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Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries
To:     Michal Hocko <mhocko@kernel.org>,
        Dave Chinner <david@fromorbit.com>
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        Linus Torvalds <torvalds@linux-foundation.org>,
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        "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
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        Jan Kara <jack@suse.cz>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Larry Woodman <lwoodman@redhat.com>,
        "Wangkai (Kevin,C)" <wangkai86@huawei.com>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
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Date:   Wed, 18 Jul 2018 12:17:24 -0400
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On 07/16/2018 05:09 AM, Michal Hocko wrote:
> On Fri 13-07-18 10:36:14, Dave Chinner wrote:
> [...]
>> By limiting the number of negative dentries in this case, internal
>> slab fragmentation is reduced such that reclaim cost never gets out
>> of control. While it appears to "fix" the symptoms, it doesn't
>> address the underlying problem. It is a partial solution at best but
>> at worst it's another opaque knob that nobody knows how or when to
>> tune.
> Would it help to put all the negative dentries into its own slab cache?
>
>> Very few microbenchmarks expose this internal slab fragmentation
>> problem because they either don't run long enough, don't create
>> memory pressure, or don't have access patterns that mix long and
>> short term slab objects together in a way that causes slab
>> fragmentation. Run some cold cache directory traversals (git
>> status?) at the same time you are creating negative dentries so you
>> create pinned partial pages in the slab cache and see how the
>> behaviour changes....
> Agreed! Slab fragmentation is a real problem we are seeing for quite
> some time. We should try to address it rather than paper over it with
> weird knobs.

I am aware that you don't like the limit knob that control how many
negative dentries are allowed as a percentage of total system memory. I
got comments in the past about doing some kind of auto-tuning. How about
consolidating the 2 knobs that I currently have in the patchset into a
single one with 3 possible values, like:

0 - no limiting
1 - set soft limit to "a constant + 4 x max # of positive dentries" and
warn if exceeded
2 - same limit but kill excess negative dentries after use.

Does that kind of knob make more sense to you?

Cheers,
Longman