From: Matthew Wilcox <willy@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>,
Dave Chinner <david@fromorbit.com>,
James Bottomley <James.Bottomley@HansenPartnership.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Waiman Long <longman@redhat.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Jonathan Corbet <corbet@lwn.net>,
"Luis R. Rodriguez" <mcgrof@kernel.org>,
Kees Cook <keescook@chromium.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
Jan Kara <jack@suse.cz>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Ingo Molnar <mingo@kernel.org>,
Miklos Szeredi <mszeredi@redhat.com>,
Larry Woodman <lwoodman@redhat.com>,
"Wangkai (Kevin,C)" <wangkai86@huawei.com>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries
Date: Mon, 16 Jul 2018 18:30:19 -0700 [thread overview]
Message-ID: <20180717013019.GA7934@bombadil.infradead.org> (raw)
In-Reply-To: <20180716164032.94e13f765c5f33c6022eca38@linux-foundation.org>
On Mon, Jul 16, 2018 at 04:40:32PM -0700, Andrew Morton wrote:
> On Mon, 16 Jul 2018 05:41:15 -0700 Matthew Wilcox <willy@infradead.org> wrote:
>
> > On Mon, Jul 16, 2018 at 11:09:01AM +0200, 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?
> >
> > Maybe the dcache should be more sensitive to its own needs. In __d_alloc,
> > it could check whether there are a high proportion of negative dentries
> > and start recycling some existing negative dentries.
>
> Well, yes.
>
> The proposed patchset adds all this background reclaiming. Problem is
> a) that background reclaiming sometimes can't keep up so a synchronous
> direct-reclaim was added on top and b) reclaiming dentries in the
> background will cause non-dentry-allocating tasks to suffer because of
> activity from the dentry-allocating tasks, which is inappropriate.
... and it's an awful lot of code (almost 600 lines!) to implement
something fairly conceptually simple.
> I expect a better design is something like
>
> __d_alloc()
> {
> ...
> while (too many dentries)
> call the dcache shrinker
> ...
> }
>
> and that's it. This way we have a hard upper limit and only the tasks
> which are creating dentries suffer the cost.
I think the "too many total dentries" is probably handled just fine
by the core MM. What the dentry cache needs to prevent is adding a
disproportionately large number of useless negative dentries.
So I'd rather see:
if (too_many_negative(nr_dentry, nr_dentry_neg))
reclaim_negative_dentries(16);
...
16 feels like a fairly natural batch size. I don't know what
too_many_negative() looks like. Maybe it's:
bool too_many_negative(unsigned int total, unsigned int neg)
{
if (neg < 100)
return false;
if (neg * 5 < total * 2)
return false;
return true;
}
but it could be almost arbitrarily complex. I do think it needs to
scale with the total number of dentries, not scale with memory size of
the machine or the number of CPUs or anything similar.
next prev parent reply other threads:[~2018-07-17 1:30 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-06 19:32 [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries Waiman Long
2018-07-06 19:32 ` [PATCH v6 1/7] fs/dcache: Track & report number " Waiman Long
2018-07-06 19:32 ` [PATCH v6 2/7] fs/dcache: Add sysctl parameter neg-dentry-pc as a soft limit on " Waiman Long
2018-07-06 19:32 ` [PATCH v6 3/7] fs/dcache: Enable automatic pruning of " Waiman Long
2018-07-06 19:32 ` [PATCH v6 4/7] fs/dcache: Spread negative dentry pruning across multiple CPUs Waiman Long
2018-07-06 19:32 ` [PATCH v6 5/7] fs/dcache: Add negative dentries to LRU head initially Waiman Long
2018-07-06 19:32 ` [PATCH v6 6/7] fs/dcache: Allow optional enforcement of negative dentry limit Waiman Long
2018-07-06 19:32 ` [PATCH v6 7/7] fs/dcache: Allow deconfiguration of negative dentry code to reduce kernel size Waiman Long
2018-07-06 21:54 ` Eric Biggers
2018-07-06 22:28 ` [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries Al Viro
2018-07-07 3:02 ` Waiman Long
2018-07-09 8:19 ` Michal Hocko
2018-07-09 16:01 ` Waiman Long
2018-07-10 14:27 ` Michal Hocko
2018-07-10 16:09 ` Waiman Long
2018-07-11 10:21 ` Michal Hocko
2018-07-11 15:13 ` Waiman Long
2018-07-11 17:42 ` James Bottomley
2018-07-11 19:07 ` Waiman Long
2018-07-11 19:21 ` James Bottomley
2018-07-12 15:54 ` Waiman Long
2018-07-12 16:04 ` James Bottomley
2018-07-12 16:26 ` Waiman Long
2018-07-12 17:33 ` James Bottomley
2018-07-13 15:32 ` Waiman Long
2018-07-12 16:49 ` Matthew Wilcox
2018-07-12 17:21 ` James Bottomley
2018-07-12 18:06 ` Linus Torvalds
2018-07-12 19:57 ` James Bottomley
2018-07-13 0:36 ` Dave Chinner
2018-07-13 15:46 ` James Bottomley
2018-07-13 23:17 ` Dave Chinner
2018-07-16 9:10 ` Michal Hocko
2018-07-16 14:42 ` James Bottomley
2018-07-16 9:09 ` Michal Hocko
2018-07-16 9:12 ` Michal Hocko
2018-07-16 12:41 ` Matthew Wilcox
2018-07-16 23:40 ` Andrew Morton
2018-07-17 1:30 ` Matthew Wilcox [this message]
2018-07-17 8:33 ` Michal Hocko
2018-07-19 0:33 ` Dave Chinner
2018-07-19 8:45 ` Michal Hocko
2018-07-19 9:13 ` Jan Kara
2018-07-18 18:39 ` Waiman Long
2018-07-18 16:17 ` Waiman Long
2018-07-19 8:48 ` Michal Hocko
2018-07-12 8:48 ` Michal Hocko
2018-07-12 16:12 ` Waiman Long
2018-07-12 23:16 ` Andrew Morton
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