Re: [PATCH v2 7/7] n_tty: Provide an informational line on VSTATUS receipt - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Arseny Maslennikov <ar@cs.msu.ru>
Cc: Jiri Slaby <jslaby@suse.com>, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Vladimir D. Seleznev" <vseleznv@altlinux.org>,
	Rob Landley <rob@landley.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Pavel Machek <pavel@ucw.cz>
Subject: Re: [PATCH v2 7/7] n_tty: Provide an informational line on VSTATUS receipt
Date: Tue, 30 Jul 2019 18:19:40 +0200	[thread overview]
Message-ID: <20190730161940.GA15798@kroah.com> (raw)
In-Reply-To: <20190625161153.29811-8-ar@cs.msu.ru>

On Tue, Jun 25, 2019 at 07:11:53PM +0300, Arseny Maslennikov wrote:
> If the three termios local flags isig, icanon, iexten are enabled
> and the local flag nokerninfo is disabled for a tty governed
> by the n_tty line discipline, then on receiving the keyboard status
> character n_tty will generate a status message and write it out to
> the tty before sending SIGINFO to the tty's foreground process group.
> 
> This kerninfo line contains information about the current system load
> as well as some properties of "the most interesting" process in the
> tty's current foreground process group, namely:
>  - its PID as seen inside its deepest PID namespace;
>    * the whole process group ought to be in a single PID namespace,
>      so this is actually deterministic
>  - its saved command name truncated to 16 bytes (task_struct::comm);
>    * at the time of writing TASK_COMM_LEN == 16
>  - its state and some related bits, procps-style;
>  - for S and D: its symbolic wait channel, if available; or a short
>    description for other process states instead;
>  - its user, system and real rusage time values;
>  - its resident set size (as well as the high watermark) in kilobytes.

Why is this really all needed as we have the SysRq handlers that report
all of this today?

> The "most interesting" process is chosen as follows:
>  - runnables over everything
>  - uninterruptibles over everything else
>  - among 2 runnables pick the biggest utime + stime
>  - any unresolved ties are decided in favour of greatest PID.

This does not feel like something that the tty core code should be doing
at all.

> While the kerninfo line is not very useful for debugging the kernel
> itself, since we have much more powerful debugging tools, it still gives
> the user behind the terminal some meaningful feedback to a VSTATUS that
> works even if no processes respond.

That's what SysRq is for.  If there's a specific set of values that we
don't currently report in that facility, why not just add the
information there?  It's much simpler and "safer" that way.

thanks,

greg k-h