From: John Villalovos <jvillalo@redhat.com> Date: Tue, 29 Sep 2009 10:26:47 -0400 Subject: [mm] fix spinlock performance issue on large systems Message-id: 20090929142647.GA7613@linuxjohn.usersys.redhat.com O-Subject: [RHEL 5.5 BZ526078] Fix spinlock performance issue on large systems Bugzilla: 526078 RH-Acked-by: Larry Woodman <lwoodman@redhat.com> RH-Acked-by: Dean Nelson <dnelson@redhat.com> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=526078 This fixes a performance issue that is being seen on Intel and AMD systems with large numbers of CPUs. It is a two line backport of an upstream commit. Basically this will cause the system to not grab a spinlock as often. Brew build: http://brewweb.devel.redhat.com/brew/taskinfo?taskID=2007033 I will also be doing an RHTS test pass but it was requested that I submit this now. This is a backport of: Upstream commit 252c5f94d944487e9f50ece7942b0fbf659c5c31 Upstream Author: Lee Schermerhorn <Lee.Schermerhorn@hp.com> Upstream Date: Mon Sep 21 17:03:40 2009 -0700 mmap: avoid unnecessary anon_vma lock acquisition in vma_adjust() We noticed very erratic behavior [throughput] with the AIM7 shared workload running on recent distro [SLES11] and mainline kernels on an 8-socket, 32-core, 256GB x86_64 platform. On the SLES11 kernel [2.6.27.19+] with Barcelona processors, as we increased the load [10s of thousands of tasks], the throughput would vary between two "plateaus"--one at ~65K jobs per minute and one at ~130K jpm. The simple patch below causes the results to smooth out at the ~130k plateau. But wait, there's more: We do not see this behavior on smaller platforms--e.g., 4 socket/8 core. This could be the result of the larger number of cpus on the larger platform--a scalability issue--or it could be the result of the larger number of interconnect "hops" between some nodes in this platform and how the tasks for a given load end up distributed over the nodes' cpus and memories--a stochastic NUMA effect. The variability in the results are less pronounced [on the same platform] with Shanghai processors and with mainline kernels. With 31-rc6 on Shanghai processors and 288 file systems on 288 fibre attached storage volumes, the curves [jpm vs load] are both quite flat with the patched kernel consistently producing ~3.9% better throughput [~80K jpm vs ~77K jpm] than the unpatched kernel. Profiling indicated that the "slow" runs were incurring high[er] contention on an anon_vma lock in vma_adjust(), apparently called from the sbrk() system call. The patch: A comment in mm/mmap.c:vma_adjust() suggests that we don't really need the anon_vma lock when we're only adjusting the end of a vma, as is the case for brk(). The comment questions whether it's worth while to optimize for this case. Apparently, on the newer, larger x86_64 platforms, with interesting NUMA topologies, it is worth while--especially considering that the patch [if correct!] is quite simple. We can detect this condition--no overlap with next vma--by noting a NULL "importer". The anon_vma pointer will also be NULL in this case, so simply avoid loading vma->anon_vma to avoid the lock. However, we DO need to take the anon_vma lock when we're inserting a vma ['insert' non-NULL] even when we have no overlap [NULL "importer"], so we need to check for 'insert', as well. And Hugh points out that we should also take it when adjusting vm_start (so that rmap.c can rely upon vma_address() while it holds the anon_vma lock). akpm: Zhang Yanmin reprts a 150% throughput improvement with aim7, so it might be -stable material even though thiss isn't a regression: "this issue is not clear on dual socket Nehalem machine (2*4*2 cpu), but is severe on large machine (4*8*2 cpu)" diff --git a/mm/mmap.c b/mm/mmap.c index 04dfc7e..d80fa9d 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -559,9 +559,9 @@ again: remove_next = 1 + (end > next->vm_end); /* * When changing only vma->vm_end, we don't really need - * anon_vma lock: but is that case worth optimizing out? + * anon_vma lock. */ - if (vma->anon_vma) + if (vma->anon_vma && (insert || importer || start != vma->vm_start)) anon_vma = vma->anon_vma; if (anon_vma) { spin_lock(&anon_vma->lock);