mm: vmpressure: allow in-kernel clients to subscribe for events

Currently, vmpressure is tied to memcg and its events are
available only to userspace clients. This patch removes
the dependency on CONFIG_MEMCG and adds a mechanism for
in-kernel clients to subscribe for vmpressure events (in
fact raw vmpressure values are delivered instead of vmpressure
levels, to provide clients more flexibility to take actions
on custom pressure levels which are not currently defined
by vmpressure module).

Change-Id: I38010f166546e8d7f12f5f355b5dbfd6ba04d587
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>

mm/vmpressure.c

@@ -429,6 +429,74 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
 
 	__vmpressure(gfp, memcg, false, tree, scanned, reclaimed);
 }
+#else
+static void vmpressure_memcg(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
+		unsigned long scanned, unsigned long reclaimed)
+{
+}
+#endif
+
+static void vmpressure_global(gfp_t gfp, unsigned long scanned,
+		unsigned long reclaimed)
+{
+	struct vmpressure *vmpr = &global_vmpressure;
+	unsigned long pressure;
+
+	if (!(gfp & (__GFP_HIGHMEM | __GFP_MOVABLE | __GFP_IO | __GFP_FS)))
+		return;
+
+	if (!scanned)
+		return;
+
+	spin_lock(&vmpr->sr_lock);
+	vmpr->scanned += scanned;
+	vmpr->reclaimed += reclaimed;
+	scanned = vmpr->scanned;
+	reclaimed = vmpr->reclaimed;
+	spin_unlock(&vmpr->sr_lock);
+
+	if (scanned < vmpressure_win)
+		return;
+
+	spin_lock(&vmpr->sr_lock);
+	vmpr->scanned = 0;
+	vmpr->reclaimed = 0;
+	spin_unlock(&vmpr->sr_lock);
+
+	pressure = vmpressure_calc_pressure(scanned, reclaimed);
+	vmpressure_notify(pressure);
+}
+
+/**
+ * vmpressure() - Account memory pressure through scanned/reclaimed ratio
+ * @gfp:	reclaimer's gfp mask
+ * @memcg:	cgroup memory controller handle
+ * @tree:	legacy subtree mode
+ * @scanned:	number of pages scanned
+ * @reclaimed:	number of pages reclaimed
+ *
+ * This function should be called from the vmscan reclaim path to account
+ * "instantaneous" memory pressure (scanned/reclaimed ratio). The raw
+ * pressure index is then further refined and averaged over time.
+ *
+ * If @tree is set, vmpressure is in traditional userspace reporting
+ * mode: @memcg is considered the pressure root and userspace is
+ * notified of the entire subtree's reclaim efficiency.
+ *
+ * If @tree is not set, reclaim efficiency is recorded for @memcg, and
+ * only in-kernel users are notified.
+ *
+ * This function does not return any value.
+ */
+void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
+		unsigned long scanned, unsigned long reclaimed)
+{
+	if (!memcg && tree)
+		vmpressure_global(gfp, scanned, reclaimed);
+
+	if (IS_ENABLED(CONFIG_MEMCG))
+		vmpressure_memcg(gfp, memcg, tree, scanned, reclaimed);
+}
 
 /**
  * vmpressure_prio() - Account memory pressure through reclaimer priority level