Re: Why sometimes count vm event with page number, sometimes not?

[Wei Yang <richardw.yang@linux.intel.com>]

On Tue, Nov 05, 2019 at 12:49:17PM -0800, Hugh Dickins wrote:
>On Tue, 5 Nov 2019, Vlastimil Babka wrote:
>> On 11/5/19 7:28 AM, Wei Yang wrote:
>> > On Mon, Nov 04, 2019 at 07:32:51PM -0800, Matthew Wilcox wrote:
>> >> On Tue, Nov 05, 2019 at 10:26:44AM +0800, Wei Yang wrote:
>> >>> Hi, All,
>> >>>
>> >>> I am curious about the semantic of __count_vm_event[s].
>> >>>
>> >>> For example, we count PGDEACTIVATE event in lru_deactivate_file_fn() and
>> >>> lru_deactivate_fn(). One of them count with number of page, the other not.
>> >>>
>> >>> Just curious about the exact value we want to count.
>> >>
>> >> I don't understand the question.  We deactivate one page
>> >> in lru_deactivate_file_fn().  We deactivate several pages in
>> >> shrink_active_list().  PGDEACTIVATE counts the number of pages which
>> >> have been deactivated.
>> >>
>> >> Does that answer your question?
>> > 
>> > Not yet.
>> > 
>> > In function, lru_deactivate_fn(), __count_vm_events's second parameter is
>> > hpage_nr_pages(page). This is the number in size of "normal" page. Per my
>> > understanding, the page deactivated in lru_deactivate_file_fn() could be a
>> > hpage too. But it just count the deactivation once instead of
>> > hpage_nr_pages().
>> > 
>> > Or you want to say the page deactivated in lru_deactivate_file_fn() must be an
>> > order 0 page?
>> 
>> I suspect that was true before THP on shmem, but now perhaps it's not
>> true anymore? CCing Kirill and Hugh.
>
>I think that you and Wei are right, that lru_deactivate_file_fn() ought
>nowadays to __count_vm_events(PGDEACTIVATE, hpage_nr_pages(page)) like
>lru_deactivate_fn() does.
>
>(Though I think the only way shmem gets there is through drop_caches
>- note the noop_backing_dev_info check in generic_fadvise().
>invalidate_mapping_pages() on shmem is rarely more than a waste of
>time, since all but mapped readonly holes are undiscardably PageDirty.
>Internally we added a shmem_mapping() check to stop tests wasting time
>on expensive repeated drop_caches of shmem. And it's debatable whether
>drop_caches updating those vm_event stats is useful or the reverse.)
>
>Except that a couple of lines above I see __count_vm_event(PGROTATED):
>shouldn't that also use hpage_nr_pages? Then looking further through
>mm/swap.c, isn't there inconsistency throughout, whether a vm_event
>on a THP should be counted as 1 or hpage_nr_pages? I think originally
>the idea was that manipulating a THP should count as a single event,
>but now we have... a muddle.
>
>And what determines whether a memcg event is counted too?
>mm/vmscan.c chooses to count PGDEACTIVATE and other memcg events,
>mm/swap.c chooses not to count memcg events - unless it's PGLAZYFREE.
>
>I don't have answers.

Yes, just as you did, I looked into several places where count vm event. Feel
confused what we really count. Hmm... this break my mind...

>
>Hugh

-- 
Wei Yang
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