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Shutemov" , Andi Kleen , dave@stgolabs.net, Jan Kara , aneesh.kumar@linux.ibm.com, Benjamin Herrenschmidt , mpe@ellerman.id.au, Paul Mackerras , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Will Deacon , Sergey Senozhatsky , sergey.senozhatsky.work@gmail.com, Andrea Arcangeli , Alexei Starovoitov , kemi.wang@intel.com, Daniel Jordan , David Rientjes , Jerome Glisse , Ganesh Mahendran , Minchan Kim , Punit Agrawal , vinayak menon , Yang Shi , zhong jiang , Haiyan Song , Balbir Singh , sj38.park@gmail.com, Mike Rapoport , LKML , linux-mm , haren@linux.vnet.ibm.com, Nick Piggin , "Paul E. McKenney" , Tim Chen , linuxppc-dev@lists.ozlabs.org, x86@kernel.org Subject: Re: [PATCH v12 00/31] Speculative page faults Message-ID: <20190423124148.GA19031@bombadil.infradead.org> References: <20190416134522.17540-1-ldufour@linux.ibm.com> <20190423104707.GK25106@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190423104707.GK25106@dhcp22.suse.cz> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 23, 2019 at 12:47:07PM +0200, Michal Hocko wrote: > On Mon 22-04-19 14:29:16, Michel Lespinasse wrote: > [...] > > I want to add a note about mmap_sem. In the past there has been > > discussions about replacing it with an interval lock, but these never > > went anywhere because, mostly, of the fact that such mechanisms were > > too expensive to use in the page fault path. I think adding the spf > > mechanism would invite us to revisit this issue - interval locks may > > be a great way to avoid blocking between unrelated mmap_sem writers > > (for example, do not delay stack creation for new threads while a > > large mmap or munmap may be going on), and probably also to handle > > mmap_sem readers that can't easily use the spf mechanism (for example, > > gup callers which make use of the returned vmas). But again that is a > > separate topic to explore which doesn't have to get resolved before > > spf goes in. > > Well, I believe we should _really_ re-evaluate the range locking sooner > rather than later. Why? Because it looks like the most straightforward > approach to the mmap_sem contention for most usecases I have heard of > (mostly a mm{unm}ap, mremap standing in the way of page faults). > On a plus side it also makes us think about the current mmap (ab)users > which should lead to an overall code improvements and maintainability. Dave Chinner recently did evaluate the range lock for solving a problem in XFS and didn't like what he saw: https://lore.kernel.org/linux-fsdevel/20190418031013.GX29573@dread.disaster.area/T/#md981b32c12a2557a2dd0f79ad41d6c8df1f6f27c I think scaling the lock needs to be tied to the actual data structure and not have a second tree on-the-side to fake-scale the locking. Anyway, we're going to have a session on this at LSFMM, right? > SPF sounds like a good idea but it is a really big and intrusive surgery > to the #PF path. And more importantly without any real world usecase > numbers which would justify this. That being said I am not opposed to > this change I just think it is a large hammer while we haven't seen > attempts to tackle problems in a simpler way. I don't think the "no real world usecase numbers" is fair. Laurent quoted: > Ebizzy: > ------- > The test is counting the number of records per second it can manage, the > higher is the best. I run it like this 'ebizzy -mTt '. To get > consistent result I repeated the test 100 times and measure the average > result. The number is the record processes per second, the higher is the best. > > BASE SPF delta > 24 CPUs x86 5492.69 9383.07 70.83% > 1024 CPUS P8 VM 8476.74 17144.38 102% and cited 30% improvement for you-know-what product from an earlier version of the patch.