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. > > @@ -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> --- 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
