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

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Date:   Fri, 18 Sep 2020 07:45:13 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Vijay Balakrishna <vijayb@...ux.microsoft.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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: [[PATCH]] mm: khugepaged: recalculate min_free_kbytes after
 memory hotplug as expected by khugepaged

On Thu 17-09-20 11:03:56, Vijay Balakrishna wrote:
[...]
> > > The auto tuned value is incorrect post hotplug memory operation, in our use
> > > case memoy hot add occurs very early during boot.
> > Define incorrect. What are the actual values? Have you tried to increase
> > the value manually after the hotplug?
> 
> In our case SoC with 8GB memory, system tuned min_free_kbytes
> - first to 22528
> - we perform memory hot add very early in boot

What was the original and after-the-hotplug size of memory and layout?
I suspect that all the hotplugged memory is in Movable zone, right?

> - now min_free_kbytes is 8703
> 
> Before looking at code, first I manually restored min_free_kbytes soon after
> boot, reran stress and didn't notice symptoms I mentioned in change log.

This is really surprising and I strongly suspect that an earlier reclaim
just changed the timing enough so that workload has spread the memory
prpessure over a longer time and that might have been enough to recycle
some of the unreclaimable memory due to its natural life time. But this
is a pure speculation. Much more data would be needed to analyze this.

In any case your stress test is oveprovisioning your Normal zone and
increased min_free_kbytes just papers over the sizing problem.
-- 
Michal Hocko
SUSE Labs