On Thu, 2026-08-27 at 09:23 +0200, Tobias Schaffner wrote:
> Add CONFIG_RV_EDGE_STAT, an optional feature that records how long a
> monitor's automaton dwells in a state and exposes it per edge through a
> per-monitor "stats" tracefs file.
>
> The core allocates a per-CPU buffer on first enable and reports, per edge
> and per CPU, the count, maximum and summed dwell time. Only the owning CPU
> writes the counters, so a reader snapshots them with local64_read() with no
> IPI and no locking on the accounting path.
>
> Signed-off-by: Tobias Schaffner <[email protected]>
In this implementation you're tying the edge stats on the struct
rv_monitor and implementing things in rv.c . Now this is targeting only
DA/HA monitors, I presume. It's probably easier directly writing the
implementation in da_monitor. That would save you a bit of headache
you're solving with function and void pointers here.
You could extend da_monitor_init()/da_monitor_destroy() to
create/destroy this tracefs file, then leave the entire matrix in the
struct da_monitor (which might be a first step to support also other
monitor types besides per-cpu).
Then all specific implementations can be done following what we do with
RV_MON_TYPE (you can only implement per-cpu for now and leave the rest
blank or whatever is cleaner).
The logic behind da_monitor.h (and friends) is to create specific
per-monitor static functions, you will get some duplication in the
object file, but the compiler can inline and optimise things better.
Now this works perfectly with handlers which are very simple, and we are
still doing it also with more complex things which could live in another
(shared) object file, you can do it too.
Does it make sense to you?
Thanks,
Gabriele
> ---
> .../trace/rv/runtime-verification.rst | 24 ++++
> include/linux/rv.h | 14 +++
> include/linux/rv_edge_stat.h | 25 +++-
> kernel/trace/rv/Kconfig | 11 ++
> kernel/trace/rv/rv.c | 116 +++++++++++++++++-
> 5 files changed, 184 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/trace/rv/runtime-verification.rst
> b/Documentation/trace/rv/runtime-verification.rst
> index c700dde9259c..194b2f9db461 100644
> --- a/Documentation/trace/rv/runtime-verification.rst
> +++ b/Documentation/trace/rv/runtime-verification.rst
> @@ -229,3 +229,27 @@ For example::
> nop
> [panic]
> printk
> +
> +**monitors/MONITOR/stats**
> +
> +Present only when the kernel is built with CONFIG_RV_EDGE_STAT=y and
> *MONITOR*
> +is a per-cpu DA/HA (automaton) monitor. It reports how long the automaton
> +dwells in each state before leaving it, timed with local_clock() and
> accounted
> +per outgoing edge and per CPU.
> +
> +- The first line is a header naming the columns.
> +- Each following line describes one edge on one CPU::
> +
> + cpu edge label count max_ns sum_ns
> +
> + *count* is the number of times the edge was taken, *max_ns* and *sum_ns*
> are
> + the worst and total dwell in nanoseconds, and *label* is "state:event".
> +
> +The counters are reset each time the monitor is enabled.
> +
> +For example::
> +
> + # cat monitors/wip/stats
> + # cpu edge label count max_ns sum_ns
> + 0 0 preemptive:preempt_disable 4210 183200 95501200
> + 0 4 non_preemptive:preempt_enable 4208 42600 3812900
> diff --git a/include/linux/rv.h b/include/linux/rv.h
> index 541ba404926a..7eeecce17e50 100644
> --- a/include/linux/rv.h
> +++ b/include/linux/rv.h
> @@ -136,6 +136,16 @@ struct rv_reactor {
> };
> #endif
>
> +/**
> + * struct rv_edge_cfg - per-edge dwell-time statistics for a monitor
> + * @n_edges: number of automaton edges (STATE_MAX * EVENT_MAX)
> + * @edge_name: optional, write a human name for @edge into @buf (may be
> NULL)
> + */
> +struct rv_edge_cfg {
> + unsigned int n_edges;
> + void (*edge_name)(unsigned int edge, char *buf, size_t
> len);
> +};
> +
> struct rv_monitor {
> const char *name;
> const char *description;
> @@ -146,6 +156,10 @@ struct rv_monitor {
> #ifdef CONFIG_RV_REACTORS
> struct rv_reactor *reactor;
> __printf(1, 0) void (*react)(const char *msg, va_list args);
> +#endif
> +#ifdef CONFIG_RV_EDGE_STAT
> + const struct rv_edge_cfg *edge_cfg;
> + void __percpu *edge_pcpu;
> #endif
> struct list_head list;
> struct rv_monitor *parent;
> diff --git a/include/linux/rv_edge_stat.h b/include/linux/rv_edge_stat.h
> index 751de8074dcc..fda30ff728a1 100644
> --- a/include/linux/rv_edge_stat.h
> +++ b/include/linux/rv_edge_stat.h
> @@ -9,14 +9,11 @@
> #define _LINUX_RV_EDGE_STAT_H
>
> #include <linux/compiler.h>
> +#include <linux/percpu.h>
> +#include <linux/rv.h>
> #include <linux/types.h>
> #include <asm/local64.h>
>
> -/*
> - * Per-CPU counters kept in local64_t so accounting is safe against interrupt
> - * and NMI nesting on the owning CPU without disabling interrupts -- the same
> - * approach the trace ring buffer uses. Only the owning CPU writes.
> - */
> struct rv_edge_stat {
> local64_t count;
> local64_t sum_ns;
> @@ -42,4 +39,22 @@ void rv_edge_stat_account(struct rv_edge_stat *s, u64
> dwell_ns)
> }
> }
>
> +#ifdef CONFIG_RV_EDGE_STAT
> +/**
> + * rv_edge_account - record a dwell of @dwell_ns on @edge of monitor @mon
> + *
> + * Cheap and lock-free: the local64_t counters make this safe against
> interrupt
> + * and NMI nesting on the current CPU without disabling interrupts, so it
> does
> + * not perturb the latency being measured. The caller only needs to stay on
> its
> + * CPU for the call (as tracepoint probes already do).
> + */
> +static __always_inline void
> +rv_edge_account(struct rv_monitor *mon, unsigned int edge, u64 dwell_ns)
> +{
> + struct rv_edge_stat *e = this_cpu_ptr(mon->edge_pcpu);
> +
> + rv_edge_stat_account(&e[edge], dwell_ns);
> +}
> +#endif /* CONFIG_RV_EDGE_STAT */
> +
> #endif /* _LINUX_RV_EDGE_STAT_H */
> diff --git a/kernel/trace/rv/Kconfig b/kernel/trace/rv/Kconfig
> index 3884b14df375..9d76dff394ca 100644
> --- a/kernel/trace/rv/Kconfig
> +++ b/kernel/trace/rv/Kconfig
> @@ -59,6 +59,17 @@ config RV_PER_TASK_MONITORS
> This option configures the maximum number of per-task RV monitors
> that can run
> simultaneously.
>
> +config RV_EDGE_STAT
> + bool "Per-edge dwell-time statistics"
> + depends on RV
> + help
> + Record per-edge dwell-time statistics for per-cpu DA/HA monitors
> and
> + expose them through a per-monitor "stats" tracefs file. This times
> + each monitored automaton transition with local_clock(), so leave it
> + off if you do not need the statistics.
> +
> + If unsure, say N.
> +
> source "kernel/trace/rv/monitors/wip/Kconfig"
> source "kernel/trace/rv/monitors/wwnr/Kconfig"
>
> diff --git a/kernel/trace/rv/rv.c b/kernel/trace/rv/rv.c
> index ee4e68102f17..88a0bbaec4d0 100644
> --- a/kernel/trace/rv/rv.c
> +++ b/kernel/trace/rv/rv.c
> @@ -142,6 +142,12 @@
> #include <linux/module.h>
> #include <linux/init.h>
> #include <linux/slab.h>
> +#include <linux/seq_file.h>
> +#ifdef CONFIG_RV_EDGE_STAT
> +#include <linux/percpu.h>
> +#include <linux/rv_edge_stat.h>
> +#include <linux/smp.h>
> +#endif
>
> #ifdef CONFIG_RV_MON_EVENTS
> #define CREATE_TRACE_POINTS
> @@ -278,6 +284,9 @@ static void rv_disable_single(struct rv_monitor *mon)
> __rv_disable_monitor(mon, true);
> }
>
> +static int rv_edge_setup(struct rv_monitor *mon);
> +static void rv_edge_reset(struct rv_monitor *mon);
> +
> static int rv_enable_single(struct rv_monitor *mon)
> {
> int retval;
> @@ -289,9 +298,15 @@ static int rv_enable_single(struct rv_monitor *mon)
>
> retval = mon->enable();
>
> - if (!retval)
> + if (!retval) {
> mon->enabled = 1;
>
> + if (rv_edge_setup(mon))
> + pr_warn("rv: %s: edge statistics unavailable (out of
> memory)\n",
> + mon->name);
> + rv_edge_reset(mon);
> + }
> +
> return retval;
> }
>
> @@ -412,6 +427,101 @@ static const struct file_operations interface_desc_fops
> = {
> .read = monitor_desc_read_data,
> };
>
> +#ifdef CONFIG_RV_EDGE_STAT
> +static size_t rv_edge_blob_size(const struct rv_monitor *mon)
> +{
> + return mon->edge_cfg->n_edges * sizeof(struct rv_edge_stat);
> +}
> +
> +static void rv_edge_reset_ipi(void *info)
> +{
> + struct rv_monitor *mon = info;
> +
> + memset(this_cpu_ptr(mon->edge_pcpu), 0, rv_edge_blob_size(mon));
> +}
> +
> +/* rv_edge_reset - zero the statistics; call from a monitor reset/enable. */
> +static void rv_edge_reset(struct rv_monitor *mon)
> +{
> + if (mon->edge_pcpu)
> + on_each_cpu(rv_edge_reset_ipi, mon, 1);
> +}
> +
> +/*
> + * The counters are per-CPU and only the owning CPU writes them, so a reader
> on
> + * any CPU can snapshot them with local64_read().
> + */
> +static int rv_edge_stats_show(struct seq_file *seq, void *v)
> +{
> + struct rv_monitor *mon = seq->private;
> + const struct rv_edge_cfg *cfg = mon->edge_cfg;
> + unsigned int e;
> + int cpu;
> +
> + seq_puts(seq, "# cpu edge label count max_ns sum_ns\n");
> +
> + if (!mon->edge_pcpu)
> + return 0;
> +
> + for_each_online_cpu(cpu) {
> + struct rv_edge_stat *s = per_cpu_ptr(mon->edge_pcpu, cpu);
> +
> + for (e = 0; e < cfg->n_edges; e++) {
> + char lbl[48] = "";
> +
> + if (cfg->edge_name)
> + cfg->edge_name(e, lbl, sizeof(lbl));
> + seq_printf(seq, "%d %u %s %llu %llu %llu\n",
> + cpu, e, lbl,
> + (u64)local64_read(&s[e].count),
> + (u64)local64_read(&s[e].max_ns),
> + (u64)local64_read(&s[e].sum_ns));
> + }
> + }
> + return 0;
> +}
> +
> +static int rv_edge_stats_open(struct inode *inode, struct file *file)
> +{
> + return single_open(file, rv_edge_stats_show, inode->i_private);
> +}
> +
> +static const struct file_operations rv_edge_stats_fops = {
> + .open = rv_edge_stats_open,
> + .read = seq_read,
> + .llseek = seq_lseek,
> + .release = single_release,
> +};
> +
> +/*
> + * Allocate the per-CPU buffer and expose stats. Done on first enable
> + * rather than at registration because a DA/HA monitor's edge_cfg is bound by
> + * da_monitor_init(), which runs from the monitor's enable path.
> + */
> +static int rv_edge_setup(struct rv_monitor *mon)
> +{
> + if (!mon->edge_cfg || !mon->edge_cfg->n_edges || mon->edge_pcpu)
> + return 0;
> +
> + mon->edge_pcpu = __alloc_percpu(rv_edge_blob_size(mon),
> + __alignof__(struct rv_edge_stat));
> + if (!mon->edge_pcpu)
> + return -ENOMEM;
> +
> + if (!rv_create_file("stats", RV_MODE_READ, mon->root_d, mon,
> + &rv_edge_stats_fops)) {
> + free_percpu(mon->edge_pcpu);
> + mon->edge_pcpu = NULL;
> + return -ENOMEM;
> + }
> +
> + return 0;
> +}
> +#else
> +static int rv_edge_setup(struct rv_monitor *mon) { return 0; }
> +static void rv_edge_reset(struct rv_monitor *mon) { }
> +#endif /* CONFIG_RV_EDGE_STAT */
> +
> /*
> * During the registration of a monitor, this function creates
> * the monitor dir, where the specific options of the monitor
> @@ -747,6 +857,10 @@ static const struct file_operations monitoring_on_fops =
> {
>
> static void destroy_monitor_dir(struct rv_monitor *mon)
> {
> +#ifdef CONFIG_RV_EDGE_STAT
> + free_percpu(mon->edge_pcpu);
> + mon->edge_pcpu = NULL;
> +#endif
> rv_remove(mon->root_d);
> }
>
