linux-kernel - Re: Issue in dmesg time with lockless ring buffer

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Message-ID: <CAGFReeNEf7a4W4hEx5bD+v0qsdszrgPfh1Sa-B-2HeaAY5natg@mail.gmail.com>
Date:   Mon, 25 Jan 2021 16:00:05 -0800
From:   "J. Avila" <elavila@...gle.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...nel.org>,
        Saravana Kannan <saravanak@...gle.com>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Issue in dmesg time with lockless ring buffer

Hello,

This dmesg uses /dev/kmsg; we've verified that we don't see this long
dmesg time when reading from syslog (via dmesg -S).

We've also tried testing this with logging daemons disabled as well as
within initrd - both result in similar behavior.

If it's relevant, this was done on a toybox shell.

Thanks,

Avila

On Mon, Jan 25, 2021 at 5:32 AM John Ogness <john.ogness@...utronix.de> wrote:
>
> On 2021-01-22, "J. Avila" <elavila@...gle.com> wrote:
> > When doing some internal testing on a 5.10.4 kernel, we found that the
> > time taken for dmesg seemed to increase from the order of milliseconds
> > to the order of seconds when the dmesg size approached the ~1.2MB
> > limit. After doing some digging, we found that by reverting all of the
> > patches in printk/ up to and including
> > 896fbe20b4e2333fb55cc9b9b783ebcc49eee7c7 ("use the lockless
> > ringbuffer"), we were able to once more see normal dmesg times.
> >
> > This kernel had no meaningful diffs in the printk/ dir when compared
> > to Linus' tree. This behavior was consistently reproducible using the
> > following steps:
> >
> > 1) In one shell, run "time dmesg > /dev/null"
> > 2) In another, constantly write to /dev/kmsg
> >
> > Within ~5 minutes, we saw that dmesg times increased to 1 second, only
> > increasing further from there. Is this a known issue?
>
> The last couple days I have tried to reproduce this issue with no
> success.
>
> Is your dmesg using /dev/kmsg or syslog() to read the buffer?
>
> Are there any syslog daemons or systemd running? Perhaps you can run
> your test within an initrd to see if this effect is still visible?
>
> John Ogness