linux-kernel - Re: [RFC v2 33/34] mm, slub: use migrate_disable() on PREEMPT

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Date:   Mon, 14 Jun 2021 13:16:19 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [RFC v2 33/34] mm, slub: use migrate_disable() on PREEMPT_RT

On 2021-06-14 13:07:14 [+0200], Vlastimil Babka wrote:
> > +#ifdef CONFIG_PREEMPT_RT
> > +#define slub_get_cpu_ptr(var)	get_cpu_ptr(var)
> > +#define slub_put_cpu_ptr(var)	put_cpu_ptr(var)
> 
> After Mel's report and bisect pointing to this patch, I realized I got the
> #ifdef wrong and it should be #ifnded

So if you got the ifdef wrong (and kept everything as-is) then you
tested the RT version on !RT. migrate_disable() behaves on !RT as on RT.
As per changelog you don't use migrate_disable() unconditionally because
it increases the overhead on !RT.
I haven't looked at the series and I have just this tiny question: why
did migrate_disable() crash for Mel on !RT and why do you expect that it
does not happen on PREEMPT_RT?

Sebastian