On Tue 07-02-17 16:47:18, vinayak menon wrote: > On Mon, Feb 6, 2017 at 8:42 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > On Mon 06-02-17 20:05:21, vinayak menon wrote: > > [...] > >> By scan I meant pages scanned by shrink_node_memcg/shrink_list > >> which is passed as nr_scanned to vmpressure. The calculation of > >> pressure for tree is done at the end of vmpressure_win and it is > >> that calculation which underflows. With this patch we want only the > >> underflow to be avoided. But if we make (reclaimed = scanned) in > >> vmpressure(), we change the vmpressure value even when there is no > >> underflow right ? > >> > >> Rewriting the above e.g again. First call to vmpressure with > >> nr_scanned=1 and nr_reclaimed=512 (THP) Second call to vmpressure > >> with nr_scanned=511 and nr_reclaimed=0 In the second call > >> vmpr->tree_scanned becomes equal to vmpressure_win and the work > >> is scheduled and it will calculate the vmpressure as 0 because > >> tree_reclaimed = 512 > >> > >> Similarly, if scanned is made equal to reclaimed in vmpressure() > >> itself as you had suggested, First call to vmpressure with > >> nr_scanned=1 and nr_reclaimed=512 (THP) And in vmpressure, we > >> make nr_scanned=1 and nr_reclaimed=1 Second call to vmpressure > >> with nr_scanned=511 and nr_reclaimed=0 In the second call > >> vmpr->tree_scanned becomes equal to vmpressure_win and the work is > >> scheduled and it will calculate the vmpressure as critical, because > >> tree_reclaimed = 1 > >> > >> So it makes a difference, no? > > > > OK, I see what you meant. Thanks for the clarification. And you are > > right that normalizing nr_reclaimed to nr_scanned is a wrong thing to > > do because that just doesn't aggregate the real work done. Normalizing > > nr_scanned to nr_reclaimed should be better - or it would be even better > > to count the scanned pages properly... > > > With the slab reclaimed issue fixed separately, only the THP case exists AFAIK. > In the case of THP, as I understand from one of Minchan's reply, the scan is > actually 1. i.e. Only a single huge page is scanned to get 512 reclaimed pages. > So the cost involved was scanning a single page. > In that case, there is no need to normalize the nr_scanned, no? Strictly speaking it is not but it has weird side effects when we basically lie about vmpressure_win. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
