linux-kernel - Re: [PATCH] sched/fair: Use printk_deferred instead of printk in pick

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Date:   Tue, 10 Oct 2023 16:12:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     Chen Yu <yu.c.chen@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Biju Das <biju.das.jz@...renesas.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-kernel@...r.kernel.org, Tim Chen <tim.c.chen@...el.com>,
        Chen Yu <yu.chen.surf@...il.com>
Subject: Re: [PATCH] sched/fair: Use printk_deferred instead of printk in
 pick_eevdf()

On Tue, Oct 10, 2023 at 08:26:00AM -0400, Phil Auld wrote:

> > No.. I detest printk_deferred with a passion. This is effectively a WARN
> > and we don't do silly buggers for them either.
> >
> 
> Sure, printk_deferred is not ideal, but is getting this message in the right
> order worth locking up people's machines?  Not sure you get the message at
> all when that happens.  I have to dig the code location out of the crash
> dump to find which sched warning fired and took down the (usually virtual)
> machine.

Same thing with WARN(), we don't have a silly bugger version of that
either. Just use a sane printk() / console or whatever.

Virt stuff has perfectly functional serial consoles that works just fine
and don't lock up the machine -- mostly. Use my early_printk hacks if
you want something reliable:

  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=debug/experimental

Boot with: "earlyprintk=serial,ttyS0,115200 force_early_printk". And all
will be well.

The fact that crashdump is more reliable than printk is a *BIG* problem
and the only solution is fixing printk() (people are sorta working on
that). We should not try and work around this problem.

I fundamentally despise the delayed stuff, I've had countless insteances
where delaying output means you have no output because the machine is
dead.