Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user

[Vijay Balakrishna]

On 9/17/2020 10:56 PM, Michal Hocko wrote:
> On Thu 17-09-20 11:16:55, Vijay Balakrishna wrote:
> > 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.
>
> This is for Kirill to comment on but my take would be that memory
> hotplug really has to alter the user defined min_free_kbytes because it
> is manipulating the amount of memory. There are usecases which are
> adding a lot of memory.
>
> We are trying to not decrease the value which is arguably a weird semantic
> but this is what've been doing for years. We would need to hear a
> specific usecase where this matters (e.g. memory hotremove heavy
> workalod with manually tuned min_free_kbytes) that misbehaves.

In our use case memory hotremove done normally during shutdown and we 
aren't manually tuning min_free_kbytes.

> > 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.
>
> I would recommend reposting the patch which adds heuristic for THP (if
> THP is enabled) into the hotplug path, arguing with the consistency and
> surprising results when adding memory decreases the value.

I hope my reposted patch
([v3 1/2] mm: khugepaged: recalculate min_free_kbytes after memory 
hotplug as expected by khugepaged)
change log is ok:

When memory is hotplug added or removed the min_free_kbytes must 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.


> Your initial
> problem is in sizing as mentioned in other email thread and you should
> be investigating more but this inconsistency might really come as a
> surprise.
>
> All that if Kirill is reconsidering his initial position of course.

Kirill, can you comment or share your opinion?

Thanks,
Vijay