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

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Message-ID: <20200921070006.GA12990@dhcp22.suse.cz>
Date:   Mon, 21 Sep 2020 09:00:06 +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 Fri 18-09-20 08:32:13, Vijay Balakrishna wrote:
> 
> 
> On 9/17/2020 10:45 PM, Michal Hocko wrote:
> > 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?
> 
> Yes, added ~1.92GB as Movable type, booting with 6GB at start.
> 
> > 
> > > - 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.
> > 
> 
> It is a synthetic workload, likely not sized I need to check.  I feel having
> higher min_free_kbytes made GFP_ATOMIC allocations not to fail.

Yes a higher min_free_kbytes will help GFP_ATOMIC. But only to some
degree. But nobody should depend on an atomic allocation for
correctness. It is just way too easy to fail under a higher memory
pressure.

> I have seen
> NETDEV WATCHDOG timeout with stacktrace trying to allocate memory, looping
> in net rx receive path.

You should talk to net folks.
-- 
Michal Hocko
SUSE Labs