The existing rcu_segcb_stats tracepoint requires active tracing, so there is no always-on, low-overhead way to inspect how many callbacks are pending on each CPU and at which stage of the grace-period pipeline.
Add a debugfs file at /sys/kernel/debug/rcu/pending_cbs that prints per-CPU callback counts broken down by segcblist segment, plus a "total" row aggregating across CPUs: - done: Callbacks ready to invoke (GP completed). - wait: Callbacks waiting for the current GP. - next_ready: Callbacks to be handled by the next GP. - next: Newly queued callbacks not yet assigned a GP. - lazy: Callbacks deferred via the RCU lazy mechanism. The interface has zero steady-state overhead: it reads the existing per-CPU rcu_segcblist.seglen[] counters on demand. These counters are already maintained by the RCU callback infrastructure for its own bookkeeping, so no new runtime accounting is introduced. Example output: cpu done wait next_ready next lazy 0 7 11 0 0 0 1 0 3 2 0 0 2 0 1 8 0 0 3 0 1 1 0 0 total 7 16 11 0 0 Signed-off-by: Gustavo Luiz Duarte <[email protected]> --- kernel/rcu/tree_stall.h | 67 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 67 insertions(+) diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index b67532cb8770..d9fc9bfdaf96 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -71,6 +71,73 @@ late_initcall(kernel_rcu_stall_sysfs_init); #endif // CONFIG_SYSFS +#ifdef CONFIG_DEBUG_FS + +#include <linux/debugfs.h> +#include <linux/seq_file.h> + +/* + * Debugfs interface for displaying per-CPU RCU callback counts broken down + * by callback-list segment. This allows monitoring how many callbacks are + * waiting for grace periods without any steady-state overhead. + */ +static int rcu_pending_cbs_show(struct seq_file *m, void *v) +{ + int cpu; + long done, wait, nxtrdy, nxt, lazy; + long total_done = 0, total_wait = 0, total_nxtrdy = 0; + long total_nxt = 0, total_lazy = 0; + struct rcu_data *rdp; + struct rcu_segcblist *rsclp; + + seq_printf(m, "%-8s %10s %10s %10s %10s %10s\n", + "cpu", "done", "wait", "next_ready", "next", "lazy"); + + for_each_possible_cpu(cpu) { + rdp = per_cpu_ptr(&rcu_data, cpu); + rsclp = &rdp->cblist; + + if (!rcu_segcblist_is_enabled(rsclp)) + continue; + + done = rcu_segcblist_get_seglen(rsclp, RCU_DONE_TAIL); + wait = rcu_segcblist_get_seglen(rsclp, RCU_WAIT_TAIL); + nxtrdy = rcu_segcblist_get_seglen(rsclp, RCU_NEXT_READY_TAIL); + nxt = rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL); + lazy = READ_ONCE(rdp->lazy_len); + + seq_printf(m, "%-8d %10ld %10ld %10ld %10ld %10ld\n", + cpu, done, wait, nxtrdy, nxt, lazy); + + total_done += done; + total_wait += wait; + total_nxtrdy += nxtrdy; + total_nxt += nxt; + total_lazy += lazy; + } + + seq_printf(m, "%-8s %10ld %10ld %10ld %10ld %10ld\n", + "total", total_done, total_wait, total_nxtrdy, + total_nxt, total_lazy); + + return 0; +} +DEFINE_SHOW_ATTRIBUTE(rcu_pending_cbs); + +static struct dentry *rcu_debugfs_dir; + +static int __init rcu_debugfs_init(void) +{ + rcu_debugfs_dir = debugfs_create_dir("rcu", NULL); + debugfs_create_file("pending_cbs", 0444, rcu_debugfs_dir, + NULL, &rcu_pending_cbs_fops); + + return 0; +} +late_initcall(rcu_debugfs_init); + +#endif // #ifdef CONFIG_DEBUG_FS + #ifdef CONFIG_PROVE_RCU #define RCU_STALL_DELAY_DELTA (5 * HZ) #else -- 2.53.0-Meta
