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From: "J. Avila" <elavila@google.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Andrea Parri <parri.andrea@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Paul McKenney <paulmck@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	Saravana Kannan <saravanak@google.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Issue in dmesg time with lockless ring buffer
Date: Fri, 22 Jan 2021 23:52:39 +0000	[thread overview]
Message-ID: <20210122235238.655049-1-elavila@google.com> (raw)
In-Reply-To: <20200709132344.760-5-john.ogness@linutronix.de>

Hello,

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?

Thank you,

Avila

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  parent reply	other threads:[~2021-01-22 23:53 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-09 13:23 [PATCH v5 0/4] printk: replace ringbuffer John Ogness
2020-07-09 13:23 ` [PATCH v5 1/4] crash: add VMCOREINFO macro to define offset in a struct declared by typedef John Ogness
2020-07-09 13:23 ` [PATCH v5 2/4] printk: add lockless ringbuffer John Ogness
2020-07-10  8:43   ` Sergey Senozhatsky
2020-07-10  8:47     ` Sergey Senozhatsky
2020-07-09 13:23 ` [PATCH v5 3/4] Revert "printk: lock/unlock console only for new logbuf entries" John Ogness
2020-07-09 13:23 ` [PATCH v5 4/4] printk: use the lockless ringbuffer John Ogness
2020-07-14  5:26   ` [printk] 96d5c61b1f: stress-ng.timerfd.ops_per_sec 8.3% improvement kernel test robot
2020-07-18 12:10   ` [PATCH v5 4/4] printk: use the lockless ringbuffer Marco Elver
2020-07-19  3:43     ` Sergey Senozhatsky
2020-07-20  6:43       ` Marco Elver
2020-07-20  8:22         ` Andy Shevchenko
2020-07-20  8:34         ` Sergey Senozhatsky
2020-07-20  8:41         ` Sergey Senozhatsky
2020-07-20  9:40           ` Marco Elver
2020-07-20 10:01             ` Dmitry Vyukov
2020-07-20 10:20     ` John Ogness
2020-07-20 10:32       ` Marco Elver
2020-08-11 20:47   ` Nick Desaulniers
2020-08-12 16:40     ` John Ogness
2021-01-22 23:52   ` J. Avila [this message]
2021-01-25 13:32     ` Issue in dmesg time with lockless ring buffer John Ogness
2021-01-26  0:00       ` J. Avila
2021-01-29  1:46         ` J. Avila
2020-07-10  8:28 ` [PATCH v5 0/4] printk: replace ringbuffer Petr Mladek
2020-07-13  2:23   ` Sergey Senozhatsky

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