[PATCH] mm: vmscan: fix missing psi annotation for node_reclaim()

[PATCH] mm: vmscan: fix missing psi annotation for node_reclaim()

[Johannes Weiner]

In a debugging session the other day, Rik noticed that node_reclaim()
was missing memstall annotations. This means we'll miss pressure and
lost productivity resulting from reclaim on an overloaded local NUMA
node when vm.zone_reclaim_mode is enabled.

There haven't been any reports, but that's likely because
vm.zone_reclaim_mode hasn't been a commonly used feature recently, and
the intersection between such setups and psi users is probably
nil. Although, secondary memory such as CXL-connected DIMMS,
persistent memory etc. and the page demotion patches that handle them
(https://lore.kernel.org/lkml/20210401183216.443C4443@viggo.jf.intel.com/)
could soon make this a more common codepath again.

Reported-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/vmscan.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 701106e1829c..8ec4412c6116 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4424,11 +4424,13 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 		.may_swap = 1,
 		.reclaim_idx = gfp_zone(gfp_mask),
 	};
+	unsigned long pflags;
 
 	trace_mm_vmscan_node_reclaim_begin(pgdat->node_id, order,
 					   sc.gfp_mask);
 
 	cond_resched();
+	psi_memstall_enter(&pflags);
 	fs_reclaim_acquire(sc.gfp_mask);
 	/*
 	 * We need to be able to allocate from the reserves for RECLAIM_UNMAP
@@ -4453,6 +4455,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	current->flags &= ~PF_SWAPWRITE;
 	memalloc_noreclaim_restore(noreclaim_flag);
 	fs_reclaim_release(sc.gfp_mask);
+	psi_memstall_leave(&pflags);
 
 	trace_mm_vmscan_node_reclaim_end(sc.nr_reclaimed);
 
-- 
2.32.0

Re: [PATCH] mm: vmscan: fix missing psi annotation for node_reclaim()

[Shakeel Butt]

On Wed, Aug 18, 2021 at 8:26 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> In a debugging session the other day, Rik noticed that node_reclaim()
> was missing memstall annotations. This means we'll miss pressure and
> lost productivity resulting from reclaim on an overloaded local NUMA
> node when vm.zone_reclaim_mode is enabled.
>
> There haven't been any reports, but that's likely because
> vm.zone_reclaim_mode hasn't been a commonly used feature recently, and
> the intersection between such setups and psi users is probably
> nil. Although, secondary memory such as CXL-connected DIMMS,
> persistent memory etc. and the page demotion patches that handle them
> (https://lore.kernel.org/lkml/20210401183216.443C4443@viggo.jf.intel.com/)
> could soon make this a more common codepath again.
>
> Reported-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Shakeel Butt <shakeelb@google.com>