[PATCH v2 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=test
    kswapd0-39      [001] .....   240.356396: mm_vmscan_memcg_shrink_end: nr_reclaimed=0 memcg=test
    kswapd0-39      [001] .....   240.356420: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=test
    kswapd0-39      [001] .....   240.356454: mm_vmscan_memcg_shrink_end: nr_reclaimed=1 memcg=test
    kswapd0-39      [001] .....   240.356479: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=test
    kswapd0-39      [001] .....   240.356506: mm_vmscan_memcg_shrink_end: nr_reclaimed=4 memcg=test
    kswapd0-39      [001] .....   240.356525: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=test
    kswapd0-39      [001] .....   240.356593: mm_vmscan_memcg_shrink_end: nr_reclaimed=11 memcg=test
    kswapd0-39      [001] .....   240.356614: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=test
    kswapd0-39      [001] .....   240.356738: mm_vmscan_memcg_shrink_end: nr_reclaimed=25 memcg=test
    kswapd0-39      [001] .....   240.356790: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=test
    kswapd0-39      [001] .....   240.357125: mm_vmscan_memcg_shrink_end: nr_reclaimed=53 memcg=test

Signed-off-by: Dmitry Rokosov <ddrokosov@xxxxxxxxxxxxxxxxx>
---
 include/trace/events/vmscan.h | 14 ++++++++++++++
 mm/vmscan.c                   | 11 +++++++++++
 2 files changed, 25 insertions(+)

diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
index 9b49cd120ae9..cb39b4f0dca9 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(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)
+);
+
 #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(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)
+);
+
 #endif /* CONFIG_MEMCG */
 
 TRACE_EVENT(mm_shrink_slab_start,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 45780952f4b5..6d89b39d9a91 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6461,6 +6461,12 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 		 */
 		cond_resched();
 
+#ifdef CONFIG_MEMCG
+		trace_mm_vmscan_memcg_shrink_begin(sc->order,
+						   sc->gfp_mask,
+						   memcg);
+#endif
+
 		mem_cgroup_calculate_protection(target_memcg, memcg);
 
 		if (mem_cgroup_below_min(target_memcg, memcg)) {
@@ -6491,6 +6497,11 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
 		shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
 			    sc->priority);
 
+#ifdef CONFIG_MEMCG
+		trace_mm_vmscan_memcg_shrink_end(sc->nr_reclaimed - reclaimed,
+						 memcg);
+#endif
+
 		/* Record the group's reclaim efficiency */
 		if (!sc->proactive)
 			vmpressure(sc->gfp_mask, memcg, false,
-- 
2.36.0