linux-kernel - Re: [PATCH v2 2/2] kernel/rcu/tree_stall: add /sys/kernel/rcu_stall

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Message-ID: <47196e67-6533-4015-95db-63a76c1c0c5c@linux.ibm.com>
Date: Tue, 3 Jun 2025 22:09:30 +0530
From: Sourabh Jain <sourabhjain@...ux.ibm.com>
To: akpm@...ux-foundation.org, Max Kellermann <max.kellermann@...os.com>,
        song@...nel.org, joel.granados@...nel.org, dianders@...omium.org,
        cminyard@...sta.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] kernel/rcu/tree_stall: add
 /sys/kernel/rcu_stall_count

Hello Andrew,

On 04/05/25 23:38, Max Kellermann wrote:
> Exposing a simple counter to userspace for monitoring tools.
>
> Signed-off-by: Max Kellermann <max.kellermann@...os.com>
> ---
> v1 -> v2: added documentation
> ---
>   .../ABI/testing/sysfs-kernel-rcu_stall_count  |  6 +++++
>   kernel/rcu/tree_stall.h                       | 26 +++++++++++++++++++
>   2 files changed, 32 insertions(+)
>   create mode 100644 Documentation/ABI/testing/sysfs-kernel-rcu_stall_count
>
> diff --git a/Documentation/ABI/testing/sysfs-kernel-rcu_stall_count b/Documentation/ABI/testing/sysfs-kernel-rcu_stall_count
> new file mode 100644
> index 000000000000..a4a97a7f4a4d
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-kernel-rcu_stall_count
> @@ -0,0 +1,6 @@
> +What:		/sys/kernel/rcu_stall_count
> +Date:		May 2025
> +KernelVersion:	6.16
> +Contact:	Linux kernel mailing list <linux-kernel@...r.kernel.org>
> +Description:
> +		Shows how many times the system has detected an RCU stall since last boot.
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 925fcdad5dea..158330524795 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -20,6 +20,28 @@
>   int sysctl_panic_on_rcu_stall __read_mostly;
>   int sysctl_max_rcu_stall_to_panic __read_mostly;
>   
> +#ifdef CONFIG_SYSFS
> +
> +static unsigned int rcu_stall_count;
> +
> +static ssize_t rcu_stall_count_show(struct kobject *kobj, struct kobj_attribute *attr,
> +				    char *page)
> +{
> +	return sysfs_emit(page, "%u\n", rcu_stall_count);
> +}
> +
> +static struct kobj_attribute rcu_stall_count_attr = __ATTR_RO(rcu_stall_count);
> +
> +static __init int kernel_rcu_stall_sysfs_init(void)
> +{
> +	sysfs_add_file_to_group(kernel_kobj, &rcu_stall_count_attr.attr, NULL);
> +	return 0;
> +}
> +
> +late_initcall(kernel_rcu_stall_sysfs_init);
> +
> +#endif // CONFIG_SYSFS
> +
>   #ifdef CONFIG_PROVE_RCU
>   #define RCU_STALL_DELAY_DELTA		(5 * HZ)
>   #else
> @@ -784,6 +806,10 @@ static void check_cpu_stall(struct rcu_data *rdp)
>   		if (kvm_check_and_clear_guest_paused())
>   			return;
>   
> +#ifdef CONFIG_SYSFS
> +		++rcu_stall_count;
> +#endif
> +
>   		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_NORM, (void *)j - gps);
>   		if (READ_ONCE(csd_lock_suppress_rcu_stall) && csd_lock_is_stuck()) {
>   			pr_err("INFO: %s detected stall, but suppressed full report due to a stuck CSD-lock.\n", rcu_state.name);

It seems like this patch was not applied properly to the upstream tree.

Out of the three hunks in this patch, only the first one is applied; the 
second
and third hunks are missing.

commit 2536c5c7d6ae5e1d844aa21f28b326b5e7f815ef
Author: Max Kellermann <max.kellermann@...os.com>
Date:   Sun May 4 20:08:31 2025 +0200

     kernel/rcu/tree_stall: add /sys/kernel/rcu_stall_count

     Expose a simple counter to userspace for monitoring tools.


Thanks,
Sourabh Jain