[PATCH v4 04/17] watchdog/perf: Ensure CPU-bound context when creating hardlockup detector event

[Douglas Anderson]

From: Pingfan Liu <kernelfans at gmail.com>

hardlockup_detector_event_create() should create perf_event on the
current CPU. Preemption could not get disabled because
perf_event_create_kernel_counter() allocates memory. Instead,
the CPU locality is achieved by processing the code in a per-CPU
bound kthread.

Add a check to prevent mistakes when calling the code in another
code path.

Signed-off-by: Pingfan Liu <kernelfans at gmail.com>
Co-developed-by: Lecopzer Chen <lecopzer.chen at mediatek.com>
Signed-off-by: Lecopzer Chen <lecopzer.chen at mediatek.com>
Reviewed-by: Petr Mladek <pmladek at suse.com>
Signed-off-by: Douglas Anderson <dianders at chromium.org>
---
I yanked this patch from the mailing lists [1] into my series just to
make it easier to avoid conflicts between my series and the one adding
the arm64 perf hardlockup detector, in case someone wanted to test
them both together. This is a nice cleanup and could land together
with the rest of my series if that makes sense.

I changed the patch prefix to match others in my series.

[1] https://lore.kernel.org/r/20220903093415.15850-4-lecopzer.chen@mediatek.com/

Changes in v4:
- Pulled ("Ensure CPU-bound context when creating ...") into my series for v4.

 kernel/watchdog_hld.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
index 247bf0b1582c..96b717205952 100644
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c
@@ -165,10 +165,16 @@ static void watchdog_overflow_callback(struct perf_event *event,
 
 static int hardlockup_detector_event_create(void)
 {
-	unsigned int cpu = smp_processor_id();
+	unsigned int cpu;
 	struct perf_event_attr *wd_attr;
 	struct perf_event *evt;
 
+	/*
+	 * Preemption is not disabled because memory will be allocated.
+	 * Ensure CPU-locality by calling this in per-CPU kthread.
+	 */
+	WARN_ON(!is_percpu_thread());
+	cpu = raw_smp_processor_id();
 	wd_attr = &wd_hw_attr;
 	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
 
-- 
2.40.1.521.gf1e218fcd8-goog