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Message-ID: <f3131178-7e69-4d38-bb99-4b063f409a66@efficios.com>
Date: Mon, 25 Aug 2025 16:30:17 -0400
From: Michael Jeanson <mjeanson@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
"Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
<seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 18/37] rseq: Provide static branch for runtime
debugging
On 2025-08-23 12:39, Thomas Gleixner wrote:
> Config based debug is rarely turned on and is not available easily when
> things go wrong.
>
> Provide a static branch to allow permanent integration of debug mechanisms
> along with the usual toggles in Kconfig, command line and debugfs.
>
> Requested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 4 +
> include/linux/rseq_entry.h | 3
> init/Kconfig | 14 ++++
> kernel/rseq.c | 73 ++++++++++++++++++++++--
> 4 files changed, 90 insertions(+), 4 deletions(-)
>
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6443,6 +6443,10 @@
> Memory area to be used by remote processor image,
> managed by CMA.
>
> + rseq_debug= [KNL] Enable or disable restartable sequence
> + debug mode. Defaults to CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE.
> + Format: <bool>
> +
> rt_group_sched= [KNL] Enable or disable SCHED_RR/FIFO group scheduling
> when CONFIG_RT_GROUP_SCHED=y. Defaults to
> !CONFIG_RT_GROUP_SCHED_DEFAULT_DISABLED.
> --- a/include/linux/rseq_entry.h
> +++ b/include/linux/rseq_entry.h
> @@ -34,6 +34,7 @@ DECLARE_PER_CPU(struct rseq_stats, rseq_
> #endif /* !CONFIG_RSEQ_STATS */
>
> #ifdef CONFIG_RSEQ
> +#include <linux/jump_label.h>
> #include <linux/rseq.h>
>
> #include <linux/tracepoint-defs.h>
> @@ -66,6 +67,8 @@ static inline void rseq_trace_ip_fixup(u
> unsigned long offset, unsigned long abort_ip) { }
> #endif /* !CONFIG_TRACEPOINT */
>
> +DECLARE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
> +
> static __always_inline void rseq_note_user_irq_entry(void)
> {
> if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY))
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1893,10 +1893,24 @@ config RSEQ_STATS
>
> If unsure, say N.
>
> +config RSEQ_DEBUG_DEFAULT_ENABLE
> + default n
> + bool "Enable restartable sequences debug mode by default" if EXPERT
> + depends on RSEQ
> + help
> + This enables the static branch for debug mode of restartable
> + sequences.
> +
> + This also can be controlled on the kernel command line via the
> + command line parameter "rseq_debug=0/1" and through debugfs.
> +
> + If unsure, say N.
> +
> config DEBUG_RSEQ
> default n
> bool "Enable debugging of rseq() system call" if EXPERT
> depends on RSEQ && DEBUG_KERNEL
> + select RSEQ_DEBUG_DEFAULT_ENABLE
> help
> Enable extra debugging checks for the rseq system call.
>
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -95,6 +95,27 @@
> RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
> RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
>
> +DEFINE_STATIC_KEY_MAYBE(CONFIG_RSEQ_DEBUG_DEFAULT_ENABLE, rseq_debug_enabled);
> +
> +static inline void rseq_control_debug(bool on)
> +{
> + if (on)
> + static_branch_enable(&rseq_debug_enabled);
> + else
> + static_branch_disable(&rseq_debug_enabled);
> +}
> +
> +static int __init rseq_setup_debug(char *str)
> +{
> + bool on;
> +
> + if (kstrtobool(str, &on))
> + return -EINVAL;
> + rseq_control_debug(on);
> + return 0;
Functions used by __setup() have to return '1' to signal that the
argument was handled, otherwise you get this in the kernel log:
kernel: Unknown kernel command line parameters "rseq_debug=1", will be
passed to user space.
> +}
> +__setup("rseq_debug=", rseq_setup_debug);
> +
> #ifdef CONFIG_TRACEPOINTS
> /*
> * Out of line, so the actual update functions can be in a header to be
> @@ -112,10 +133,11 @@ void __rseq_trace_ip_fixup(unsigned long
> }
> #endif /* CONFIG_TRACEPOINTS */
>
> +#ifdef CONFIG_DEBUG_FS
> #ifdef CONFIG_RSEQ_STATS
> DEFINE_PER_CPU(struct rseq_stats, rseq_stats);
>
> -static int rseq_debug_show(struct seq_file *m, void *p)
> +static int rseq_stats_show(struct seq_file *m, void *p)
> {
> struct rseq_stats stats = { };
> unsigned int cpu;
> @@ -140,14 +162,56 @@ static int rseq_debug_show(struct seq_fi
> return 0;
> }
>
> +static int rseq_stats_open(struct inode *inode, struct file *file)
> +{
> + return single_open(file, rseq_stats_show, inode->i_private);
> +}
> +
> +static const struct file_operations stat_ops = {
> + .open = rseq_stats_open,
> + .read = seq_read,
> + .llseek = seq_lseek,
> + .release = single_release,
> +};
> +
> +static int __init rseq_stats_init(struct dentry *root_dir)
> +{
> + debugfs_create_file("stats", 0444, root_dir, NULL, &stat_ops);
> + return 0;
> +}
> +#else
> +static inline void rseq_stats_init(struct dentry *root_dir) { }
> +#endif /* CONFIG_RSEQ_STATS */
> +
> +static int rseq_debug_show(struct seq_file *m, void *p)
> +{
> + bool on = static_branch_unlikely(&rseq_debug_enabled);
> +
> + seq_printf(m, "%d\n", on);
> + return 0;
> +}
> +
> +static ssize_t rseq_debug_write(struct file *file, const char __user *ubuf,
> + size_t count, loff_t *ppos)
> +{
> + bool on;
> +
> + if (kstrtobool_from_user(ubuf, count, &on))
> + return -EINVAL;
> +
> + rseq_control_debug(on);
> + return count;
> +}
> +
> static int rseq_debug_open(struct inode *inode, struct file *file)
> {
> return single_open(file, rseq_debug_show, inode->i_private);
> }
>
> -static const struct file_operations dfs_ops = {
> +static const struct file_operations debug_ops = {
> .open = rseq_debug_open,
> .read = seq_read,
> + .write = rseq_debug_write,
> .llseek = seq_lseek,
> .release = single_release,
> };
> @@ -156,11 +220,12 @@ static int __init rseq_debugfs_init(void
> {
> struct dentry *root_dir = debugfs_create_dir("rseq", NULL);
>
> - debugfs_create_file("stats", 0444, root_dir, NULL, &dfs_ops);
> + debugfs_create_file("debug", 0644, root_dir, NULL, &debug_ops);
> + rseq_stats_init(root_dir);
> return 0;
> }
> __initcall(rseq_debugfs_init);
> -#endif /* CONFIG_RSEQ_STATS */
> +#endif /* CONFIG_DEBUG_FS */
>
> #ifdef CONFIG_DEBUG_RSEQ
> static struct rseq *rseq_kernel_fields(struct task_struct *t)
>
>
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