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Date:   Mon, 13 Jul 2020 12:42:35 +0100
From:   Chris Down <chris@chrisdown.name>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: [PATCH v2 1/2] mm, memcg: reclaim more aggressively before high
 allocator throttling
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In Facebook production, we've seen cases where cgroups have been put
into allocator throttling even when they appear to have a lot of slack
file caches which should be trivially reclaimable.

Looking more closely, the problem is that we only try a single cgroup
reclaim walk for each return to usermode before calculating whether or
not we should throttle. This single attempt doesn't produce enough
pressure to shrink for cgroups with a rapidly growing amount of file
caches prior to entering allocator throttling.

As an example, we see that threads in an affected cgroup are stuck in
allocator throttling:

    # for i in $(cat cgroup.threads); do
    >     grep over_high "/proc/$i/stack"
    > done
    [<0>] mem_cgroup_handle_over_high+0x10b/0x150
    [<0>] mem_cgroup_handle_over_high+0x10b/0x150
    [<0>] mem_cgroup_handle_over_high+0x10b/0x150

...however, there is no I/O pressure reported by PSI, despite a lot of
slack file pages:

    # cat memory.pressure
    some avg10=78.50 avg60=84.99 avg300=84.53 total=5702440903
    full avg10=78.50 avg60=84.99 avg300=84.53 total=5702116959
    # cat io.pressure
    some avg10=0.00 avg60=0.00 avg300=0.00 total=78051391
    full avg10=0.00 avg60=0.00 avg300=0.00 total=78049640
    # grep _file memory.stat
    inactive_file 1370939392
    active_file 661635072

This patch changes the behaviour to retry reclaim either until the
current task goes below the 10ms grace period, or we are making no
reclaim progress at all. In the latter case, we enter reclaim throttling
as before.

To a user, there's no intuitive reason for the reclaim behaviour to
differ from hitting memory.high as part of a new allocation, as opposed
to hitting memory.high because someone lowered its value. As such this
also brings an added benefit: it unifies the reclaim behaviour between
the two.

There's precedent for this behaviour: we already do reclaim retries when
writing to memory.{high,max}, in max reclaim, and in the page allocator
itself.

Signed-off-by: Chris Down <chris@chrisdown.name>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
---
 mm/memcontrol.c | 42 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0145a77aa074..d4b0d8af3747 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -73,6 +73,7 @@ EXPORT_SYMBOL(memory_cgrp_subsys);
 
 struct mem_cgroup *root_mem_cgroup __read_mostly;
 
+/* The number of times we should retry reclaim failures before giving up. */
 #define MEM_CGROUP_RECLAIM_RETRIES	5
 
 /* Socket memory accounting disabled? */
@@ -2365,18 +2366,23 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
 	return 0;
 }
 
-static void reclaim_high(struct mem_cgroup *memcg,
-			 unsigned int nr_pages,
-			 gfp_t gfp_mask)
+static unsigned long reclaim_high(struct mem_cgroup *memcg,
+				  unsigned int nr_pages,
+				  gfp_t gfp_mask)
 {
+	unsigned long nr_reclaimed = 0;
+
 	do {
 		if (page_counter_read(&memcg->memory) <=
 		    READ_ONCE(memcg->memory.high))
 			continue;
 		memcg_memory_event(memcg, MEMCG_HIGH);
-		try_to_free_mem_cgroup_pages(memcg, nr_pages, gfp_mask, true);
+		nr_reclaimed += try_to_free_mem_cgroup_pages(memcg, nr_pages,
+							     gfp_mask, true);
 	} while ((memcg = parent_mem_cgroup(memcg)) &&
 		 !mem_cgroup_is_root(memcg));
+
+	return nr_reclaimed;
 }
 
 static void high_work_func(struct work_struct *work)
@@ -2532,16 +2538,32 @@ void mem_cgroup_handle_over_high(void)
 {
 	unsigned long penalty_jiffies;
 	unsigned long pflags;
+	unsigned long nr_reclaimed;
 	unsigned int nr_pages = current->memcg_nr_pages_over_high;
+	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
 	struct mem_cgroup *memcg;
+	bool in_retry = false;
 
 	if (likely(!nr_pages))
 		return;
 
 	memcg = get_mem_cgroup_from_mm(current->mm);
-	reclaim_high(memcg, nr_pages, GFP_KERNEL);
 	current->memcg_nr_pages_over_high = 0;
 
+retry_reclaim:
+	/*
+	 * The allocating task should reclaim at least the batch size, but for
+	 * subsequent retries we only want to do what's necessary to prevent oom
+	 * or breaching resource isolation.
+	 *
+	 * This is distinct from memory.max or page allocator behaviour because
+	 * memory.high is currently batched, whereas memory.max and the page
+	 * allocator run every time an allocation is made.
+	 */
+	nr_reclaimed = reclaim_high(memcg,
+				    in_retry ? SWAP_CLUSTER_MAX : nr_pages,
+				    GFP_KERNEL);
+
 	/*
 	 * memory.high is breached and reclaim is unable to keep up. Throttle
 	 * allocators proactively to slow down excessive growth.
@@ -2568,6 +2590,16 @@ void mem_cgroup_handle_over_high(void)
 	if (penalty_jiffies <= HZ / 100)
 		goto out;
 
+	/*
+	 * If reclaim is making forward progress but we're still over
+	 * memory.high, we want to encourage that rather than doing allocator
+	 * throttling.
+	 */
+	if (nr_reclaimed || nr_retries--) {
+		in_retry = true;
+		goto retry_reclaim;
+	}
+
 	/*
 	 * If we exit early, we're guaranteed to die (since
 	 * schedule_timeout_killable sets TASK_KILLABLE). This means we don't
-- 
2.27.0