Re: [PATCH v5 2/2] mm: add swapiness= arg to memory.reclaim

[Michal Hocko]

On Wed 20-12-23 07:26:51, Dan Schatzberg wrote:
> Allow proactive reclaimers to submit an additional swappiness=<val>
> argument to memory.reclaim. This overrides the global or per-memcg
> swappiness setting for that reclaim attempt.
> 
> For example:
> 
> echo "2M swappiness=0" > /sys/fs/cgroup/memory.reclaim
> 
> will perform reclaim on the rootcg with a swappiness setting of 0 (no
> swap) regardless of the vm.swappiness sysctl setting.
> 
> Userspace proactive reclaimers use the memory.reclaim interface to
> trigger reclaim. The memory.reclaim interface does not allow for any way
> to effect the balance of file vs anon during proactive reclaim. The only
> approach is to adjust the vm.swappiness setting. However, there are a
> few reasons we look to control the balance of file vs anon during
> proactive reclaim, separately from reactive reclaim:
> 
> * Swapout should be limited to manage SSD write endurance. In near-OOM
> situations we are fine with lots of swap-out to avoid OOMs. As these are
> typically rare events, they have relatively little impact on write
> endurance. However, proactive reclaim runs continuously and so its
> impact on SSD write endurance is more significant. Therefore it is
> desireable to control swap-out for proactive reclaim separately from
> reactive reclaim
> 
> * Some userspace OOM killers like systemd-oomd[1] support OOM killing on
> swap exhaustion. This makes sense if the swap exhaustion is triggered
> due to reactive reclaim but less so if it is triggered due to proactive
> reclaim (e.g. one could see OOMs when free memory is ample but anon is
> just particularly cold). Therefore, it's desireable to have proactive
> reclaim reduce or stop swap-out before the threshold at which OOM
> killing occurs.
> 
> In the case of Meta's Senpai proactive reclaimer, we adjust
> vm.swappiness before writes to memory.reclaim[2]. This has been in
> production for nearly two years and has addressed our needs to control
> proactive vs reactive reclaim behavior but is still not ideal for a
> number of reasons:
> 
> * vm.swappiness is a global setting, adjusting it can race/interfere
> with other system administration that wishes to control vm.swappiness.
> In our case, we need to disable Senpai before adjusting vm.swappiness.
> 
> * vm.swappiness is stateful - so a crash or restart of Senpai can leave
> a misconfigured setting. This requires some additional management to
> record the "desired" setting and ensure Senpai always adjusts to it.
> 
> With this patch, we avoid these downsides of adjusting vm.swappiness
> globally.

Thank you for extending the changelog with usecases!

> [1]https://www.freedesktop.org/software/systemd/man/latest/systemd-oomd.service.html
> [2]https://github.com/facebookincubator/oomd/blob/main/src/oomd/plugins/Senpai.cpp#L585-L598
> 
> Signed-off-by: Dan Schatzberg <schatzberg.dan@xxxxxxxxx>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 18 ++++----
>  include/linux/swap.h                    |  3 +-
>  mm/memcontrol.c                         | 56 ++++++++++++++++++++-----
>  mm/vmscan.c                             | 13 +++++-
>  4 files changed, 69 insertions(+), 21 deletions(-)

LGTM
Acked-by: Michal Hocko <mhocko@xxxxxxxx.

Just one minor thing. It would be really great to prevent from potential
incorrect use of mem_cgroup_swappiness. This should be internal function
to memcg. Now, having scan_control internal to vmscan.c makes that
harder and moving it out to swap.h or internal.h sounds overreaching.

We could do this at least to reduce those mistakes. I can make it a
proper patch if this seems reasonable or you can fold it into your patch
directly.
--- 

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f98dff23b758..5f3a182e9515 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -92,8 +92,10 @@ struct scan_control {
 	unsigned long	anon_cost;
 	unsigned long	file_cost;
 
-	/* Swappiness value for reclaim. NULL will fall back to per-memcg/global value */
+#ifdef CONFIG_MEMCG
+	/* Swappiness value for reclaim. Always use sc_swappiness()! */
 	int *swappiness;
+#endif
 
 	/* Can active folios be deactivated as part of reclaim? */
 #define DEACTIVATE_ANON 1
@@ -230,6 +232,13 @@ static bool writeback_throttling_sane(struct scan_control *sc)
 #endif
 	return false;
 }
+
+static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
+{
+	if (sc->swappiness)
+		return *sc->swappiness;
+	return mem_cgroup_swappiness(memcg);
+}
 #else
 static bool cgroup_reclaim(struct scan_control *sc)
 {
@@ -245,6 +254,10 @@ static bool writeback_throttling_sane(struct scan_control *sc)
 {
 	return true;
 }
+static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
+{
+	return READ_ONCE(vm_swappiness);
+}
 #endif
 
 static void set_task_reclaim_state(struct task_struct *task,
@@ -2330,8 +2343,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 	unsigned long anon_cost, file_cost, total_cost;
-	int swappiness = sc->swappiness ?
-		*sc->swappiness : mem_cgroup_swappiness(memcg);
+	int swappiness = sc_swappiness(sc, memcg);
 	u64 fraction[ANON_AND_FILE];
 	u64 denominator = 0;	/* gcc */
 	enum scan_balance scan_balance;
@@ -2612,10 +2624,7 @@ static int get_swappiness(struct lruvec *lruvec, struct scan_control *sc)
 	    mem_cgroup_get_nr_swap_pages(memcg) < MIN_LRU_BATCH)
 		return 0;
 
-	if (sc->swappiness)
-		return *sc->swappiness;
-
-	return mem_cgroup_swappiness(memcg);
+	return sc_swappiness(sc, memcg);
 }
 
 static int get_nr_gens(struct lruvec *lruvec, int type)
-- 
Michal Hocko
SUSE Labs