Re: [PATCH v9 0/8] stg mail -e --version=v9 \

From: Nitesh Narayan Lal
Date: Wed Sep 11 2019 - 10:05:11 EST

On 9/11/19 9:20 AM, Michal Hocko wrote:
> On Wed 11-09-19 15:03:39, David Hildenbrand wrote:
>> On 11.09.19 14:54, Michal Hocko wrote:
>>> On Wed 11-09-19 14:42:41, David Hildenbrand wrote:
>>>> On 11.09.19 14:25, Michal Hocko wrote:
>>>>> On Wed 11-09-19 14:19:41, Michal Hocko wrote:
>>>>>> On Wed 11-09-19 08:08:38, Michael S. Tsirkin wrote:
>>>>>>> On Wed, Sep 11, 2019 at 01:36:19PM +0200, Michal Hocko wrote:
>>>>>>>> On Tue 10-09-19 14:23:40, Alexander Duyck wrote:
>>>>>>>> [...]
>>>>>>>>> We don't put any limitations on the allocator other then that it needs to
>>>>>>>>> clean up the metadata on allocation, and that it cannot allocate a page
>>>>>>>>> that is in the process of being reported since we pulled it from the
>>>>>>>>> free_list. If the page is a "Reported" page then it decrements the
>>>>>>>>> reported_pages count for the free_area and makes sure the page doesn't
>>>>>>>>> exist in the "Boundary" array pointer value, if it does it moves the
>>>>>>>>> "Boundary" since it is pulling the page.
>>>>>>>> This is still a non-trivial limitation on the page allocation from an
>>>>>>>> external code IMHO. I cannot give any explicit reason why an ordering on
>>>>>>>> the free list might matter (well except for page shuffling which uses it
>>>>>>>> to make physical memory pattern allocation more random) but the
>>>>>>>> architecture seems hacky and dubious to be honest. It shoulds like the
>>>>>>>> whole interface has been developed around a very particular and single
>>>>>>>> purpose optimization.
>>>>>>>> I remember that there was an attempt to report free memory that provided
>>>>>>>> a callback mechanism [1], which was much less intrusive to the internals
>>>>>>>> of the allocator yet it should provide a similar functionality. Did you
>>>>>>>> see that approach? How does this compares to it? Or am I completely off
>>>>>>>> when comparing them?
>>>>>>>> [1] mostly likely not the latest version of the patchset
>>>>>>> Linus nacked that one. He thinks invoking callbacks with lots of
>>>>>>> internal mm locks is too fragile.
>>>>>> I would be really curious how much he would be happy about injecting
>>>>>> other restrictions on the allocator like this patch proposes. This is
>>>>>> more intrusive as it has a higher maintenance cost longterm IMHO.
>>>>> Btw. I do agree that callbacks with internal mm locks are not great
>>>>> either. We do have a model for that in mmu_notifiers and it is something
>>>>> I do consider PITA, on the other hand it is mostly sleepable part of the
>>>>> interface which makes it the real pain. The above callback mechanism was
>>>>> explicitly documented with restrictions and that the context is
>>>>> essentially atomic with no access to particular struct pages and no
>>>>> expensive operations possible. So in the end I've considered it
>>>>> acceptably painful. Not that I want to override Linus' nack but if
>>>>> virtualization usecases really require some form of reporting and no
>>>>> other way to do that push people to invent even more interesting
>>>>> approaches then we should simply give them/you something reasonable
>>>>> and least intrusive to our internals.
>>>> The issue with "[PATCH v14 4/5] mm: support reporting free page blocks"
>>>> is that it cannot really handle the use case we have here if I am not
>>>> wrong. While a page is getting processed by the hypervisor (e.g.
>>>> MADV_DONTNEED), it must not get reused.
>>> What prevents to use the callback to get a list of pfn ranges to work on
>>> and then use something like start_isolate_page_range on the collected
>>> pfn ranges to make sure nobody steals pages from under your feet, do
>>> your thing and drop the isolated state afterwards.
>>> I am saying somethig like because you wouldn't really want a generic
>>> has_unmovable_pages but rather
>>> if (!page_ref_count(page)) {
>>> if (PageBuddy(page))
>>> iter += (1 << page_order(page)) - 1;
>>> continue;
>>> }
>>> subset of it.
>> Something slightly similar is being performed by Nitesh's patch set. On
>> every free of a certain granularity, he records it in the bitmap. These
>> bits are "hints of free pages".
>> A thread then walks over the bitmap and tries to allocate the "hints".
>> If the pages were already reused, the bit is silently cleared.
>> Instead of allocating/freeing, we could only try to isolate the
>> pageblock, then test if free. (One of the usual issues to work around is
>> MAX_ORDER-1 crossing pageblocks, that might need special care)
> OK, cool that I have reinvented the wheel ;). Allocation is indeed not
> necessary as long as pages are isolated because nobody will allocate
> them.
>> I think you should have a look at the rough idea of Nitesh's patch set
>> to see if something like that is going into a better direction. The
>> bitmap part is in place to do bulk reporting and avoid duplicate reports.
> Let's see how much time I can find for that in my endless inbox whack a mole.
>> I think main points we want (and what I am missing from callback idea
>> being discussed) are
>> 1. Do bulk reporting only when a certain threshold is reached
> Is a time based approach too coarse?

I haven't looked into it yet. One situation which I would definitely
want to avoid is to unnecessary invoke bitmap scans when there are not
sufficient free pages available in the zone.
I can take a look at it if required.

>> 2. Report only bigger granularities (especially, avoid THP splits in the
>> hypervisor - >= 2MB proofed to be effective)
> the callback has supported order based scan in some of its iteration.
>> 3. Avoid reporting what has just been reported.
> Is the overhead of checking a pfn range in a bitmask that much of an
> overhead to really care?

In most of the cases No. Similar to Alexander I was also running will-it-scale/
page_fault1 to analyze the performance impact of the patch series and haven't
noticed any significant degradation.
In some specific cases, we may see noticeable degradation due to the scanning

>> 4. Continuously report, not the "one time report everything" approach.
> So you mean the allocator reporting this rather than an external code to
> poll right? I do not know, how much this is nice to have than must have?

Not sure if I understood the question completely.
But yes in my case any workload which is allocating and freeing pages will end
up initiating reporting requests based on the number of free pages in the zone.