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@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
mm/vmscan.c | 51 +++++++++++++++++++++++++++++++++++++++++----------
1 file changed, 41 insertions(+), 10 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f463398..8cb16eb 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)
@@ -941,10 +972,10 @@ 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
@@ -958,7 +989,7 @@ 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
@@ -973,7 +1004,7 @@ 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()
@@ -1422,7 +1453,7 @@ 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) {
@@ -1614,10 +1645,10 @@ 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
--
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