID: <19008afb-bfbb-35e2-3bd5-e7fd1b7355cc@gmail.com>
Date:   Wed, 10 Jun 2020 17:24:57 +0100
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     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

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).

If one function call actually matters here, than should
uart_insert_char() also go into header?

I see quite common pattern in drivers:
: if (!uart_handle_sysrq_char(&up->port, ch))
: 	uart_insert_char(&up->port, byte, 0, ch, TTY_NORMAL);

Don't misunderstand me, but I would prefer keeping headers cleaner
without realization details with the exception if function calls
actually hurts the performance.

Probably, a comment like
/*
 * Keeping these functions in the header improves performance by X% on
 * YYY platform by letting the compiler inline them.
 */
would also help.

Thanks for working on this,
          Dmitry