On Sat,  3 Aug 2019 07:01:55 -0700 Tejun Heo [off-list ref] wrote:

There's an inherent mismatch between memcg and writeback.  The former
trackes ownership per-page while the latter per-inode.  This was a
deliberate design decision because honoring per-page ownership in the
writeback path is complicated, may lead to higher CPU and IO overheads
and deemed unnecessary given that write-sharing an inode across
different cgroups isn't a common use-case.

Combined with inode majority-writer ownership switching, this works
well enough in most cases but there are some pathological cases.  For
example, let's say there are two cgroups A and B which keep writing to
different but confined parts of the same inode.  B owns the inode and
A's memory is limited far below B's.  A's dirty ratio can rise enough
to trigger balance_dirty_pages() sleeps but B's can be low enough to
avoid triggering background writeback.  A will be slowed down without
a way to make writeback of the dirty pages happen.

This patch implements foreign dirty recording and foreign mechanism so
that when a memcg encounters a condition as above it can trigger
flushes on bdi_writebacks which can clean its pages.  Please see the
comment on top of mem_cgroup_track_foreign_dirty_slowpath() for
details.

...

+void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
+					     struct bdi_writeback *wb)
+{
+	struct mem_cgroup *memcg = page->mem_cgroup;
+	struct memcg_cgwb_frn *frn;
+	u64 now = jiffies_64;
+	u64 oldest_at = now;
+	int oldest = -1;
+	int i;
+
+	/*
+	 * Pick the slot to use.  If there is already a slot for @wb, keep
+	 * using it.  If not replace the oldest one which isn't being
+	 * written out.
+	 */
+	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
+		frn = &memcg->cgwb_frn[i];
+		if (frn->bdi_id == wb->bdi->id &&
+		    frn->memcg_id == wb->memcg_css->id)
+			break;
+		if (frn->at < oldest_at && atomic_read(&frn->done.cnt) == 1) {
+			oldest = i;
+			oldest_at = frn->at;
+		}
+	}
+
+	if (i < MEMCG_CGWB_FRN_CNT) {
+		unsigned long update_intv =
+			min_t(unsigned long, HZ,
+			      msecs_to_jiffies(dirty_expire_interval * 10) / 8);

An explanation of what's going on here would be helpful.

Why "* 1.25" and not, umm "* 1.24"?

+		/*
+		 * Re-using an existing one.  Let's update timestamp lazily
+		 * to avoid making the cacheline hot.
+		 */
+		if (frn->at < now - update_intv)
+			frn->at = now;
+	} else if (oldest >= 0) {
+		/* replace the oldest free one */
+		frn = &memcg->cgwb_frn[oldest];
+		frn->bdi_id = wb->bdi->id;
+		frn->memcg_id = wb->memcg_css->id;
+		frn->at = now;
+	}
+}
+
+/*
+ * Issue foreign writeback flushes for recorded foreign dirtying events
+ * which haven't expired yet and aren't already being written out.
+ */
+void mem_cgroup_flush_foreign(struct bdi_writeback *wb)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
+	unsigned long intv = msecs_to_jiffies(dirty_expire_interval * 10);

Ditto.

+	u64 now = jiffies_64;
+	int i;
+
+	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
+		struct memcg_cgwb_frn *frn = &memcg->cgwb_frn[i];
+
+		if (frn->at > now - intv && atomic_read(&frn->done.cnt) == 1) {
+			frn->at = 0;
+			cgroup_writeback_by_id(frn->bdi_id, frn->memcg_id,
+					       LONG_MAX, WB_REASON_FOREIGN_FLUSH,
+					       &frn->done);
+		}
+	}
+}
+