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

[Vijay Balakrishna <vijayb@linux.microsoft.com>]

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

Thanks,
Vijay