Re: [PATCHv2 2/4] kernel/watchdog_hld: clarify the condition in hardlockup_detector_event_create()

On Mon, Oct 04, 2021 at 02:32:47PM +0200, Petr Mladek wrote:

On Thu 2021-09-23 22:09:49, Pingfan Liu wrote:

quoted

As for the context, there are two arguments to change
debug_smp_processor_id() to is_percpu_thread().

  -1. watchdog_ev is percpu, and migration will frustrate the attempt
which try to bind a watchdog_ev to a cpu by protecting this func inside
the pair of preempt_disable()/preempt_enable().

  -2. hardlockup_detector_event_create() indirectly calls
kmem_cache_alloc_node(), which is blockable.

So here, spelling out the really planned context "is_percpu_thread()".

The description is pretty hard to understand. I would suggest
something like:

Subject: kernel/watchdog_hld: Ensure CPU-bound context when creating
hardlockup detector event

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.

Appreciate for that. I will use it.

quoted

Signed-off-by: Pingfan Liu <redacted>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Wang Qing <redacted>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Santosh Sivaraj <redacted>
Cc: linux-arm-kernel@lists.infradead.org
To: linux-kernel@vger.kernel.org
---
 kernel/watchdog_hld.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
index 247bf0b1582c..df010df76576 100644
--- a/kernel/watchdog_hld.c
+++ b/kernel/watchdog_hld.c

@@ -165,10 +165,13 @@ 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;
 
+	/* This function plans to execute in cpu bound kthread */

This does not explain why it is needed. I suggest something like:

	/*
	 * Preemption is not disabled because memory will be allocated.
	 * Ensure CPU-locality by calling this in per-CPU kthread.
	 */

It sounds good. I will use it.

quoted

+	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);
 

Othrewise the change looks good to me.

Thank for your help.

Regards,

	Pingfan

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