Re: [[PATCH]] mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged

From: Vijay Balakrishna
Date: Thu Sep 10 2020 - 18:57:02 EST

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@xxxxxxxxxxxxxxx> 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

Fixes: f000565adb77 ("thp: set recommended min free kbytes")

Signed-off-by: Vijay Balakrishna <vijayb@xxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

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:


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,

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.