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Date:   Tue, 10 Oct 2023 08:26:00 -0400
From:   Phil Auld <pauld@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Chen Yu <yu.c.chen@intel.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Biju Das <biju.das.jz@bp.renesas.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        linux-kernel@vger.kernel.org, Tim Chen <tim.c.chen@intel.com>,
        Chen Yu <yu.chen.surf@gmail.com>
Subject: Re: [PATCH] sched/fair: Use printk_deferred instead of printk in
 pick_eevdf()
Message-ID: <20231010122600.GA477540@lorien.usersys.redhat.com>
References: <20231010032541.339606-1-yu.c.chen@intel.com>
 <20231010075928.GA377@noisy.programming.kicks-ass.net>
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On Tue, Oct 10, 2023 at 09:59:28AM +0200 Peter Zijlstra wrote:
> On Tue, Oct 10, 2023 at 11:25:41AM +0800, Chen Yu wrote:
> > When no eligible entity is found in pick_eevdf(), it has to pick
> > the entity with smallest vruntime. This indicates a potential issue
> > and scheduler will print this error.
> > 
> > However this printk could introduce possible circular locking issue
> > because when the code path reaches here with the rq lock held, the
> > printk could trigger further scheduling which loops back to the
> > scheduler.
> > 
> > Use printk_deferred() to defer the console write from current context
> > to the irq work in the next tick.

Chen: I was not actually suggesting you make this change, just answering your
question about printk/rq lock.  You don't need to put that line in there.

> 
> 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.


Cheers,
Phil

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