Messages in this thread | | | Subject | Re: [v4] mm: khugepaged: avoid overriding min_free_kbytes set by user | From | Vijay Balakrishna <> | Date | Mon, 21 Sep 2020 12:07:23 -0700 |
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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
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