lkml.org 
[lkml]   [2020]   [Sep]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user
From
Date


On 9/17/2020 10:52 AM, Michal Hocko wrote:
> On Thu 17-09-20 10:27:16, Vijay Balakrishna wrote:
>>
>>
>> On 9/17/2020 2:28 AM, Michal Hocko wrote:
>>> On Wed 16-09-20 23:39:39, Vijay Balakrishna wrote:
>>>> set_recommended_min_free_kbytes need to honor min_free_kbytes set by the
>>>> user. Post start-of-day THP enable or memory hotplug operations can
>>>> lose user specified min_free_kbytes, in particular when it is higher than
>>>> calculated recommended value.
>>>
>>> I was about to recommend a more detailed explanation when I have
>>> realized that this patch is not really needed after all. Unless I am
>>> missing something.
>>>
>>> init_per_zone_wmark_min ignores the newly calculated min_free_kbytes if
>>> it is lower than user_min_free_kbytes. So calculated min_free_kbytes >=
>>> user_min_free_kbytes.
>>>
>>> Except for value clamping when the value is reduced and this likely
>>> needs fixing. But set_recommended_min_free_kbytes should be fine.
>>>
>>
>> IIUC, after start-of-day if a user performs
>> - THP disable
>> - modifies min_free_bytes
>> - THP enable
>> above sequence currently wouldn't result in calling init_per_zone_wmark_min.
>
> I will not, but why do you think this matters? All we should care about
> is that auto-tuning shouldn't reduce user provided value [1] and that
> the memory hotplug should be consistent with the boot time heuristic.
> init_per_zone_wmark_min should make sure that the user value is not
> reduced and thp heuristic makes sure it will not reduce this value.
> So the property should be transitive with the existing code (modulo the
> problem I have highlighted).
>
> [1] one could argue that it shouldn't even increase the value strictly
> speaking because an admin might have a very good reason to decrease the
> value but this has never been the semantic and changing it now might be
> problematic
>

I made an attempt to address Kirill A. Shutemov's comment. And incrased
min_free_kbytes to see the issue in my testing and attempted a fix. I'm
ok leaving as it is. Do not want introduce any changes that may cause
regression.

Thanks,
Vijay

\
 
 \ /
  Last update: 2020-09-17 20:21    [W:0.217 / U:0.316 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site