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.
WARNING: multiple messages have this Message-ID (diff)
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. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
next prev parent reply other threads:[~2018-07-17 1:30 UTC|newest] Thread overview: 114+ 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 ` Waiman Long 2018-07-06 19:32 ` [PATCH v6 1/7] fs/dcache: Track & report number " Waiman Long 2018-07-06 19:32 ` 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 ` Waiman Long 2018-07-06 19:32 ` [PATCH v6 3/7] fs/dcache: Enable automatic pruning of " Waiman Long 2018-07-06 19:32 ` 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 ` 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 ` 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 ` 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 19:32 ` Waiman Long 2018-07-06 21:54 ` Eric Biggers 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 2018-07-06 22:28 ` Al Viro 2018-07-07 3:02 ` Waiman Long 2018-07-07 3:02 ` Waiman Long 2018-07-09 8:19 ` Michal Hocko 2018-07-09 8:19 ` Michal Hocko 2018-07-09 16:01 ` Waiman Long 2018-07-09 16:01 ` Waiman Long 2018-07-10 14:27 ` Michal Hocko 2018-07-10 14:27 ` Michal Hocko 2018-07-10 16:09 ` Waiman Long 2018-07-10 16:09 ` Waiman Long 2018-07-11 10:21 ` Michal Hocko 2018-07-11 10:21 ` Michal Hocko 2018-07-11 15:13 ` Waiman Long 2018-07-11 15:13 ` Waiman Long 2018-07-11 17:42 ` James Bottomley 2018-07-11 17:42 ` James Bottomley 2018-07-11 17:42 ` James Bottomley 2018-07-11 19:07 ` Waiman Long 2018-07-11 19:07 ` Waiman Long 2018-07-11 19:21 ` James Bottomley 2018-07-11 19:21 ` James Bottomley 2018-07-11 19:21 ` James Bottomley 2018-07-11 19:21 ` James Bottomley 2018-07-12 15:54 ` Waiman Long 2018-07-12 15:54 ` Waiman Long 2018-07-12 16:04 ` James Bottomley 2018-07-12 16:04 ` James Bottomley 2018-07-12 16:04 ` James Bottomley 2018-07-12 16:04 ` James Bottomley 2018-07-12 16:26 ` Waiman Long 2018-07-12 16:26 ` Waiman Long 2018-07-12 17:33 ` James Bottomley 2018-07-12 17:33 ` James Bottomley 2018-07-12 17:33 ` James Bottomley 2018-07-12 17:33 ` James Bottomley 2018-07-13 15:32 ` Waiman Long 2018-07-13 15:32 ` Waiman Long 2018-07-12 16:49 ` Matthew Wilcox 2018-07-12 16:49 ` Matthew Wilcox 2018-07-12 17:21 ` James Bottomley 2018-07-12 17:21 ` James Bottomley 2018-07-12 17:21 ` James Bottomley 2018-07-12 17:21 ` James Bottomley 2018-07-12 18:06 ` Linus Torvalds 2018-07-12 19:57 ` James Bottomley 2018-07-12 19:57 ` James Bottomley 2018-07-12 19:57 ` James Bottomley 2018-07-12 19:57 ` James Bottomley 2018-07-13 0:36 ` Dave Chinner 2018-07-13 0:36 ` Dave Chinner 2018-07-13 15:46 ` James Bottomley 2018-07-13 15:46 ` James Bottomley 2018-07-13 15:46 ` James Bottomley 2018-07-13 15:46 ` James Bottomley 2018-07-13 23:17 ` Dave Chinner 2018-07-13 23:17 ` Dave Chinner 2018-07-13 23:17 ` Dave Chinner 2018-07-13 23:17 ` Dave Chinner 2018-07-16 9:10 ` Michal Hocko 2018-07-16 9:10 ` Michal Hocko 2018-07-16 14:42 ` James Bottomley 2018-07-16 14:42 ` James Bottomley 2018-07-16 14:42 ` James Bottomley 2018-07-16 14:42 ` James Bottomley 2018-07-16 9:09 ` Michal Hocko 2018-07-16 9:09 ` Michal Hocko 2018-07-16 9:12 ` Michal Hocko 2018-07-16 9:12 ` Michal Hocko 2018-07-16 12:41 ` Matthew Wilcox 2018-07-16 12:41 ` Matthew Wilcox 2018-07-16 23:40 ` Andrew Morton 2018-07-16 23:40 ` Andrew Morton 2018-07-17 1:30 ` Matthew Wilcox [this message] 2018-07-17 1:30 ` Matthew Wilcox 2018-07-17 8:33 ` Michal Hocko 2018-07-17 8:33 ` Michal Hocko 2018-07-19 0:33 ` Dave Chinner 2018-07-19 0:33 ` Dave Chinner 2018-07-19 8:45 ` Michal Hocko 2018-07-19 8:45 ` Michal Hocko 2018-07-19 9:13 ` Jan Kara 2018-07-19 9:13 ` Jan Kara 2018-07-18 18:39 ` Waiman Long 2018-07-18 18:39 ` Waiman Long 2018-07-18 16:17 ` Waiman Long 2018-07-18 16:17 ` Waiman Long 2018-07-19 8:48 ` Michal Hocko 2018-07-19 8:48 ` Michal Hocko 2018-07-12 8:48 ` Michal Hocko 2018-07-12 8:48 ` Michal Hocko 2018-07-12 16:12 ` Waiman Long 2018-07-12 16:12 ` Waiman Long 2018-07-12 23:16 ` Andrew Morton 2018-07-12 23:16 ` Andrew Morton
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