[PATCH] psi: Don't account force reclaim as memory pressure

[PATCH] psi: Don't account force reclaim as memory pressure

[Xunlei Pang]

There're several cases like resize and force_empty that don't
need to account to psi, otherwise is misleading.

We also have a module reclaiming dying memcgs at background to
avoid too many dead memcgs which can cause lots of trouble, then
it makes the psi inaccuracy even worse without this patch.

Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
---
 include/linux/swap.h |  3 ++-
 mm/memcontrol.c      | 13 +++++++------
 mm/vmscan.c          |  9 ++++++---
 3 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 4bfb5c4ac108..74b5443877d4 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -354,7 +354,8 @@ extern int __isolate_lru_page(struct page *page, isolate_mode_t mode);
 extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 						  unsigned long nr_pages,
 						  gfp_t gfp_mask,
-						  bool may_swap);
+						  bool may_swap,
+						  bool force_reclaim);
 extern unsigned long mem_cgroup_shrink_node(struct mem_cgroup *mem,
 						gfp_t gfp_mask, bool noswap,
 						pg_data_t *pgdat,
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f1dfa651f55d..f4ec57876ada 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2237,7 +2237,8 @@ static void reclaim_high(struct mem_cgroup *memcg,
 		if (page_counter_read(&memcg->memory) <= memcg->high)
 			continue;
 		memcg_memory_event(memcg, MEMCG_HIGH);
-		try_to_free_mem_cgroup_pages(memcg, nr_pages, gfp_mask, true);
+		try_to_free_mem_cgroup_pages(memcg, nr_pages,
+				gfp_mask, true, false);
 	} while ((memcg = parent_mem_cgroup(memcg)));
 }
 
@@ -2330,7 +2331,7 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	memcg_memory_event(mem_over_limit, MEMCG_MAX);
 
 	nr_reclaimed = try_to_free_mem_cgroup_pages(mem_over_limit, nr_pages,
-						    gfp_mask, may_swap);
+					 gfp_mask, may_swap, false);
 
 	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
 		goto retry;
@@ -2860,7 +2861,7 @@ static int mem_cgroup_resize_max(struct mem_cgroup *memcg,
 		}
 
 		if (!try_to_free_mem_cgroup_pages(memcg, 1,
-					GFP_KERNEL, !memsw)) {
+					GFP_KERNEL, !memsw, true)) {
 			ret = -EBUSY;
 			break;
 		}
@@ -2993,7 +2994,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
 			return -EINTR;
 
 		progress = try_to_free_mem_cgroup_pages(memcg, 1,
-							GFP_KERNEL, true);
+							GFP_KERNEL, true, true);
 		if (!progress) {
 			nr_retries--;
 			/* maybe some writeback is necessary */
@@ -5549,7 +5550,7 @@ static ssize_t memory_high_write(struct kernfs_open_file *of,
 	nr_pages = page_counter_read(&memcg->memory);
 	if (nr_pages > high)
 		try_to_free_mem_cgroup_pages(memcg, nr_pages - high,
-					     GFP_KERNEL, true);
+					     GFP_KERNEL, true, true);
 
 	memcg_wb_domain_size_changed(memcg);
 	return nbytes;
@@ -5596,7 +5597,7 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
 
 		if (nr_reclaims) {
 			if (!try_to_free_mem_cgroup_pages(memcg, nr_pages - max,
-							  GFP_KERNEL, true))
+						GFP_KERNEL, true, true))
 				nr_reclaims--;
 			continue;
 		}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7acd0afdfc2a..3831848fca5a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3212,7 +3212,8 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
 unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 					   unsigned long nr_pages,
 					   gfp_t gfp_mask,
-					   bool may_swap)
+					   bool may_swap,
+					   bool force_reclaim)
 {
 	struct zonelist *zonelist;
 	unsigned long nr_reclaimed;
@@ -3243,13 +3244,15 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 
 	trace_mm_vmscan_memcg_reclaim_begin(0, sc.gfp_mask);
 
-	psi_memstall_enter(&pflags);
+	if (!force_reclaim)
+		psi_memstall_enter(&pflags);
 	noreclaim_flag = memalloc_noreclaim_save();
 
 	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
 
 	memalloc_noreclaim_restore(noreclaim_flag);
-	psi_memstall_leave(&pflags);
+	if (!force_reclaim)
+		psi_memstall_leave(&pflags);
 
 	trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
 
-- 
2.14.4.44.g2045bb6

Re: [PATCH] psi: Don't account force reclaim as memory pressure

[Chris Down]

Hi Xunlei,

Xunlei Pang writes:
>There're several cases like resize and force_empty that don't
>need to account to psi, otherwise is misleading.

I'm afraid I'm quite confused by this patch. Why do you think accounting for 
force reclaim in PSI is misleading? I completely expect that force reclaim 
should still be accounted for as memory pressure, can you present some reason 
why it shouldn't be?

Re: [PATCH] psi: Don't account force reclaim as memory pressure

[Xunlei Pang]

Hi Chris,

On 2019/6/15 PM 11:58, Chris Down wrote:
> Hi Xunlei,
> 
> Xunlei Pang writes:
>> There're several cases like resize and force_empty that don't
>> need to account to psi, otherwise is misleading.
> 
> I'm afraid I'm quite confused by this patch. Why do you think accounting
> for force reclaim in PSI is misleading? I completely expect that force
> reclaim should still be accounted for as memory pressure, can you
> present some reason why it shouldn't be?

We expect psi stands for negative factors to applications
which affect their response time, but force reclaims are
behaviours triggered on purpose like "/proc/sys/vm/drop_caches",
not the real negative pressure.

e.g. my module force reclaims the dead memcgs, there's no
application attached to it, and its memory(page caches) is
usually useless, force reclaiming them doesn't mean the
system or parent memcg is under memory pressure, while
actually the whole system or the parent memcg has plenty
of free memory. If the force reclaim causes further memory
pressure like hot page cache miss, then the workingset
refault psi will catch that.