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. Hugh
