Re: [v5] mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged

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

On 9/30/2020 11:20 AM, Mike Kravetz wrote:
> On 9/29/20 9:49 AM, Vijay Balakrishna wrote:
>> When memory is hotplug added or removed the min_free_kbytes should 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 change restores min_free_kbytes
>> as expected for THP consumers.
>>
>> Fixes: f000565adb77 ("thp: set recommended min free kbytes")
>>
>> Signed-off-by: Vijay Balakrishna <vijayb@linux.microsoft.com>
>> Cc: stable@vger.kernel.org
>> Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
>> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Sorry for jumping in so late.  Should we use this as an opportunity to
> also fix up the messages logged when (re)calculating mfk?  They are wrong
> and could be quite confusing.


Sure.  Please share your thoughts regarding appropriate message.  Here 
is what I'm thinking

pr_warn("min_free_kbytes is not updated to %d because current value %d 
is preferred\n", new_min_free_kbytes, min_free_kbytes);

If above message is reasonable I can post a new revision (v6).

Thanks,
Vijay

> For example consider the following sequence
> of operations and corresponding log messages produced.
> 
> Freshly booted VM with 2 nodes and 8GB memory:
> # cat /proc/sys/vm/min_free_kbytes
> 90112
> # echo 90000 > /proc/sys/vm/min_free_kbytes
> # cat /proc/sys/vm/min_free_kbytes
> 90000
> # echo 0 > /sys/devices/system/node/node1/memory56/online
> [  135.099947] Offlined Pages 32768
> [  135.102362] min_free_kbytes is not updated to 11241 because user defined value 90000 is preferred
> [  135.109070] khugepaged: raising min_free_kbytes from 90000 to 90112 to help t
> ransparent hugepage allocations
> # cat /proc/sys/vm/min_free_kbytes
> 90112
> # echo 1 > /sys/devices/system/node/node1/memory56/online
> [  231.656075] min_free_kbytes is not updated to 11334 because user defined value 90000 is preferred
> # cat /proc/sys/vm/min_free_kbytes
> 90112
>