From: "Huang\, Ying" <ying.huang@intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Huang\, Ying" <ying.huang@intel.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Rik van Riel <riel@redhat.com>, Michal Hocko <mhocko@suse.com>,
LKML <linux-kernel@vger.kernel.org>,
Michal Hocko <mhocko@kernel.org>,
Minchan Kim <minchan@kernel.org>,
Vinayak Menon <vinmenon@codeaurora.org>,
Mel Gorman <mgorman@suse.de>,
Andrew Morton <akpm@linux-foundation.org>, LKP <lkp@01.org>
Subject: Re: [LKP] [lkp] [mm] 5c0a85fad9: unixbench.score -6.3% regression
Date: Mon, 13 Jun 2016 17:02:15 +0800 [thread overview]
Message-ID: <87bn35zcko.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <CA+55aFy4oYis6HTu7o4YwiFawRtDOPO=87v8oHZdTFS+BjnA8g@mail.gmail.com> (Linus Torvalds's message of "Sat, 11 Jun 2016 18:02:57 -0700")
Linus Torvalds <torvalds@linux-foundation.org> writes:
> On Sat, Jun 11, 2016 at 5:49 PM, Huang, Ying <ying.huang@intel.com> wrote:
>>
>> From perf profile, the time spent in page_fault and its children
>> functions are almost same (7.85% vs 7.81%). So the time spent in page
>> fault and page table operation itself doesn't changed much. So, you
>> mean CPU may be slower to load the page table entry to TLB if accessed
>> bit is not set?
>
> So the CPU does take a microfault internally when it needs to set the
> accessed/dirty bit. It's not architecturally visible, but you can see
> it when you do timing loops.
>
> I've timed it at over a thousand cycles on at least some CPU's, but
> that's still peanuts compared to a real page fault. It shouldn't be
> *that* noticeable, ie no way it's a 6% regression on its own.
I done some simple counting, and found that about 3.15e9 PTE are set to
old during the test after the commit. This may interpret the user_time
increase as below, because these accessed bit microfault is accounted as
user time.
387.66 . 0% +5.4% 408.49 . 0% unixbench.time.user_time
I also make a one line debug patch as below on top of the commit to set
the PTE to young unconditionally, which recover the regression.
modified mm/filemap.c
@@ -2193,7 +2193,7 @@ repeat:
if (file->f_ra.mmap_miss > 0)
file->f_ra.mmap_miss--;
addr = address + (page->index - vmf->pgoff) * PAGE_SIZE;
- do_set_pte(vma, addr, page, pte, false, false, true);
+ do_set_pte(vma, addr, page, pte, false, false, false);
unlock_page(page);
atomic64_inc(&old_pte_count);
goto next;
Best Regards,
Huang, Ying
next prev parent reply other threads:[~2016-06-13 9:02 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-06 2:27 [lkp] [mm] 5c0a85fad9: unixbench.score -6.3% regression kernel test robot
2016-06-06 9:51 ` Kirill A. Shutemov
2016-06-08 7:21 ` [LKP] " Huang, Ying
2016-06-08 8:41 ` Huang, Ying
2016-06-08 8:58 ` Kirill A. Shutemov
2016-06-12 0:49 ` Huang, Ying
2016-06-12 1:02 ` Linus Torvalds
2016-06-13 9:02 ` Huang, Ying [this message]
2016-06-14 13:38 ` Minchan Kim
2016-06-15 23:42 ` Huang, Ying
2016-06-13 12:52 ` Kirill A. Shutemov
2016-06-14 6:11 ` Linus Torvalds
2016-06-14 8:26 ` Kirill A. Shutemov
2016-06-14 16:07 ` Rik van Riel
2016-06-14 14:03 ` Christian Borntraeger
2016-06-14 8:57 ` Minchan Kim
2016-06-14 14:34 ` Kirill A. Shutemov
2016-06-15 23:52 ` Huang, Ying
2016-06-16 0:13 ` Minchan Kim
2016-06-16 22:27 ` Huang, Ying
2016-06-17 5:41 ` Minchan Kim
2016-06-17 19:26 ` Huang, Ying
2016-06-20 0:06 ` Minchan Kim
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