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

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Date:   Wed, 11 Oct 2023 18:27:28 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Phil Auld <pauld@...hat.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 2023-10-10 at 16:12:44 +0200, Peter Zijlstra wrote:
> 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.

I see, thanks for this information.

thanks,
Chenyu