[PATCH v3 2/2] mm: memcg: introduce new event to trace shrink_memcg

[Dmitry Rokosov]

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: order=0 gfp_flags=GFP_KERNEL memcg=16
    kswapd0-39      [001] .....   240.356396: mm_vmscan_memcg_shrink_end: nr_reclaimed=0 memcg=16
    kswapd0-39      [001] .....   240.356420: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
    kswapd0-39      [001] .....   240.356454: mm_vmscan_memcg_shrink_end: nr_reclaimed=1 memcg=16
    kswapd0-39      [001] .....   240.356479: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
    kswapd0-39      [001] .....   240.356506: mm_vmscan_memcg_shrink_end: nr_reclaimed=4 memcg=16
    kswapd0-39      [001] .....   240.356525: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
    kswapd0-39      [001] .....   240.356593: mm_vmscan_memcg_shrink_end: nr_reclaimed=11 memcg=16
    kswapd0-39      [001] .....   240.356614: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
    kswapd0-39      [001] .....   240.356738: mm_vmscan_memcg_shrink_end: nr_reclaimed=25 memcg=16
    kswapd0-39      [001] .....   240.356790: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
    kswapd0-39      [001] .....   240.357125: mm_vmscan_memcg_shrink_end: nr_reclaimed=53 memcg=16

Signed-off-by: Dmitry Rokosov <ddrokosov@xxxxxxxxxxxxxxxxx>
---
 include/trace/events/vmscan.h | 22 ++++++++++++++++++++++
 mm/vmscan.c                   |  7 +++++++
 2 files changed, 29 insertions(+)

diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
index e9093fa1c924..a4686afe571d 100644
--- a/include/trace/events/vmscan.h
+++ b/include/trace/events/vmscan.h
@@ -180,6 +180,17 @@ DEFINE_EVENT(mm_vmscan_memcg_reclaim_begin_template, mm_vmscan_memcg_softlimit_r
 	TP_ARGS(order, gfp_flags, memcg)
 );
 
+DEFINE_EVENT(mm_vmscan_memcg_reclaim_begin_template, mm_vmscan_memcg_shrink_begin,
+
+	TP_PROTO(int order, gfp_t gfp_flags, const struct mem_cgroup *memcg),
+
+	TP_ARGS(order, gfp_flags, memcg)
+);
+
+#else
+
+#define trace_mm_vmscan_memcg_shrink_begin(...)
+
 #endif /* CONFIG_MEMCG */
 
 DECLARE_EVENT_CLASS(mm_vmscan_direct_reclaim_end_template,
@@ -243,6 +254,17 @@ DEFINE_EVENT(mm_vmscan_memcg_reclaim_end_template, mm_vmscan_memcg_softlimit_rec
 	TP_ARGS(nr_reclaimed, memcg)
 );
 
+DEFINE_EVENT(mm_vmscan_memcg_reclaim_end_template, mm_vmscan_memcg_shrink_end,
+
+	TP_PROTO(unsigned long nr_reclaimed, const struct mem_cgroup *memcg),
+
+	TP_ARGS(nr_reclaimed, memcg)
+);
+
+#else
+
+#define trace_mm_vmscan_memcg_shrink_end(...)
+
 #endif /* CONFIG_MEMCG */
 
 TRACE_EVENT(mm_shrink_slab_start,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 45780952f4b5..f7e3ddc5a7ad 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(sc->order,
+						   sc->gfp_mask,
+						   memcg);
+
 		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(sc->nr_reclaimed - reclaimed,
+						 memcg);
+
 		/* Record the group's reclaim efficiency */
 		if (!sc->proactive)
 			vmpressure(sc->gfp_mask, memcg, false,
-- 
2.36.0