On Wed 03-06-20 17:48:04, Feng Tang wrote: > On Tue, Jun 02, 2020 at 12:02:22AM -0400, Qian Cai wrote: > > > > > > > On Jun 1, 2020, at 11:37 PM, Feng Tang <feng.t...@intel.com> wrote: > > > > > > I re-run the same benchmark with v5.7 and 5.7+remove_warning kernels, > > > the overall performance change is trivial (which is expected) > > > > > > 1330147 +0.1% 1331032 will-it-scale.72.processes > > > > > > But the perf stats of "self" shows big change for __vm_enough_memory() > > > > > > 0.27 -0.3 0.00 pp.self.__vm_enough_memory > > > > > > I post the full compare result in the end. > > > > I don’t really see what that means exactly, but I suppose the warning is > > there for so long and no one seems notice much trouble (or benefit) because > > of it, so I think you will probably need to come up with a proper > > justification to explain why it is a trouble now, and how your patchset > > suddenly start to trigger the warning as well as why it is no better way > > but to suffer this debuggability regression (probably tiny but still). > > Thanks for the suggestion, and I updated the commit log. > > > >From 1633da8228bd3d0dcbbd8df982977ad4594962a1 Mon Sep 17 00:00:00 2001 > From: Feng Tang <feng.t...@intel.com> > Date: Fri, 29 May 2020 08:48:48 +0800 > Subject: [PATCH] mm/util.c: remove the VM_WARN_ONCE for vm_committed_as > underflow check > > This check was added by 82f71ae4a2b8 ("mm: catch memory commitment underflow") > in 2014 to have a safety check for issues which have been fixed. > And there has been few report caught by it, as described in its > commit log: > > : This shouldn't happen any more - the previous two patches fixed > : the committed_as underflow issues. > > But it was really found by Qian Cai when he used the LTP memory > stress suite to test a RFC patchset, which tries to improve scalability > of per-cpu counter 'vm_committed_as', by chosing a bigger 'batch' number > for loose overcommit policies (OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS), > while keeping current number for OVERCOMMIT_NEVER. > > With that patchset, when system firstly uses a loose policy, the > 'vm_committed_as' count could be a big negative value, as its big 'batch' > number allows a big deviation, then when the policy is changed to > OVERCOMMIT_NEVER, the 'batch' will be decreased to a much smaller value, > thus hits this WARN check. > > To mitigate this, one proposed solution is to queue work on all online > CPUs to do a local sync for 'vm_committed_as' when changing policy to > OVERCOMMIT_NEVER, plus some global syncing to garante the case won't > be hit. > > But this solution is costy and slow, given this check hasn't shown real > trouble or benefit, simply drop it from one hot path of MM. And perf > stats does show some tiny saving for removing it. > > Reported-by: Qian Cai <c...@lca.pw> > Signed-off-by: Feng Tang <feng.t...@intel.com> > Cc: Konstantin Khlebnikov <koc...@gmail.com> > Cc: Michal Hocko <mho...@suse.com> > Cc: Andi Kleen <andi.kl...@intel.com>
Acked-by: Michal Hocko <mho...@suse.com> > --- > mm/util.c | 8 -------- > 1 file changed, 8 deletions(-) > > diff --git a/mm/util.c b/mm/util.c > index 9b3be03..c63c8e4 100644 > --- a/mm/util.c > +++ b/mm/util.c > @@ -814,14 +814,6 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, > int cap_sys_admin) > { > long allowed; > > - /* > - * A transient decrease in the value is unlikely, so no need > - * READ_ONCE() for vm_committed_as.count. > - */ > - VM_WARN_ONCE(data_race(percpu_counter_read(&vm_committed_as) < > - -(s64)vm_committed_as_batch * num_online_cpus()), > - "memory commitment underflow"); > - > vm_acct_memory(pages); > > /* > -- > 2.7.4 > -- Michal Hocko SUSE Labs