linux-kernel - RE: 5.19 printk breaks message ordering

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Message-ID: <87edzlrrva.fsf@jogness.linutronix.de>
Date:   Sun, 19 Jun 2022 10:21:37 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>,
        'Petr Mladek' <pmladek@...e.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Marco Elver <elver@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: 5.19 printk breaks message ordering

On 2022-06-17, David Laight <David.Laight@...LAB.COM> wrote:
> What priority do these kthreads run at?

120 (SCHED_OTHER, nice=0)

> I'd have thought they ought to run at a high priority?
> That should tend to give kernel messages priority over user ones.
>
> Quite how high is another matter.
> Probably a bit below the RT/FIFO:50 of threaded ISR.

As a default value, I recommend keeping to the SCHED_OTHER policy as a
default. Perhaps a nice value of -20? There are quite a few kernel
threads using that as their default:

# ps -Leo ni,command | grep ^-20 | sort
-20 [acpi_thermal_pm]
-20 [ata_sff]
-20 [blkcg_punt_bio]
-20 [cfg80211]
-20 [inet_frag_wq]
-20 [ipv6_addrconf]
-20 [kblockd]
-20 [kworker/0:0H-events_highpri]
-20 [kworker/0:1H-events_highpri]
-20 [md]
-20 [mld]
-20 [mm_percpu_wq]
-20 [netns]
-20 [nfsiod]
-20 [rcu_gp]
-20 [rcu_par_gp]
-20 [rpciod]
-20 [scsi_tmf_0]
-20 [scsi_tmf_1]
-20 [writeback]
-20 [xprtiod]

John Ogness