Re: [lkp-robot] [mm] 7674270022: will-it-scale.per_process_ops -19.3% regression

Minchan Kim
2017-08-08T08:08:26Z

On Mon, Aug 07, 2017 at 10:51:00PM -0700, Nadav Amit wrote:
> Nadav Amit <nadav.amit@gmail.com> wrote:
>
> > Minchan Kim <minchan@kernel.org> wrote:
> >
> >> Hi,
> >>
> >> On Tue, Aug 08, 2017 at 09:19:23AM +0800, kernel test robot wrote:
> >>> Greeting,
> >>>
> >>> FYI, we noticed a -19.3% regression of will-it-scale.per_process_ops due to commit:
> >>>
> >>>
> >>> commit: 76742700225cad9df49f05399381ac3f1ec3dc60 ("mm: fix MADV_[FREE|DONTNEED] TLB flush miss problem")
> >>> url: https://github.com/0day-ci/linux/commits/Nadav-Amit/mm-migrate-prevent-racy-access-to-tlb_flush_pending/20170802-205715
> >>>
> >>>
> >>> in testcase: will-it-scale
> >>> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory
> >>> with following parameters:
> >>>
> >>> nr_task: 16
> >>> mode: process
> >>> test: brk1
> >>> cpufreq_governor: performance
> >>>
> >>> test-description: Will It Scale takes a testcase and runs it from 1 through to n parallel copies to see if the testcase will scale. It builds both a process and threads based test in order to see any differences between the two.
> >>> test-url: https://github.com/antonblanchard/will-it-scale
> >>
> >> Thanks for the report.
> >> Could you explain what kinds of workload you are testing?
> >>
> >> Does it calls frequently madvise(MADV_DONTNEED) in parallel on multiple
> >> threads?
> >
> > According to the description it is "testcase:brk increase/decrease of one
> > page”. According to the mode it spawns multiple processes, not threads.
> >
> > Since a single page is unmapped each time, and the iTLB-loads increase
> > dramatically, I would suspect that for some reason a full TLB flush is
> > caused during do_munmap().
> >
> > If I find some free time, I’ll try to profile the workload - but feel free
> > to beat me to it.
>
> The root-cause appears to be that tlb_finish_mmu() does not call
> dec_tlb_flush_pending() - as it should. Any chance you can take care of it?

Oops, but with second looking, it seems it's not my fault. ;-)
https://marc.info/?l=linux-mm&m=150156699114088&w=2

Anyway, thanks for the pointing out.
xiaolong.ye, could you retest with this fix?

>From 83012114c9cd9304f0d55d899bb4b9329d0e22ac Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Tue, 8 Aug 2017 17:05:19 +0900
Subject: [PATCH] mm: decrease tlb flush pending count in tlb_finish_mmu

The tlb pending count increased by tlb_gather_mmu should be decreased
at tlb_finish_mmu. Otherwise, A lot of TLB happens which makes
performance regression.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
mm/memory.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/mm/memory.c b/mm/memory.c
index 34b1fcb829e4..ad2617552f55 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -423,6 +423,7 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
bool force = mm_tlb_flush_nested(tlb->mm);

arch_tlb_finish_mmu(tlb, start, end, force);
+ dec_tlb_flush_pending(tlb->mm);
}

/*
--
2.7.4