linux-kernel - Re: [PATCH -mm] mm, huge page: Copy to access sub-page last when copy huge page

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Date:   Mon, 21 May 2018 09:51:50 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, Andi Kleen <andi.kleen@...el.com>,
        Jan Kara <jack@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>, Shaohua Li <shli@...com>,
        Christopher Lameter <cl@...ux.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH -mm] mm, huge page: Copy to access sub-page last when copy huge page

Hi, Michal,

Michal Hocko <mhocko@...nel.org> writes:

> On Fri 18-05-18 11:03:16, Huang, Ying wrote:
> [...]
>> The patch is a generic optimization which should benefit quite some
>> workloads, not for a specific use case.  To demonstrate the performance
>> benefit of the patch, we tested it with vm-scalability run on
>> transparent huge page.
>
> It is also adds quite some non-intuitive code. So is this worth? Does
> any _real_ workload benefits from the change?

I don't have any _real_ workload which benefits from this.  But I think
this is the right way to copy the huge page.  It should benefit many
workloads with heavy cache contention, as illustrated in the
micro-benchmark.  But the performance benefit may be small or
non-measurable for the _real_ workload.

The code does become not as intuitive as before.  But fortunately, all
non-intuitive code are in copy_user_huge_page(), which is a leaf
function with well defined interface and semantics.  And with the help
of the code comments, at least the intention of the code is clear.

Best Regards,
Huang, Ying

>>  include/linux/mm.h |  3 ++-
>>  mm/huge_memory.c   |  3 ++-
>>  mm/memory.c        | 43 +++++++++++++++++++++++++++++++++++++++----
>>  3 files changed, 43 insertions(+), 6 deletions(-)