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

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Message-ID: <87bmd9ka8e.fsf@yhuang-dev.intel.com>
Date:   Mon, 21 May 2018 10:34:25 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        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>
Subject: Re: [PATCH -mm] mm, huge page: Copy to access sub-page last when copy huge page

Mike Kravetz <mike.kravetz@...cle.com> writes:

> On 05/17/2018 11:24 PM, Michal Hocko wrote:
>> 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?
>
> One way to 'add less code' would be to create a helper routine that
> indicates the order in which sub-pages are to be copied.  IIUC, you
> added the same algorithm for sub-page ordering to copy_huge_page()
> that was previously added to clear_huge_page().  Correct?

Yes.

> If so, then perhaps a common helper could be used by both the clear
> and copy huge page routines.  It would also make maintenance easier.

That's a good idea.  But this may need to turn
copy_user_highpage()/clear_user_highpage() calling in
copy_user_huge_page()/clear_huge_page() from direct call to indirect
call.  I don't know whether this will incur some overhead.  Will try to
measure this.

Best Regards,
Huang, Ying