linux-kernel - [PATCH 06/46] sched_ext: Make qmap dump operation non-destructive

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250920005931.2753828-7-tj@kernel.org>
Date: Fri, 19 Sep 2025 14:58:29 -1000
From: Tejun Heo <tj@...nel.org>
To: void@...ifault.com,
	arighi@...dia.com,
	multics69@...il.com
Cc: linux-kernel@...r.kernel.org,
	sched-ext@...ts.linux.dev,
	memxor@...il.com,
	bpf@...r.kernel.org,
	Tejun Heo <tj@...nel.org>
Subject: [PATCH 06/46] sched_ext: Make qmap dump operation non-destructive

The qmap dump operation was destructively consuming queue entries while
displaying them. As dump can be triggered anytime, this can easily lead to
stalls. Add a temporary dump_store queue and modify the dump logic to pop
entries, display them, and then restore them back to the original queue.
This allows dump operations to be performed without affecting the
scheduler's queue state.

Note that if racing against new enqueues during dump, ordering can get
mixed up, but this is acceptable for debugging purposes.

Signed-off-by: Tejun Heo <tj@...nel.org>
---
 tools/sched_ext/scx_qmap.bpf.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index 69d877501cb7..cd50a94326e3 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -56,7 +56,8 @@ struct qmap {
   queue1 SEC(".maps"),
   queue2 SEC(".maps"),
   queue3 SEC(".maps"),
-  queue4 SEC(".maps");
+  queue4 SEC(".maps"),
+  dump_store SEC(".maps");
 
 struct {
 	__uint(type, BPF_MAP_TYPE_ARRAY_OF_MAPS);
@@ -578,11 +579,26 @@ void BPF_STRUCT_OPS(qmap_dump, struct scx_dump_ctx *dctx)
 			return;
 
 		scx_bpf_dump("QMAP FIFO[%d]:", i);
+
+		/*
+		 * Dump can be invoked anytime and there is no way to iterate in
+		 * a non-destructive way. Pop and store in dump_store and then
+		 * restore afterwards. If racing against new enqueues, ordering
+		 * can get mixed up.
+		 */
 		bpf_repeat(4096) {
 			if (bpf_map_pop_elem(fifo, &pid))
 				break;
+			bpf_map_push_elem(&dump_store, &pid, 0);
 			scx_bpf_dump(" %d", pid);
 		}
+
+		bpf_repeat(4096) {
+			if (bpf_map_pop_elem(&dump_store, &pid))
+				break;
+			bpf_map_push_elem(fifo, &pid, 0);
+		}
+
 		scx_bpf_dump("\n");
 	}
 }
-- 
2.51.0