linux-kernel - Re: [v4] mm: khugepaged: avoid overriding min_free

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Message-ID: <a55f1f0b-d26e-3cc3-370f-bd6f50692e34@linux.microsoft.com>
Date:   Tue, 22 Sep 2020 09:10:46 -0700
From:   Vijay Balakrishna <vijayb@...ux.microsoft.com>
To:     Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Song Liu <songliubraving@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Allen Pais <apais@...rosoft.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user



On 9/22/2020 3:07 AM, Michal Hocko wrote:
> On Tue 22-09-20 10:07:26, Kirill A. Shutemov wrote:
>> On Mon, Sep 21, 2020 at 12:07:23PM -0700, Vijay Balakrishna wrote:
>>>>
>>>> I would recommend reposting the patch which adds heuristic for THP (if
>>>> THP is enabled) into the hotplug path, arguing with the consistency and
>>>> surprising results when adding memory decreases the value.
>>>
>>> I hope my reposted patch
>>> ([v3 1/2] mm: khugepaged: recalculate min_free_kbytes after memory hotplug
>>> as expected by khugepaged)
>>> change log is ok:
>>>
>>> When memory is hotplug added or removed the min_free_kbytes must be
>>> recalculated based on what is expected by khugepaged.  Currently
>>> after hotplug, min_free_kbytes will be set to a lower default and higher
>>> default set when THP enabled is lost.  This change restores min_free_kbytes
>>> as expected for THP consumers.
>>
>> Any scenario when hotremove would result in changing min_free_kbytes?
> 
> init_per_zone_wmark_min is called from both online and offline path. But
> I believe the problem is not in the offlining path. A decrease wrt
> previous auto tuned value is to be expected. The primary problem is that
> the hotadding memory after boot (without any user configured value) will
> decrease the value effectively because khugepaged tuning
> (set_recommended_min_free_kbytes) is not called.

Thank you Michal and Kirill.

Vijay
>