linux-kernel - Re: [RFC v2 00/34] SLUB: reduce irq disabled scope and make it RT compatible

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Date:   Sun, 04 Jul 2021 07:37:53 +0200
From:   Mike Galbraith <efault@....de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 00/34] SLUB: reduce irq disabled scope and make it RT
 compatible

On Sat, 2021-07-03 at 17:47 +0200, Mike Galbraith wrote:
> On Sat, 2021-07-03 at 09:24 +0200, Mike Galbraith wrote:
> > It also appears to be saying that there's something RT specific to
> > stare at in addition to the list_lock business.
>
> The what is ___slab_alloc() consuming 3.9% CPU in tip-rt-slub whereas
> it consumes < 1% in both tip-rt (sans slub patches) and tip-slub.
>
> The why remains to ponder.

Ignoring odd distribution in the profile (red herring whackable via
prefetch games), and removing perf/debug options from the picture, the
remaining RT specific cost is just that of forced slow path.  For
hackbench in my little box, it adds up to right at 3%.

	-Mike