On Fri, Nov 15, 2019 at 8:07 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Thu, Nov 14, 2019 at 03:47:59PM -0800, Shakeel Butt wrote: > > On Thu, Nov 7, 2019 at 12:53 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > > > We use refault information to determine whether the cache workingset > > > is stable or transitioning, and dynamically adjust the inactive:active > > > file LRU ratio so as to maximize protection from one-off cache during > > > stable periods, and minimize IO during transitions. > > > > > > With cgroups and their nested LRU lists, we currently don't do this > > > correctly. While recursive cgroup reclaim establishes a relative LRU > > > order among the pages of all involved cgroups, refaults only affect > > > the local LRU order in the cgroup in which they are occuring. As a > > > result, cache transitions can take longer in a cgrouped system as the > > > active pages of sibling cgroups aren't challenged when they should be. > > > > > > [ Right now, this is somewhat theoretical, because the siblings, under > > > continued regular reclaim pressure, should eventually run out of > > > inactive pages - and since inactive:active *size* balancing is also > > > done on a cgroup-local level, we will challenge the active pages > > > eventually in most cases. But the next patch will move that relative > > > size enforcement to the reclaim root as well, and then this patch > > > here will be necessary to propagate refault pressure to siblings. ] > > > > > > This patch moves refault detection to the root of reclaim. Instead of > > > remembering the cgroup owner of an evicted page, remember the cgroup > > > that caused the reclaim to happen. When refaults later occur, they'll > > > correctly influence the cross-cgroup LRU order that reclaim follows. > > > > Can you please explain how "they'll correctly influence"? I see that > > if the refaulted page was evicted due to pressure in some ancestor, > > then that's ancestor's refault distance and active file size will be > > used to decide to activate the refaulted page but how that is > > influencing cross-cgroup LRUs? > > I take it the next patch answered your question: Activating a page > inside a cgroup has an effect on how it's reclaimed relative to pages > in sibling cgroups. So the influence part isn't new with this change - > it's about recognizing that an activation has an effect on a wider > scope than just the local cgroup, and considering that scope when > making the decision whether to activate or not. > Thanks for the clarification. > > > @@ -302,6 +330,17 @@ void workingset_refault(struct page *page, void *shadow) > > > */ > > > refault_distance = (refault - eviction) & EVICTION_MASK; > > > > > > + /* > > > + * The activation decision for this page is made at the level > > > + * where the eviction occurred, as that is where the LRU order > > > + * during page reclaim is being determined. > > > + * > > > + * However, the cgroup that will own the page is the one that > > > + * is actually experiencing the refault event. > > > + */ > > > + memcg = get_mem_cgroup_from_mm(current->mm); > > > > Why not page_memcg(page)? page is locked. > > Nice catch! Indeed, the page is charged and locked at this point, so > we don't have to do another lookup and refcounting dance here. > > Delta patch: > > --- > > From 8984f37f3e88b1b39c7d6470b313730093b24474 Mon Sep 17 00:00:00 2001 > From: Johannes Weiner <hannes@xxxxxxxxxxx> > Date: Fri, 15 Nov 2019 09:14:04 -0500 > Subject: [PATCH] mm: vmscan: detect file thrashing at the reclaim root fix > > Shakeel points out that the page is locked and already charged by the > time we call workingset_refault(). Instead of doing another cgroup > lookup and reference from current->mm we can simply use page_memcg(). > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> For the complete patch: Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > --- > mm/workingset.c | 7 ++----- > 1 file changed, 2 insertions(+), 5 deletions(-) > > diff --git a/mm/workingset.c b/mm/workingset.c > index f0885d9f41cd..474186b76ced 100644 > --- a/mm/workingset.c > +++ b/mm/workingset.c > @@ -338,7 +338,7 @@ void workingset_refault(struct page *page, void *shadow) > * However, the cgroup that will own the page is the one that > * is actually experiencing the refault event. > */ > - memcg = get_mem_cgroup_from_mm(current->mm); > + memcg = page_memcg(page); > lruvec = mem_cgroup_lruvec(memcg, pgdat); > > inc_lruvec_state(lruvec, WORKINGSET_REFAULT); > @@ -349,7 +349,7 @@ void workingset_refault(struct page *page, void *shadow) > * the memory was available to the page cache. > */ > if (refault_distance > active_file) > - goto out_memcg; > + goto out; > > SetPageActive(page); > advance_inactive_age(memcg, pgdat); > @@ -360,9 +360,6 @@ void workingset_refault(struct page *page, void *shadow) > SetPageWorkingset(page); > inc_lruvec_state(lruvec, WORKINGSET_RESTORE); > } > - > -out_memcg: > - mem_cgroup_put(memcg); > out: > rcu_read_unlock(); > } > -- > 2.24.0 >
