[PATCH v2 18/18] mm: vmscan: disable memcg direct reclaim stalling if cgroup writeback support is in use

Tejun Heo
2015-03-25T22:26:20Z

>From 3cde6e12925bff066d2bb6332df323fb38de979e Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 25 Mar 2015 18:17:13 -0400

Because writeback wasn't cgroup aware before, the usual dirty
throttling mechanism in balance_dirty_pages() didn't work for
processes under memcg limit. The writeback path didn't know how much
memory is available or how fast the dirty pages are being written out
for a given memcg and balance_dirty_pages() didn't have any measure of
IO back pressure for the memcg.

To work around the issue, memcg implemented an ad-hoc dirty throttling
mechanism in the direct reclaim path by stalling on pages under
writeback which are encountered during direct reclaim scan. This is
rather ugly and crude - none of the configurability, fairness, or
bandwidth-proportional distribution of the normal path.

The previous patches implemented proper memcg aware dirty throttling
when cgroup writeback is in use making the ad-hoc mechanism
unnecessary. This patch disables direct reclaim stalling for such
case.

Note: I disabled the parts which seemed obvious and it behaves fine
while testing but my understanding of this code path is
rudimentary and it's quite possible that I got something wrong.
Please let me know if I got some wrong or more global_reclaim()
sites should be updated.

v2: The original patch removed the direct stalling mechanism which
breaks legacy hierarchies. Conditionalize instead of removing.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
---
The rewview git branch has been updated accordingly.

git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-cgroup-writeback-backpressure-20150322

Thanks.

mm/vmscan.c | 51 +++++++++++++++++++++++++++++++++++++++++----------
1 file changed, 41 insertions(+), 10 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9f8d3c0..cc76c66 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -154,11 +154,42 @@ static bool global_reclaim(struct scan_control *sc)
{
return !sc->target_mem_cgroup;
}
+
+/**
+ * sane_reclaim - is the usual dirty throttling mechanism operational?
+ * @sc: scan_control in question
+ *
+ * The normal page dirty throttling mechanism in balance_dirty_pages() is
+ * completely broken with the legacy memcg and direct stalling in
+ * shrink_page_list() is used for throttling instead, which lacks all the
+ * niceties such as fairness, adaptive pausing, bandwidth proportional
+ * allocation and configurability.
+ *
+ * This function tests whether the vmscan currently in progress can assume
+ * that the normal dirty throttling mechanism is operational.
+ */
+static bool sane_reclaim(struct scan_control *sc)
+{
+ struct mem_cgroup *memcg = sc->target_mem_cgroup;
+
+ if (!memcg)
+ return true;
+#ifdef CONFIG_CGROUP_WRITEBACK
+ if (cgroup_on_dfl(mem_cgroup_css(memcg)->cgroup))
+ return true;
+#endif
+ return false;
+}
#else
static bool global_reclaim(struct scan_control *sc)
{
return true;
}
+
+static bool sane_reclaim(struct scan_control *sc)
+{
+ return true;
+}
#endif

static unsigned long zone_reclaimable_pages(struct zone *zone)
@@ -942,10 +973,10 @@ static unsigned long shrink_page_list(struct list_head *page_list,
* note that the LRU is being scanned too quickly and the
* caller can stall after page list has been processed.
*
- * 2) Global reclaim encounters a page, memcg encounters a
- * page that is not marked for immediate reclaim or
- * the caller does not have __GFP_IO. In this case mark
- * the page for immediate reclaim and continue scanning.
+ * 2) Global or new memcg reclaim encounters a page that is
+ * not marked for immediate reclaim or the caller does not
+ * have __GFP_IO. In this case mark the page for immediate
+ * reclaim and continue scanning.
*
* __GFP_IO is checked because a loop driver thread might
* enter reclaim, and deadlock if it waits on a page for
@@ -959,7 +990,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
* grab_cache_page_write_begin(,,AOP_FLAG_NOFS), so testing
* may_enter_fs here is liable to OOM on them.
*
- * 3) memcg encounters a page that is not already marked
+ * 3) Legacy memcg encounters a page that is not already marked
* PageReclaim. memcg does not have any dirty pages
* throttling so we could easily OOM just because too many
* pages are in writeback and there is nothing else to
@@ -974,7 +1005,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
goto keep_locked;

/* Case 2 above */
- } else if (global_reclaim(sc) ||
+ } else if (sane_reclaim(sc) ||
!PageReclaim(page) || !(sc->gfp_mask & __GFP_IO)) {
/*
* This is slightly racy - end_page_writeback()
@@ -1423,7 +1454,7 @@ static int too_many_isolated(struct zone *zone, int file,
if (current_is_kswapd())
return 0;

- if (!global_reclaim(sc))
+ if (!sane_reclaim(sc))
return 0;

if (file) {
@@ -1615,10 +1646,10 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
set_bit(ZONE_WRITEBACK, &zone->flags);

/*
- * memcg will stall in page writeback so only consider forcibly
- * stalling for global reclaim
+ * Legacy memcg will stall in page writeback so avoid forcibly
+ * stalling here.
*/
- if (global_reclaim(sc)) {
+ if (sane_reclaim(sc)) {
/*
* Tag a zone as congested if all the dirty pages scanned were
* backed by a congested BDI and wait_iff_congested will stall.
--
2.1.0