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

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Date:   Mon, 1 Mar 2021 14:21:11 +0100
From:   Greg KH <greg@...ah.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     stable@...r.kernel.org, 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 Tue, Feb 23, 2021 at 10:02:14AM +0100, Petr Mladek wrote:
> 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.

Already queued up, thanks.

greg k-h