On 4 October 2026 02:14:25 BST, Cunlong Li <[email protected]> wrote: >The kernel.panic_on_rcu_stall and kernel.max_rcu_stall_to_panic sysctls >count RCU CPU stalls since system boot, and invoke panic() once >max_rcu_stall_to_panic stalls have elapsed. This count is never reset, >so stalls caused by transient incidents keep consuming the budget of a >long-running system, and a later unrelated stall can immediately >trigger panic() instead of allowing the intended fresh window of >stalls. > >This commit therefore introduces the kernel.rcu_stall_panic_count >sysctl. Reading this file reports the number of stalls counted so far, >and writing 0 to it resets the count. This allows the cumulative stall >history to be cleared after an incident has been resolved, and also >allows userspace to clear the count periodically, so that panic() is >only triggered by a burst of stalls occurring within a short period. >
Hi, LGTM! Reviewed-by: Bradley Morgan <[email protected]> >Signed-off-by: Cunlong Li <[email protected]> >--- > Documentation/admin-guide/sysctl/kernel.rst | 15 ++++++++++++++- > kernel/rcu/tree_stall.h | 28 > ++++++++++++++++++++++++++-- > 2 files changed, 40 insertions(+), 3 deletions(-) > >diff --git a/Documentation/admin-guide/sysctl/kernel.rst >b/Documentation/admin-guide/sysctl/kernel.rst >index ffea61d448eb..64fe2985e358 100644 >--- a/Documentation/admin-guide/sysctl/kernel.rst >+++ b/Documentation/admin-guide/sysctl/kernel.rst >@@ -959,7 +959,20 @@ max_rcu_stall_to_panic > When ``panic_on_rcu_stall`` is set to 1, this value determines the > number of times that RCU can stall before panic() is called. > >-When ``panic_on_rcu_stall`` is set to 0, this value is has no effect. >+When ``panic_on_rcu_stall`` is set to 0, this value has no effect. >+ >+rcu_stall_panic_count >+===================== >+ >+Indicates the number of RCU CPU stalls that have been counted since >+system boot or since the counter was reset. When ``panic_on_rcu_stall`` >+is set to 1, this count is compared against ``max_rcu_stall_to_panic`` >+to decide whether panic() should be called. >+ >+Writing 0 to this file resets the counter to zero, which restarts the >+``max_rcu_stall_to_panic`` window of stalls. This allows system >+administrators to clear the cumulative stall count after an incident >+has been resolved, without requiring a system restart. > > perf_cpu_time_max_percent > ========================= >diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h >index 091e7850ab6e..a80f03e1c7ea 100644 >--- a/kernel/rcu/tree_stall.h >+++ b/kernel/rcu/tree_stall.h >@@ -19,6 +19,20 @@ > /* panic() on RCU Stall sysctl. */ > static int sysctl_panic_on_rcu_stall __read_mostly; > static int sysctl_max_rcu_stall_to_panic __read_mostly; >+static unsigned long sysctl_rcu_stall_panic_count; >+ >+/* Reset the RCU stall panic count when written to. */ >+static int proc_do_rcu_stall_panic_count(const struct ctl_table *table, int >write, >+ void *buffer, size_t *lenp, loff_t >*ppos) >+{ >+ if (!write) >+ return proc_doulongvec_minmax(table, write, buffer, lenp, ppos); >+ >+ WRITE_ONCE(sysctl_rcu_stall_panic_count, 0); >+ *ppos += *lenp; >+ >+ return 0; >+} > > static const struct ctl_table rcu_stall_sysctl_table[] = { > { >@@ -39,6 +53,13 @@ static const struct ctl_table rcu_stall_sysctl_table[] = { > .extra1 = SYSCTL_ONE, > .extra2 = SYSCTL_INT_MAX, > }, >+ { >+ .procname = "rcu_stall_panic_count", >+ .data = &sysctl_rcu_stall_panic_count, >+ .maxlen = sizeof(sysctl_rcu_stall_panic_count), >+ .mode = 0644, >+ .proc_handler = proc_do_rcu_stall_panic_count, >+ }, > }; > > static int __init init_rcu_stall_sysctl(void) >@@ -161,7 +182,7 @@ early_initcall(check_cpu_stall_init); > /* If so specified via sysctl, panic, yielding cleaner stall-warning > output. */ > static void panic_on_rcu_stall(const struct cpumask *stalled_mask) > { >- static int cpu_stall; >+ unsigned long count; > > /* > * Attempt to kick out the BPF scheduler if it's installed and defer >@@ -170,7 +191,10 @@ static void panic_on_rcu_stall(const struct cpumask >*stalled_mask) > if (scx_rcu_cpu_stall(stalled_mask)) > return; > >- if (++cpu_stall < sysctl_max_rcu_stall_to_panic) >+ /* A lost RMW update only delays the panic by one stall. */ >+ count = READ_ONCE(sysctl_rcu_stall_panic_count) + 1; >+ WRITE_ONCE(sysctl_rcu_stall_panic_count, count); >+ if (count < (unsigned long)READ_ONCE(sysctl_max_rcu_stall_to_panic)) > return; > > if (sysctl_panic_on_rcu_stall) > >--- >base-commit: ce1e0223d8ad4211275c82a17ed6d43ab81e13d9 >change-id: 20261003-rcu-375496d7704e > >Best regards, > --- Thanks! "I'm not a very positive person" - Linus torvalds
