linux-kernel - Re: [PATCH v2] sched/all: Change all BUG_ON() instances in the scheduler to WARN_ON

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Message-ID: <20220822084019.wcezy5dyy5slg4k7@suse.de>
Date:   Mon, 22 Aug 2022 09:40:19 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2] sched/all: Change all BUG_ON() instances in the
 scheduler to WARN_ON_ONCE()

On Sun, Aug 21, 2022 at 01:28:58PM +0200, Ingo Molnar wrote:
> 
> * Mel Gorman <mgorman@...e.de> wrote:
> 
> > For the rest, I didn't see obvious recovery paths that would allow the 
> > system to run predictably. Any of them firing will have unpredictable 
> > consequences (e.g. move_queued_task firing would be fun if it was a 
> > per-cpu kthread). Depending on which warning triggers, the remaining life 
> > of the system may be very short but maybe long enough to be logged even 
> > if system locks up shortly afterwards.
> 
> Correct. I'd prefer to keep all these warnings 'simple' - i.e. no attempted 
> recovery & control flow, unless we ever expect these to trigger.
> 

I generally hope we never expect a warning to trigger until of course it
does :P

> I.e. instead of adding a 'goto' I'd prefer if we removed most of the ones 
> you highlighted. But wanted to keep this first patch simple.
> 

The only one I would push back on is claming cpupri_find_fitness() so it
does not access outside the bounds of an array after a warning triggers.

For the rest, I've no strong objection to recovering given the nature of
the BUGs you are converting. If any of them *did* trigger, it would be
more important to fix the issue than recover the warning. So other than
the out-of-bounds array access which I'd like to see clamped just in case;

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs