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Subject: Re: [[PATCH]] mm: khugepaged: recalculate min_free_kbytes after
 memory hotplug as expected by khugepaged
To:     Pavel Tatashin <pasha.tatashin@soleen.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Allen Pais <apais@microsoft.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
References: <1599770859-14826-1-git-send-email-vijayb@linux.microsoft.com>
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 <CA+CK2bACLNVqKP9Mgr37HQKOY=7+-PwuLCtqN6+saW7uuiMCfw@mail.gmail.com>
From:   Vijay Balakrishna <vijayb@linux.microsoft.com>
Message-ID: <a153dae8-77c2-f298-0ae5-9956560f9382@linux.microsoft.com>
Date:   Thu, 10 Sep 2020 15:56:57 -0700
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On 9/10/2020 3:28 PM, Pavel Tatashin wrote:
> Hi Kirill,
> 
> On Thu, Sep 10, 2020 at 6:01 PM Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
>>
>> On Thu, Sep 10, 2020 at 01:47:39PM -0700, Vijay Balakrishna wrote:
>>> 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 leaves the system with small
>>> min_free_kbytes which isn't suitable for systems especially with network
>>> intensive loads.  Typical failure symptoms include HW WATCHDOG reset,
>>> soft lockup hang notices, NETDEVICE WATCHDOG timeouts, and OOM process
>>> kills.
>>>
>>> Fixes: f000565adb77 ("thp: set recommended min free kbytes")
>>>
>>> Signed-off-by: Vijay Balakrishna <vijayb@linux.microsoft.com>
>>> Cc: stable@vger.kernel.org
>>
>> NAK. It would override min_free_kbytes set by user.
> 
> Hi Kirill,
> 
> Thank you for looking into this. How is this different from when
> khugepaged modifies it?
> 
> echo always >/sys/kernel/mm/transparent_hugepage/enabled
> 
> Which results in:
> 
> start_stop_khugepaged
>    set_recommended_min_free_kbytes
> 
> Which will also adjust min_free_kbytes according to hugepaged requirement?
> 
> This bug that Vijay described is another hot-plug related issue that
> we have found on our system where we perform memory hot add and hot
> remove on every reboot.
> 
> Thank you,
> Pasha
> 

Thanks Kirill for taking a look and spotting it.

IIUC, it is an existing issue, we should fix it while we are at it. 
Otherwise with my propsed patch introduces a regression for hotplug 
memory consumers with THP enabled and min_free_kbytes set by user higher 
than calculated by THP.

Vijay