Re: [PATCH] tracing: Add common_comm to histograms

From: Tom Zanussi
Date: Mon Apr 14 2025 - 17:14:38 EST

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In reply to: Steven Rostedt: "Re: [PATCH] tracing: Add common_comm to histograms"
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Hi Steve,

On Mon, 2025-04-07 at 15:49 -0400, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
>
> If one wants to trace the name of the task that wakes up a process and
> pass that to the synthetic events, there's nothing currently that lets the
> synthetic events do that. Add a "common_comm" to the histogram logic that
> allows histograms save the current->comm as a variable that can be passed
> through and added to a synthetic event:
>
> # cd /sys/kernel/tracing
> # echo 's:wake_lat char[] waker; char[] wakee; u64 delta;' >> dynamic_events
> # echo 'hist:keys=pid:comm=common_comm:ts=common_timestamp.usecs if !(common_flags & 0x18)' > events/sched/sched_waking/trigger
> # echo 'hist:keys=next_pid:wake_comm=$comm:delta=common_timestamp.usecs-$ts:onmatch(sched.sched_waking).trace(wake_lat,$wake_comm,next_comm,$delta)' > events/sched/sched_switch/trigger
>
> The above will create a synthetic trace event that will save both the name
> of the waker and the wakee but only if the wakeup did not happen in a hard
> or soft interrupt context.
>
> The "common_comm" is used to save the task->comm at the time of the
> initial event and is passed via the "comm" variable to the second event,
> and that is saved as the "waker" field in the "wake_lat" synthetic event.
>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

Very nice, looks good to me.

Reviewed-by: Tom Zanussi <zanussi@xxxxxxxxxx>

Thanks,

Tom

> ---
> kernel/trace/trace_events_hist.c | 51 ++++++++++++++++++++++++++------
> 1 file changed, 42 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index e85bc59c0421..58c9535f61df 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -114,6 +114,7 @@ enum hist_field_fn {
> HIST_FIELD_FN_BUCKET,
> HIST_FIELD_FN_TIMESTAMP,
> HIST_FIELD_FN_CPU,
> + HIST_FIELD_FN_COMM,
> HIST_FIELD_FN_STRING,
> HIST_FIELD_FN_DYNSTRING,
> HIST_FIELD_FN_RELDYNSTRING,
> @@ -506,6 +507,7 @@ enum hist_field_flags {
> HIST_FIELD_FL_CONST = 1 << 18,
> HIST_FIELD_FL_PERCENT = 1 << 19,
> HIST_FIELD_FL_GRAPH = 1 << 20,
> + HIST_FIELD_FL_COMM = 1 << 21,
> };
>
> struct var_defs {
> @@ -885,6 +887,15 @@ static u64 hist_field_cpu(struct hist_field *hist_field,
> return cpu;
> }
>
> +static u64 hist_field_comm(struct hist_field *hist_field,
> + struct tracing_map_elt *elt,
> + struct trace_buffer *buffer,
> + struct ring_buffer_event *rbe,
> + void *event)
> +{
> + return (u64)(unsigned long)current->comm;
> +}
> +
> /**
> * check_field_for_var_ref - Check if a VAR_REF field references a variable
> * @hist_field: The VAR_REF field to check
> @@ -1338,6 +1349,8 @@ static const char *hist_field_name(struct hist_field *field,
> field_name = hist_field_name(field->operands[0], ++level);
> else if (field->flags & HIST_FIELD_FL_CPU)
> field_name = "common_cpu";
> + else if (field->flags & HIST_FIELD_FL_COMM)
> + field_name = "common_comm";
> else if (field->flags & HIST_FIELD_FL_EXPR ||
> field->flags & HIST_FIELD_FL_VAR_REF) {
> if (field->system) {
> @@ -2015,6 +2028,13 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
> goto out;
> }
>
> + if (flags & HIST_FIELD_FL_COMM) {
> + hist_field->fn_num = HIST_FIELD_FN_COMM;
> + hist_field->size = MAX_FILTER_STR_VAL;
> + hist_field->type = "char[]";
> + goto out;
> + }
> +
> if (WARN_ON_ONCE(!field))
> goto out;
>
> @@ -2359,9 +2379,11 @@ parse_field(struct hist_trigger_data *hist_data, struct trace_event_file *file,
> hist_data->attrs->ts_in_usecs = true;
> } else if (strcmp(field_name, "common_stacktrace") == 0) {
> *flags |= HIST_FIELD_FL_STACKTRACE;
> - } else if (strcmp(field_name, "common_cpu") == 0)
> + } else if (strcmp(field_name, "common_cpu") == 0) {
> *flags |= HIST_FIELD_FL_CPU;
> - else if (strcmp(field_name, "hitcount") == 0)
> + } else if (strcmp(field_name, "common_comm") == 0) {
> + *flags |= HIST_FIELD_FL_COMM | HIST_FIELD_FL_STRING;
> + } else if (strcmp(field_name, "hitcount") == 0)
> *flags |= HIST_FIELD_FL_HITCOUNT;
> else {
> field = trace_find_event_field(file->event_call, field_name);
> @@ -2377,6 +2399,8 @@ parse_field(struct hist_trigger_data *hist_data, struct trace_event_file *file,
> *flags |= HIST_FIELD_FL_CPU;
> } else if (field && field->filter_type == FILTER_STACKTRACE) {
> *flags |= HIST_FIELD_FL_STACKTRACE;
> + } else if (field && field->filter_type == FILTER_COMM) {
> + *flags |= HIST_FIELD_FL_COMM | HIST_FIELD_FL_STRING;
> } else {
> hist_err(tr, HIST_ERR_FIELD_NOT_FOUND,
> errpos(field_name));
> @@ -4327,6 +4351,8 @@ static u64 hist_fn_call(struct hist_field *hist_field,
> return hist_field_timestamp(hist_field, elt, buffer, rbe, event);
> case HIST_FIELD_FN_CPU:
> return hist_field_cpu(hist_field, elt, buffer, rbe, event);
> + case HIST_FIELD_FN_COMM:
> + return hist_field_comm(hist_field, elt, buffer, rbe, event);
> case HIST_FIELD_FN_STRING:
> return hist_field_string(hist_field, elt, buffer, rbe, event);
> case HIST_FIELD_FN_DYNSTRING:
> @@ -5212,14 +5238,19 @@ static inline void add_to_key(char *compound_key, void *key,
> size_t size = key_field->size;
>
> if (key_field->flags & HIST_FIELD_FL_STRING) {
> - struct ftrace_event_field *field;
>
> - field = key_field->field;
> - if (field->filter_type == FILTER_DYN_STRING ||
> - field->filter_type == FILTER_RDYN_STRING)
> - size = *(u32 *)(rec + field->offset) >> 16;
> - else if (field->filter_type == FILTER_STATIC_STRING)
> - size = field->size;
> + if (key_field->flags & HIST_FIELD_FL_COMM) {
> + size = strlen((char *)key);
> + } else {
> + struct ftrace_event_field *field;
> +
> + field = key_field->field;
> + if (field->filter_type == FILTER_DYN_STRING ||
> + field->filter_type == FILTER_RDYN_STRING)
> + size = *(u32 *)(rec + field->offset) >> 16;
> + else if (field->filter_type == FILTER_STATIC_STRING)
> + size = field->size;
> + }
>
> /* ensure NULL-termination */
> if (size > key_field->size - 1)
> @@ -6097,6 +6128,8 @@ static void hist_field_print(struct seq_file *m, struct hist_field *hist_field)
>
> if (hist_field->flags & HIST_FIELD_FL_CPU)
> seq_puts(m, "common_cpu");
> + if (hist_field->flags & HIST_FIELD_FL_COMM)
> + seq_puts(m, "common_comm");
> else if (hist_field->flags & HIST_FIELD_FL_CONST)
> seq_printf(m, "%llu", hist_field->constant);
> else if (field_name) {

Next message: Adam Xue: "[PATCH v2] USB: serial: option: Add Sierra Wireless EM9291"
Previous message: Kaplan, David: "RE: [PATCH v4 16/36] x86/bugs: Restructure srso mitigation"
In reply to: Steven Rostedt: "Re: [PATCH] tracing: Add common_comm to histograms"
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