From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 16 Jul 2018 18:30:19 -0700 From: Matthew Wilcox To: Andrew Morton Cc: Michal Hocko , Dave Chinner , James Bottomley , Linus Torvalds , Waiman Long , Al Viro , Jonathan Corbet , "Luis R. Rodriguez" , Kees Cook , Linux Kernel Mailing List , linux-fsdevel , linux-mm , "open list:DOCUMENTATION" , Jan Kara , Paul McKenney , Ingo Molnar , Miklos Szeredi , Larry Woodman , "Wangkai (Kevin,C)" Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries Message-ID: <20180717013019.GA7934@bombadil.infradead.org> References: <4d49a270-23c9-529f-f544-65508b6b53cc@redhat.com> <1531411494.18255.6.camel@HansenPartnership.com> <20180712164932.GA3475@bombadil.infradead.org> <1531416080.18255.8.camel@HansenPartnership.com> <1531425435.18255.17.camel@HansenPartnership.com> <20180713003614.GW2234@dastard> <20180716090901.GG17280@dhcp22.suse.cz> <20180716124115.GA7072@bombadil.infradead.org> <20180716164032.94e13f765c5f33c6022eca38@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180716164032.94e13f765c5f33c6022eca38@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: On Mon, Jul 16, 2018 at 04:40:32PM -0700, Andrew Morton wrote: > On Mon, 16 Jul 2018 05:41:15 -0700 Matthew Wilcox 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.