linux-kernel - Re: [PATCH v2] printk: avoid prb_first_valid

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Date:   Tue, 23 Feb 2021 10:02:14 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     stable@...r.kernel.org
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "J. Avila" <elavila@...gle.com>,
        kernel test robot <oliver.sang@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] printk: avoid prb_first_valid_seq() where possible

On Thu 2021-02-11 18:37:52, John Ogness wrote:
> If message sizes average larger than expected (more than 32
> characters), the data_ring will wrap before the desc_ring. Once the
> data_ring wraps, it will start invalidating descriptors. These
> invalid descriptors hang around until they are eventually recycled
> when the desc_ring wraps. Readers do not care about invalid
> descriptors, but they still need to iterate past them. If the
> average message size is much larger than 32 characters, then there
> will be many invalid descriptors preceding the valid descriptors.
> 
> The function prb_first_valid_seq() always begins at the oldest
> descriptor and searches for the first valid descriptor. This can
> be rather expensive for the above scenario. And, in fact, because
> of its heavy usage in /dev/kmsg, there have been reports of long
> delays and even RCU stalls.
> 
> For code that does not need to search from the oldest record,
> replace prb_first_valid_seq() usage with prb_read_valid_*()
> functions, which provide a start sequence number to search from.
> 
> Fixes: 896fbe20b4e2333fb55 ("printk: use the lockless ringbuffer")
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Reported-by: J. Avila <elavila@...gle.com>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

Could you please push this fix into the stable releases
based on 5.10 and 5.11, please?

The patch fixes a visible performance regression. It has
landed in the mainline as the commit
13791c80b0cdf54d92fc542 ("printk: avoid prb_first_valid_seq() where
possible").

It should apply cleanly.

Best Regards,
Petr