From: Andrew Morton <akpm@linux-foundation.org>
To: Matthew Wilcox <willy@infradead.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 16:40:32 -0700 [thread overview]
Message-ID: <20180716164032.94e13f765c5f33c6022eca38@linux-foundation.org> (raw)
In-Reply-To: <20180716124115.GA7072@bombadil.infradead.org>
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.
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.
Regarding the slab page fragmentation issue: I'm wondering if the whole
idea of balancing the slab scan rates against the page scan rates isn't
really working out. Maybe shrink_slab() should be sitting there
hammering the caches until they have freed up a particular number of
pages. Quite a big change, conceptually and implementationally.
Aside: about a billion years ago we were having issues with processes
getting stuck in direct reclaim because other processes were coming in
and stealing away the pages which the direct-reclaimer had just freed.
One possible solution to that was to make direct-reclaiming tasks
release the freed pages into a list on the task_struct. So those pages
were invisible to other allocating tasks and were available to the
direct-reclaimer when it returned from the reclaim effort. I forget
what happened to this.
It's quite a small code change and would provide a mechanism for
implementing the hammer-cache-until-youve-freed-enough design above.
Aside 2: if we *do* do something like the above __d_alloc() pseudo code
then perhaps it could be cast in terms of pages, not dentries. ie,
__d_alloc()
{
...
while (too many pages in dentry_cache)
call the dcache shrinker
...
}
and, apart from the external name thing (grr), that should address
these fragmentation issues, no? I assume it's easy to ask slab how
many pages are presently in use for a particular cache.
next prev parent reply other threads:[~2018-07-16 23:40 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 [this message]
2018-07-17 1:30 ` Matthew Wilcox
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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