* [PATCH v2 1/2] tracing: Non-consuming read for trace remotes with an offline CPU
2026-04-01 4:50 [PATCH v2 0/2] Fix trace remotes read with an offline CPU Vincent Donnefort
@ 2026-04-01 4:50 ` Vincent Donnefort
2026-04-01 4:51 ` [PATCH v2 2/2] tracing: selftests: Extend hotplug testing for trace remotes Vincent Donnefort
2026-04-02 13:37 ` [PATCH v2 0/2] Fix trace remotes read with an offline CPU Marc Zyngier
2 siblings, 0 replies; 4+ messages in thread
From: Vincent Donnefort @ 2026-04-01 4:50 UTC (permalink / raw)
To: rostedt, mhiramat, mathieu.desnoyers, linux-trace-kernel, maz
Cc: kernel-team, linux-kernel, Vincent Donnefort
When a trace_buffer is created while a CPU is offline, this CPU is
cleared from the trace_buffer CPU mask, preventing the creation of a
non-consuming iterator (ring_buffer_iter). For trace remotes, it means
the iterator fails to be allocated (-ENOMEM) even though there are
available ring buffers in the trace_buffer.
For non-consuming reads of trace remotes, skip missing ring_buffer_iter
to allow reading the available ring buffers.
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
index 0d78e5f5fe98..d6c3f94d67cd 100644
--- a/kernel/trace/trace_remote.c
+++ b/kernel/trace/trace_remote.c
@@ -282,6 +282,14 @@ static void trace_remote_put(struct trace_remote *remote)
trace_remote_try_unload(remote);
}
+static bool trace_remote_has_cpu(struct trace_remote *remote, int cpu)
+{
+ if (cpu == RING_BUFFER_ALL_CPUS)
+ return true;
+
+ return ring_buffer_poll_remote(remote->trace_buffer, cpu) == 0;
+}
+
static void __poll_remote(struct work_struct *work)
{
struct delayed_work *dwork = to_delayed_work(work);
@@ -324,6 +332,10 @@ static int __alloc_ring_buffer_iter(struct trace_remote_iterator *iter, int cpu)
iter->rb_iters[cpu] = ring_buffer_read_start(iter->remote->trace_buffer, cpu,
GFP_KERNEL);
if (!iter->rb_iters[cpu]) {
+ /* This CPU isn't part of trace_buffer. Skip it */
+ if (!trace_remote_has_cpu(iter->remote, cpu))
+ continue;
+
__free_ring_buffer_iter(iter, RING_BUFFER_ALL_CPUS);
return -ENOMEM;
}
@@ -347,10 +359,10 @@ static struct trace_remote_iterator
if (ret)
return ERR_PTR(ret);
- /* Test the CPU */
- ret = ring_buffer_poll_remote(remote->trace_buffer, cpu);
- if (ret)
+ if (!trace_remote_has_cpu(remote, cpu)) {
+ ret = -ENODEV;
goto err;
+ }
iter = kzalloc_obj(*iter);
if (iter) {
@@ -361,6 +373,7 @@ static struct trace_remote_iterator
switch (type) {
case TRI_CONSUMING:
+ ring_buffer_poll_remote(remote->trace_buffer, cpu);
INIT_DELAYED_WORK(&iter->poll_work, __poll_remote);
schedule_delayed_work(&iter->poll_work, msecs_to_jiffies(remote->poll_ms));
break;
@@ -476,6 +489,9 @@ __peek_event(struct trace_remote_iterator *iter, int cpu, u64 *ts, unsigned long
return ring_buffer_peek(iter->remote->trace_buffer, cpu, ts, lost_events);
case TRI_NONCONSUMING:
rb_iter = __get_rb_iter(iter, cpu);
+ if (!rb_iter)
+ return NULL;
+
rb_evt = ring_buffer_iter_peek(rb_iter, ts);
if (!rb_evt)
return NULL;
--
2.53.0.1118.gaef5881109-goog
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v2 2/2] tracing: selftests: Extend hotplug testing for trace remotes
2026-04-01 4:50 [PATCH v2 0/2] Fix trace remotes read with an offline CPU Vincent Donnefort
2026-04-01 4:50 ` [PATCH v2 1/2] tracing: Non-consuming read for trace remotes " Vincent Donnefort
@ 2026-04-01 4:51 ` Vincent Donnefort
2026-04-02 13:37 ` [PATCH v2 0/2] Fix trace remotes read with an offline CPU Marc Zyngier
2 siblings, 0 replies; 4+ messages in thread
From: Vincent Donnefort @ 2026-04-01 4:51 UTC (permalink / raw)
To: rostedt, mhiramat, mathieu.desnoyers, linux-trace-kernel, maz
Cc: kernel-team, linux-kernel, Vincent Donnefort
The hotplug testing only tries reading a trace remote buffer, loaded
before a CPU is offline. Extend this testing to cover:
* A trace remote buffer loaded after a CPU is offline.
* A trace remote buffer loaded before a CPU is online.
Because of these added test cases, move the hotplug testing into a
separate hotplug.tc file.
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
diff --git a/tools/testing/selftests/ftrace/test.d/remotes/functions b/tools/testing/selftests/ftrace/test.d/remotes/functions
index 97a09d564a34..05224fac3653 100644
--- a/tools/testing/selftests/ftrace/test.d/remotes/functions
+++ b/tools/testing/selftests/ftrace/test.d/remotes/functions
@@ -24,12 +24,21 @@ setup_remote_test()
assert_loaded()
{
- grep -q "(loaded)" buffer_size_kb
+ grep -q "(loaded)" buffer_size_kb || return 1
}
assert_unloaded()
{
- grep -q "(unloaded)" buffer_size_kb
+ grep -q "(unloaded)" buffer_size_kb || return 1
+}
+
+reload_remote()
+{
+ echo 0 > tracing_on
+ clear_trace
+ assert_unloaded
+ echo 1 > tracing_on
+ assert_loaded
}
dump_trace_pipe()
@@ -79,10 +88,12 @@ get_cpu_ids()
sed -n 's/^processor\s*:\s*\([0-9]\+\).*/\1/p' /proc/cpuinfo
}
-get_page_size() {
+get_page_size()
+{
sed -ne 's/^.*data.*size:\([0-9][0-9]*\).*/\1/p' events/header_page
}
-get_selftest_event_size() {
+get_selftest_event_size()
+{
sed -ne 's/^.*field:.*;.*size:\([0-9][0-9]*\);.*/\1/p' events/*/selftest/format | awk '{s+=$1} END {print s}'
}
diff --git a/tools/testing/selftests/ftrace/test.d/remotes/hotplug.tc b/tools/testing/selftests/ftrace/test.d/remotes/hotplug.tc
new file mode 100644
index 000000000000..145617eb8061
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/remotes/hotplug.tc
@@ -0,0 +1,88 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: Test trace remote read with an offline CPU
+# requires: remotes/test
+
+. $TEST_DIR/remotes/functions
+
+hotunplug_one_cpu()
+{
+ [ "$(get_cpu_ids | wc -l)" -ge 2 ] || return 1
+
+ for cpu in $(get_cpu_ids); do
+ echo 0 > /sys/devices/system/cpu/cpu$cpu/online || return 1
+ break
+ done
+
+ echo $cpu
+}
+
+# Check non-consuming and consuming read
+check_read()
+{
+ for i in $(seq 1 8); do
+ echo $i > write_event
+ done
+
+ check_trace 1 8 trace
+
+ output=$(dump_trace_pipe)
+ check_trace 1 8 $output
+ rm $output
+}
+
+test_hotplug()
+{
+ echo 0 > trace
+ assert_loaded
+
+ #
+ # Test a trace buffer containing an offline CPU
+ #
+
+ cpu=$(hotunplug_one_cpu) || exit_unsupported
+ trap "echo 1 > /sys/devices/system/cpu/cpu$cpu/online" EXIT
+
+ check_read
+
+ #
+ # Test a trace buffer with a missing CPU
+ #
+
+ reload_remote
+
+ check_read
+
+ #
+ # Test a trace buffer with a CPU added later
+ #
+
+ echo 1 > /sys/devices/system/cpu/cpu$cpu/online
+ trap "" EXIT
+ assert_loaded
+
+ check_read
+
+ # Test if the ring-buffer for the newly added CPU is both writable and
+ # readable
+ for i in $(seq 1 8); do
+ taskset -c $cpu echo $i > write_event
+ done
+
+ cd per_cpu/cpu$cpu/
+
+ check_trace 1 8 trace
+
+ output=$(dump_trace_pipe)
+ check_trace 1 8 $output
+ rm $output
+
+ cd -
+}
+
+if [ -z "$SOURCE_REMOTE_TEST" ]; then
+ set -e
+
+ setup_remote_test
+ test_hotplug
+fi
diff --git a/tools/testing/selftests/ftrace/test.d/remotes/hypervisor/hotplug.tc b/tools/testing/selftests/ftrace/test.d/remotes/hypervisor/hotplug.tc
new file mode 100644
index 000000000000..580ec32c8f81
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/remotes/hypervisor/hotplug.tc
@@ -0,0 +1,11 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: Test hypervisor trace read with an offline CPU
+# requires: remotes/hypervisor/write_event
+
+SOURCE_REMOTE_TEST=1
+. $TEST_DIR/remotes/hotplug.tc
+
+set -e
+setup_remote "hypervisor"
+test_hotplug
diff --git a/tools/testing/selftests/ftrace/test.d/remotes/trace.tc b/tools/testing/selftests/ftrace/test.d/remotes/trace.tc
index 170f7648732a..bc9377a70e8d 100644
--- a/tools/testing/selftests/ftrace/test.d/remotes/trace.tc
+++ b/tools/testing/selftests/ftrace/test.d/remotes/trace.tc
@@ -58,11 +58,7 @@ test_trace()
#
# Ensure the writer is not on the reader page by reloading the buffer
- echo 0 > tracing_on
- echo 0 > trace
- assert_unloaded
- echo 1 > tracing_on
- assert_loaded
+ reload_remote
# Ensure ring-buffer overflow by emitting events from the same CPU
for cpu in $(get_cpu_ids); do
@@ -96,27 +92,6 @@ test_trace()
cd - > /dev/null
done
-
- #
- # Test with hotplug
- #
-
- [ "$(get_cpu_ids | wc -l)" -ge 2 ] || return 0
-
- echo 0 > trace
-
- for cpu in $(get_cpu_ids); do
- echo 0 > /sys/devices/system/cpu/cpu$cpu/online || return 0
- break
- done
-
- for i in $(seq 1 8); do
- echo $i > write_event
- done
-
- check_trace 1 8 trace
-
- echo 1 > /sys/devices/system/cpu/cpu$cpu/online
}
if [ -z "$SOURCE_REMOTE_TEST" ]; then
diff --git a/tools/testing/selftests/ftrace/test.d/remotes/trace_pipe.tc b/tools/testing/selftests/ftrace/test.d/remotes/trace_pipe.tc
index 669a7288ed7c..7f7b7b79c490 100644
--- a/tools/testing/selftests/ftrace/test.d/remotes/trace_pipe.tc
+++ b/tools/testing/selftests/ftrace/test.d/remotes/trace_pipe.tc
@@ -92,31 +92,6 @@ test_trace_pipe()
rm $output
cd - > /dev/null
done
-
- #
- # Test interaction with hotplug
- #
-
- [ "$(get_cpu_ids | wc -l)" -ge 2 ] || return 0
-
- echo 0 > trace
-
- for cpu in $(get_cpu_ids); do
- echo 0 > /sys/devices/system/cpu/cpu$cpu/online || return 0
- break
- done
-
- for i in $(seq 1 8); do
- echo $i > write_event
- done
-
- output=$(dump_trace_pipe)
-
- check_trace 1 8 $output
-
- rm $output
-
- echo 1 > /sys/devices/system/cpu/cpu$cpu/online
}
if [ -z "$SOURCE_REMOTE_TEST" ]; then
--
2.53.0.1118.gaef5881109-goog
^ permalink raw reply related [flat|nested] 4+ messages in thread* Re: [PATCH v2 0/2] Fix trace remotes read with an offline CPU
2026-04-01 4:50 [PATCH v2 0/2] Fix trace remotes read with an offline CPU Vincent Donnefort
2026-04-01 4:50 ` [PATCH v2 1/2] tracing: Non-consuming read for trace remotes " Vincent Donnefort
2026-04-01 4:51 ` [PATCH v2 2/2] tracing: selftests: Extend hotplug testing for trace remotes Vincent Donnefort
@ 2026-04-02 13:37 ` Marc Zyngier
2 siblings, 0 replies; 4+ messages in thread
From: Marc Zyngier @ 2026-04-02 13:37 UTC (permalink / raw)
To: rostedt, mhiramat, mathieu.desnoyers, linux-trace-kernel,
Vincent Donnefort
Cc: kernel-team, linux-kernel
On Wed, 01 Apr 2026 05:50:58 +0100, Vincent Donnefort wrote:
> This small series is fixing non-consuming read of a trace remote when the
> trace_buffer is created after a CPU is offline.
>
> It also extends hotplug testing coverage to include this test case.
>
> I have based this series on top of kvmarm/next which contains the hypervisor
> tracing patches.
>
> [...]
Applied to next, thanks!
[1/2] tracing: Non-consuming read for trace remotes with an offline CPU
commit: ce47b798ed1e44a6ae2c2966cdf7cba6b428083e
[2/2] tracing: selftests: Extend hotplug testing for trace remotes
commit: ec07906bdc52848bd7dc93d1d44e642dcdc7a15a
Cheers,
M.
--
Without deviation from the norm, progress is not possible.
^ permalink raw reply [flat|nested] 4+ messages in thread