[PATCH 1/1] sched_ext: Add error logging for dsq creation failures

[PATCH 1/1] sched_ext: Add error logging for dsq creation failures

[George Guo]

From: George Guo <guodongtai@kylinos.cn>

Add scx_bpf_error() calls when scx_bpf_create_dsq() fails in multiple
schedulers to improve debuggability:

- scx_central.bpf.c: central_init()
- scx_flatcg.bpf.c: fcg_cgroup_init() and fcg_init()
- scx_qmap.bpf.c: qmap_init()
- scx_simple.bpf.c: simple_init()

Signed-off-by: George Guo <guodongtai@kylinos.cn>
---
 tools/sched_ext/scx_central.bpf.c |  4 +++-
 tools/sched_ext/scx_flatcg.bpf.c  | 12 ++++++++++--
 tools/sched_ext/scx_qmap.bpf.c    |  8 ++++++--
 tools/sched_ext/scx_simple.bpf.c  |  8 +++++++-
 4 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
index 55df8b798865..1c2376b75b5d 100644
--- a/tools/sched_ext/scx_central.bpf.c
+++ b/tools/sched_ext/scx_central.bpf.c
@@ -301,8 +301,10 @@ int BPF_STRUCT_OPS_SLEEPABLE(central_init)
 	int ret;
 
 	ret = scx_bpf_create_dsq(FALLBACK_DSQ_ID, -1);
-	if (ret)
+	if (ret) {
+		scx_bpf_error("scx_bpf_create_dsq failed (%d)", ret);
 		return ret;
+	}
 
 	timer = bpf_map_lookup_elem(&central_timer, &key);
 	if (!timer)
diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
index 2c720e3ecad5..f7830e14b9af 100644
--- a/tools/sched_ext/scx_flatcg.bpf.c
+++ b/tools/sched_ext/scx_flatcg.bpf.c
@@ -842,8 +842,10 @@ int BPF_STRUCT_OPS_SLEEPABLE(fcg_cgroup_init, struct cgroup *cgrp,
 	 * unlikely case that it breaks.
 	 */
 	ret = scx_bpf_create_dsq(cgid, -1);
-	if (ret)
+	if (ret) {
+		scx_bpf_error("scx_bpf_create_dsq failed (%d)", ret);
 		return ret;
+	}
 
 	cgc = bpf_cgrp_storage_get(&cgrp_ctx, cgrp, 0,
 				   BPF_LOCAL_STORAGE_GET_F_CREATE);
@@ -927,7 +929,13 @@ void BPF_STRUCT_OPS(fcg_cgroup_move, struct task_struct *p,
 
 s32 BPF_STRUCT_OPS_SLEEPABLE(fcg_init)
 {
-	return scx_bpf_create_dsq(FALLBACK_DSQ, -1);
+	s32 ret;
+
+	ret = scx_bpf_create_dsq(FALLBACK_DSQ, -1);
+	if (ret)
+		scx_bpf_error("scx_bpf_create_dsq failed (%d)", ret);
+
+	return ret;
 }
 
 void BPF_STRUCT_OPS(fcg_exit, struct scx_exit_info *ei)
diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index 3072b593f898..16ed8f867ad9 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -852,12 +852,16 @@ s32 BPF_STRUCT_OPS_SLEEPABLE(qmap_init)
 		print_cpus();
 
 	ret = scx_bpf_create_dsq(SHARED_DSQ, -1);
-	if (ret)
+	if (ret) {
+		scx_bpf_error("scx_bpf_create_dsq failed (%d)", ret);
 		return ret;
+	}
 
 	ret = scx_bpf_create_dsq(HIGHPRI_DSQ, -1);
-	if (ret)
+	if (ret) {
+		scx_bpf_error("scx_bpf_create_dsq failed (%d)", ret);
 		return ret;
+	}
 
 	timer = bpf_map_lookup_elem(&monitor_timer, &key);
 	if (!timer)
diff --git a/tools/sched_ext/scx_simple.bpf.c b/tools/sched_ext/scx_simple.bpf.c
index e6de99dba7db..50e4555459de 100644
--- a/tools/sched_ext/scx_simple.bpf.c
+++ b/tools/sched_ext/scx_simple.bpf.c
@@ -131,7 +131,13 @@ void BPF_STRUCT_OPS(simple_enable, struct task_struct *p)
 
 s32 BPF_STRUCT_OPS_SLEEPABLE(simple_init)
 {
-	return scx_bpf_create_dsq(SHARED_DSQ, -1);
+	s32 ret;
+
+	ret = scx_bpf_create_dsq(SHARED_DSQ, -1);
+	if (ret)
+		scx_bpf_error("scx_bpf_create_dsq failed (%d)", ret);
+
+	return ret;
 }
 
 void BPF_STRUCT_OPS(simple_exit, struct scx_exit_info *ei)
-- 
2.43.0

Re: [PATCH 1/1] sched_ext: Add error logging for dsq creation failures

[Emil Tsalapatis]

Hi George,

(sorry for the formatting, my email client has decided not to indent the email)

________________________________________
From: George Guo <dongtai.guo@linux.dev>
Sent: Wednesday, January 7, 2026 1:28 AM
To: tj@kernel.org; void@manifault.com; arighi@nvidia.com; changwoo@igalia.com
Cc: sched-ext@lists.linux.dev; linux-kernel@vger.kernel.org; George Guo
Subject: [PATCH 1/1] sched_ext: Add error logging for dsq creation failures

From: George Guo <guodongtai@kylinos.cn>

Add scx_bpf_error() calls when scx_bpf_create_dsq() fails in multiple
schedulers to improve debuggability:

- scx_central.bpf.c: central_init()
- scx_flatcg.bpf.c: fcg_cgroup_init() and fcg_init()
- scx_qmap.bpf.c: qmap_init()
- scx_simple.bpf.c: simple_init()

Signed-off-by: George Guo <guodongtai@kylinos.cn>
---

While the change looks correct, is there any benefit to it? 

- Failing to create a dsq will already cause init() to fail with the error code 
from the failed call. In scx_simple in particular, the  change adds no new 
info because the function amounts to a single scx_bpf_create_dsq() call.

- Regressions to scx_bpf_dsq_create() from kernel-side (as opposed to
BPF scheduler-side) changes will be caught by the selftests.

Not opposed to it, just curious what the advantages are.

Re: [PATCH 1/1] sched_ext: Add error logging for dsq creation failures

[George Guo]

Hi Etsal,
Thanks for reviewing!

> While the change looks correct, is there any benefit to it?
Yes, definitely. I found this issue while adding sched_ext support for
LoongArch. When scx_bpf_create_dsq() failed during my development, there
were no error messages at all, which made debugging difficult.

Specifically:
1. The failure was silent - no log, no error message in dmesg
2. I had to add bpf_printk() temporarily to confirm it was dsq creation failing

So the benefit is improved debuggability and faster issue diagnosis.

Thanks!

Re: [PATCH 1/1] sched_ext: Add error logging for dsq creation failures

[Andrea Righi]

Hi George,

On Thu, Jan 08, 2026 at 11:23:28AM +0800, George Guo wrote:
> Hi Etsal,
> Thanks for reviewing!
> 
> > While the change looks correct, is there any benefit to it?
> Yes, definitely. I found this issue while adding sched_ext support for
> LoongArch. When scx_bpf_create_dsq() failed during my development, there
> were no error messages at all, which made debugging difficult.
> 
> Specifically:
> 1. The failure was silent - no log, no error message in dmesg

Which kernel is that?

In theory you should get a clear message in dmesg. For example, if I try to
create an invalid DSQ in scx_simple I get this in dmesg:

  [690301.673313] sched_ext: BPF scheduler "simple" disabled (runtime error)
  [690301.673324] sched_ext: simple: ops.init() failed (-22)
  [690301.673327]    scx_enable.isra.0+0xc8b/0x11f0
  [690301.673343]    bpf_struct_ops_link_create+0x141/0x1b0
  [690301.673352]    __sys_bpf+0x1c0c/0x2de0
  [690301.673357]    __x64_sys_bpf+0x21/0x30
  [690301.673359]    do_syscall_64+0x81/0x820
  [690301.673368]    entry_SYSCALL_64_after_hwframe+0x76/0x7e

And in stdout:

DEBUG DUMP
================================================================================

scx_simple[364999] triggered exit kind 1024:
  runtime error (ops.init() failed (-22))

Backtrace:
  scx_enable.isra.0+0xc8b/0x11f0
  bpf_struct_ops_link_create+0x141/0x1b0
  __sys_bpf+0x1c0c/0x2de0
  __x64_sys_bpf+0x21/0x30
  do_syscall_64+0x81/0x820
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

CPU states
----------

CPU 2   : nr_run=0 flags=0x1 cpu_rel=0 ops_qseq=87907495 pnt_seq=66861592
          curr=scx_simple[364999] class=fair_sched_class

CPU 3   : nr_run=0 flags=0x1 cpu_rel=0 ops_qseq=85461608 pnt_seq=64229201
          curr=pacman[364201] class=fair_sched_class

Event counters
--------------
              SCX_EV_SELECT_CPU_FALLBACK:                0
       SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE:                0
               SCX_EV_DISPATCH_KEEP_LAST:                0
                 SCX_EV_ENQ_SKIP_EXITING:                0
      SCX_EV_ENQ_SKIP_MIGRATION_DISABLED:                0
                 SCX_EV_REFILL_SLICE_DFL:                0
                  SCX_EV_BYPASS_DURATION:                0
                  SCX_EV_BYPASS_DISPATCH:                0
                  SCX_EV_BYPASS_ACTIVATE:                0

================================================================================

EXIT: runtime error (ops.init() failed (-22))

> 2. I had to add bpf_printk() temporarily to confirm it was dsq creation failing
> 
> So the benefit is improved debuggability and faster issue diagnosis.
> 
> Thanks!
> 

Thanks,
-Andrea

Re: [PATCH 1/1] sched_ext: Add error logging for dsq creation failures

[George Guo]

Hi Andrea,
I got the similar dmesg too.
It seems that it's not necessry in scx_simple.bpf.c indeed.
I will send v2 later for your review.