Subject: Re: Low TCP throughput due to vmpressure with swap enabled

From: Johannes Weiner <[email-masked]>
Date: Tue, 22 Nov 2022 15:05:10 -0500

On Mon, Nov 21, 2022 at 04:53:43PM -0800, Ivan Babrou wrote:
> Hello,
> 
> We have observed a negative TCP throughput behavior from the following commit:
> 
> * 8e8ae645249b mm: memcontrol: hook up vmpressure to socket pressure
> 
> It landed back in 2016 in v4.5, so it's not exactly a new issue.
> 
> The crux of the issue is that in some cases with swap present the
> workload can be unfairly throttled in terms of TCP throughput.

Thanks for the detailed analysis, Ivan.

Originally, we pushed back on sockets only when regular page reclaim
had completely failed and we were about to OOM. This patch was an
attempt to be smarter about it and equalize pressure more smoothly
between socket memory, file cache, anonymous pages.

After a recent discussion with Shakeel, I'm no longer quite sure the
kernel is the right place to attempt this sort of balancing. It kind
of depends on the workload which type of memory is more imporant. And
your report shows that vmpressure is a flawed mechanism to implement
this, anyway.

So I'm thinking we should delete the vmpressure thing, and go back to
socket throttling only if an OOM is imminent. This is in line with
what we do at the system level: sockets get throttled only after
reclaim fails and we hit hard limits. It's then up to the users and
sysadmin to allocate a reasonable amount of buffers given the overall
memory budget.

Cgroup accounting, limiting and OOM enforcement is still there for the
socket buffers, so misbehaving groups will be contained either way.

What do you think? Something like the below patch?

---

>From 67757f78d8b4412b72fe1583ebaf13cfd0fc03b0 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <[email-masked]>
Date: Tue, 22 Nov 2022 14:40:50 -0500
Subject: [PATCH] Revert "mm: memcontrol: hook up vmpressure to socket
 pressure"

This reverts commit 8e8ae645249b85c8ed6c178557f8db8613a6bcc7.

NOT-Signed-off-by: Johannes Weiner <[email-masked]>
---
 include/linux/memcontrol.h |  6 ++--
 include/linux/vmpressure.h |  7 ++---
 mm/memcontrol.c            | 19 +++++++++----
 mm/vmpressure.c            | 58 ++++++--------------------------------
 mm/vmscan.c                | 15 +---------
 5 files changed, 29 insertions(+), 76 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e1644a24009c..e7369363d4c2 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -283,11 +283,11 @@ struct mem_cgroup {
 	atomic_long_t		memory_events[MEMCG_NR_MEMORY_EVENTS];
 	atomic_long_t		memory_events_local[MEMCG_NR_MEMORY_EVENTS];
 
+	/* Socket memory allocations have failed */
 	unsigned long		socket_pressure;
 
 	/* Legacy tcp memory accounting */
 	bool			tcpmem_active;
-	int			tcpmem_pressure;
 
 #ifdef CONFIG_MEMCG_KMEM
 	int kmemcg_id;
@@ -1701,10 +1701,10 @@ void mem_cgroup_sk_alloc(struct sock *sk);
 void mem_cgroup_sk_free(struct sock *sk);
 static inline bool mem_cgroup_under_socket_pressure(struct mem_cgroup *memcg)
 {
-	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && memcg->tcpmem_pressure)
+	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && memcg->socket_pressure)
 		return true;
 	do {
-		if (time_before(jiffies, READ_ONCE(memcg->socket_pressure)))
+		if (memcg->socket_pressure)
 			return true;
 	} while ((memcg = parent_mem_cgroup(memcg)));
 	return false;
diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
index 6a2f51ebbfd3..20d93de37a17 100644
--- a/include/linux/vmpressure.h
+++ b/include/linux/vmpressure.h
@@ -11,9 +11,6 @@
 #include <linux/eventfd.h>
 
 struct vmpressure {
-	unsigned long scanned;
-	unsigned long reclaimed;
-
 	unsigned long tree_scanned;
 	unsigned long tree_reclaimed;
 	/* The lock is used to keep the scanned/reclaimed above in sync. */
@@ -30,7 +27,7 @@ struct vmpressure {
 struct mem_cgroup;
 
 #ifdef CONFIG_MEMCG
-extern void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
+extern void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
 		       unsigned long scanned, unsigned long reclaimed);
 extern void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio);
 
@@ -44,7 +41,7 @@ extern int vmpressure_register_event(struct mem_cgroup *memcg,
 extern void vmpressure_unregister_event(struct mem_cgroup *memcg,
 					struct eventfd_ctx *eventfd);
 #else
-static inline void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
+static inline void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
 			      unsigned long scanned, unsigned long reclaimed) {}
 static inline void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg,
 				   int prio) {}
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2d8549ae1b30..066166aebbef 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7195,10 +7195,10 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages,
 		struct page_counter *fail;
 
 		if (page_counter_try_charge(&memcg->tcpmem, nr_pages, &fail)) {
-			memcg->tcpmem_pressure = 0;
+			memcg->socket_pressure = 0;
 			return true;
 		}
-		memcg->tcpmem_pressure = 1;
+		memcg->socket_pressure = 1;
 		if (gfp_mask & __GFP_NOFAIL) {
 			page_counter_charge(&memcg->tcpmem, nr_pages);
 			return true;
@@ -7206,12 +7206,21 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages,
 		return false;
 	}
 
-	if (try_charge(memcg, gfp_mask, nr_pages) == 0) {
-		mod_memcg_state(memcg, MEMCG_SOCK, nr_pages);
-		return true;
+	if (try_charge(memcg, gfp_mask & ~__GFP_NOFAIL, nr_pages) == 0) {
+		memcg->socket_pressure = 0;
+		goto success;
+	}
+	memcg->socket_pressure = 1;
+	if (gfp_mask & __GFP_NOFAIL) {
+		try_charge(memcg, gfp_mask, nr_pages);
+		goto success;
 	}
 
 	return false;
+
+success:
+	mod_memcg_state(memcg, MEMCG_SOCK, nr_pages);
+	return true;
 }
 
 /**
diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index b52644771cc4..4cec90711cf4 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -219,7 +219,6 @@ static void vmpressure_work_fn(struct work_struct *work)
  * 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
  *
@@ -227,16 +226,9 @@ static void vmpressure_work_fn(struct work_struct *work)
  * "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,
+void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
 		unsigned long scanned, unsigned long reclaimed)
 {
 	struct vmpressure *vmpr;
@@ -271,46 +263,14 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
 	if (!scanned)
 		return;
 
-	if (tree) {
-		spin_lock(&vmpr->sr_lock);
-		scanned = vmpr->tree_scanned += scanned;
-		vmpr->tree_reclaimed += reclaimed;
-		spin_unlock(&vmpr->sr_lock);
-
-		if (scanned < vmpressure_win)
-			return;
-		schedule_work(&vmpr->work);
-	} else {
-		enum vmpressure_levels level;
-
-		/* For now, no users for root-level efficiency */
-		if (!memcg || mem_cgroup_is_root(memcg))
-			return;
-
-		spin_lock(&vmpr->sr_lock);
-		scanned = vmpr->scanned += scanned;
-		reclaimed = vmpr->reclaimed += reclaimed;
-		if (scanned < vmpressure_win) {
-			spin_unlock(&vmpr->sr_lock);
-			return;
-		}
-		vmpr->scanned = vmpr->reclaimed = 0;
-		spin_unlock(&vmpr->sr_lock);
+	spin_lock(&vmpr->sr_lock);
+	scanned = vmpr->tree_scanned += scanned;
+	vmpr->tree_reclaimed += reclaimed;
+	spin_unlock(&vmpr->sr_lock);
 
-		level = vmpressure_calc_level(scanned, reclaimed);
-
-		if (level > VMPRESSURE_LOW) {
-			/*
-			 * Let the socket buffer allocator know that
-			 * we are having trouble reclaiming LRU pages.
-			 *
-			 * For hysteresis keep the pressure state
-			 * asserted for a second in which subsequent
-			 * pressure events can occur.
-			 */
-			WRITE_ONCE(memcg->socket_pressure, jiffies + HZ);
-		}
-	}
+	if (scanned < vmpressure_win)
+		return;
+	schedule_work(&vmpr->work);
 }
 
 /**
@@ -340,7 +300,7 @@ void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio)
 	 * to the vmpressure() basically means that we signal 'critical'
 	 * level.
 	 */
-	vmpressure(gfp, memcg, true, vmpressure_win, 0);
+	vmpressure(gfp, memcg, vmpressure_win, 0);
 }
 
 #define MAX_VMPRESSURE_ARGS_LEN	(strlen("critical") + strlen("hierarchy") + 2)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 04d8b88e5216..d348366d58d4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6035,8 +6035,6 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 	memcg = mem_cgroup_iter(target_memcg, NULL, NULL);
 	do {
 		struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
-		unsigned long reclaimed;
-		unsigned long scanned;
 
 		/*
 		 * This loop can become CPU-bound when target memcgs
@@ -6068,20 +6066,9 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 			memcg_memory_event(memcg, MEMCG_LOW);
 		}
 
-		reclaimed = sc->nr_reclaimed;
-		scanned = sc->nr_scanned;
-
 		shrink_lruvec(lruvec, sc);
-
 		shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
 			    sc->priority);
-
-		/* Record the group's reclaim efficiency */
-		if (!sc->proactive)
-			vmpressure(sc->gfp_mask, memcg, false,
-				   sc->nr_scanned - scanned,
-				   sc->nr_reclaimed - reclaimed);
-
 	} while ((memcg = mem_cgroup_iter(target_memcg, memcg, NULL)));
 }
 
@@ -6111,7 +6098,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 
 	/* Record the subtree's reclaim efficiency */
 	if (!sc->proactive)
-		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
+		vmpressure(sc->gfp_mask, sc->target_mem_cgroup,
 			   sc->nr_scanned - nr_scanned,
 			   sc->nr_reclaimed - nr_reclaimed);
 
-- 
2.38.1