[PATCH v1 2/2] mm: memcg: introduce new event to trace shrink_memcg

[Dmitry Rokosov <ddrokosov@salutedevices.com>]

The shrink_memcg flow plays a crucial role in memcg reclamation.
Currently, it is not possible to trace this point from non-direct
reclaim paths. However, direct reclaim has its own tracepoint, so there
is no issue there. In certain cases, when debugging memcg pressure,
developers may need to identify all potential requests for memcg
reclamation including kswapd(). The patchset introduces the tracepoints
mm_vmscan_memcg_shrink_{begin|end}() to address this problem.

Example of output in the kswapd context (non-direct reclaim):
    kswapd0-39      [001] .....   240.356378: mm_vmscan_memcg_shrink_begin: memcg=test order=0 gfp_flags=GFP_KERNEL
    kswapd0-39      [001] .....   240.356396: mm_vmscan_memcg_shrink_end: memcg=test nr_reclaimed=0
    kswapd0-39      [001] .....   240.356420: mm_vmscan_memcg_shrink_begin: memcg=test oorder=0 gfp_flags=GFP_KERNEL
    kswapd0-39      [001] .....   240.356454: mm_vmscan_memcg_shrink_end: memcg=test nr_reclaimed=1
    kswapd0-39      [001] .....   240.356479: mm_vmscan_memcg_shrink_begin: memcg=test oorder=0 gfp_flags=GFP_KERNEL
    kswapd0-39      [001] .....   240.356506: mm_vmscan_memcg_shrink_end: memcg=test nr_reclaimed=4
    kswapd0-39      [001] .....   240.356525: mm_vmscan_memcg_shrink_begin: memcg=test oorder=0 gfp_flags=GFP_KERNEL
    kswapd0-39      [001] .....   240.356593: mm_vmscan_memcg_shrink_end: memcg=test nr_reclaimed=11
    kswapd0-39      [001] .....   240.356614: mm_vmscan_memcg_shrink_begin: memcg=test oorder=0 gfp_flags=GFP_KERNEL
    kswapd0-39      [001] .....   240.356738: mm_vmscan_memcg_shrink_end: memcg=test nr_reclaimed=25
    kswapd0-39      [001] .....   240.356790: mm_vmscan_memcg_shrink_begin: memcg=test oorder=0 gfp_flags=GFP_KERNEL
    kswapd0-39      [001] .....   240.357125: mm_vmscan_memcg_shrink_end: memcg=test nr_reclaimed=53

Signed-off-by: Dmitry Rokosov <ddrokosov@salutedevices.com>
---
 include/trace/events/vmscan.h | 14 ++++++++++++++
 mm/vmscan.c                   |  7 +++++++
 2 files changed, 21 insertions(+)

diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
index 124bc22866c8..518e7232c9eb 100644
--- a/include/trace/events/vmscan.h
+++ b/include/trace/events/vmscan.h
@@ -182,6 +182,13 @@ DEFINE_EVENT(mm_vmscan_memcg_reclaim_begin_template, mm_vmscan_memcg_softlimit_r
 	TP_ARGS(memcg, order, gfp_flags)
 );
 
+DEFINE_EVENT(mm_vmscan_memcg_reclaim_begin_template, mm_vmscan_memcg_shrink_begin,
+
+	TP_PROTO(const struct mem_cgroup *memcg, int order, gfp_t gfp_flags),
+
+	TP_ARGS(memcg, order, gfp_flags)
+);
+
 #endif /* CONFIG_MEMCG */
 
 DECLARE_EVENT_CLASS(mm_vmscan_direct_reclaim_end_template,
@@ -247,6 +254,13 @@ DEFINE_EVENT(mm_vmscan_memcg_reclaim_end_template, mm_vmscan_memcg_softlimit_rec
 	TP_ARGS(memcg, nr_reclaimed)
 );
 
+DEFINE_EVENT(mm_vmscan_memcg_reclaim_end_template, mm_vmscan_memcg_shrink_end,
+
+	TP_PROTO(const struct mem_cgroup *memcg, unsigned long nr_reclaimed),
+
+	TP_ARGS(memcg, nr_reclaimed)
+);
+
 #endif /* CONFIG_MEMCG */
 
 TRACE_EVENT(mm_shrink_slab_start,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4309eaf188b4..6b9619922dfb 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6461,6 +6461,10 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 		 */
 		cond_resched();
 
+		trace_mm_vmscan_memcg_shrink_begin(memcg,
+						   sc->order,
+						   sc->gfp_mask);
+
 		mem_cgroup_calculate_protection(target_memcg, memcg);
 
 		if (mem_cgroup_below_min(target_memcg, memcg)) {
@@ -6491,6 +6495,9 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 		shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
 			    sc->priority);
 
+		trace_mm_vmscan_memcg_shrink_end(memcg,
+						 sc->nr_reclaimed - reclaimed);
+
 		/* Record the group's reclaim efficiency */
 		if (!sc->proactive)
 			vmpressure(sc->gfp_mask, memcg, false,
-- 
2.25.1