On Tue, Jun 02, 2020 at 12:02:22AM -0400, Qian Cai wrote: > > > > On Jun 1, 2020, at 11:37 PM, Feng Tang <[email protected]> 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 <[email protected]> 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 <[email protected]> Signed-off-by: Feng Tang <[email protected]> Cc: Konstantin Khlebnikov <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Andi Kleen <[email protected]> --- 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
