linux-kernel - Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects

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Date:   Fri, 7 Aug 2020 20:28:27 +0300
From:   Pekka Enberg <penberg@...il.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Xunlei Pang <xlpang@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wen Yang <wenyang@...ux.alibaba.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Roman Gushchin <guro@...com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects

Hi Christopher,

On Fri, 7 Aug 2020, Pekka Enberg wrote:
> > I think we can just default to the counters. After all, if I
> > understood correctly, we're talking about up to 100 ms time period
> > with IRQs disabled when count_partial() is called. As this is
> > triggerable from user space, that's a performance bug whatever way you
> > look at it.

On Fri, Aug 7, 2020 at 4:02 PM Christopher Lameter <cl@...ux.com> wrote:
> Well yes under extreme conditions and this is only happening for sysfs
> counter retrieval.

You will likely get some stall even in less extreme conditions, and in
any case, the kernel should not allow user space to trigger such a
stall.

On Fri, Aug 7, 2020 at 4:02 PM Christopher Lameter <cl@...ux.com> wrote:
> There could be other solutions to this. This solution here is penalizing
> evertu hotpath slab allocation for the sake of relatively infrequently
> used counter monitoring. There the possibility of not traversing the list
> ande simply estimating the value based on the number of slab pages
> allocated on that node.

Why do you consider this to be a fast path? This is all partial list
accounting when we allocate/deallocate a slab, no? Just like
___slab_alloc() says, I assumed this to be the slow path... What am I
missing?

No objections to alternative fixes, of course, but wrapping the
counters under CONFIG_DEBUG seems like just hiding the actual issue...

- Pekka