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Sun, 28 Dec 2025 14:59:36 -0800 (PST) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Sun, 28 Dec 2025 23:59:34 +0100 To: Steven Rostedt Cc: LKML , Linux Trace Kernel , Masami Hiramatsu , Mathieu Desnoyers , Ian Rogers , Arnaldo Carvalho de Melo , Namhyung Kim , Peter Zijlstra Subject: Re: [PATCH] tracing: Allow perf to read synthetic events Message-ID: References: <20251217113920.50b56246@gandalf.local.home> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251217113920.50b56246@gandalf.local.home> On Wed, Dec 17, 2025 at 11:39:20AM -0500, Steven Rostedt wrote: > From: Steven Rostedt > > Currently, perf can not enable synthetic events. When it does, it either > causes a warning in the kernel or errors with "no such device". > > Add the necessary code to allow perf to also attach to synthetic events. > > Reported-by: Ian Rogers > Signed-off-by: Steven Rostedt (Google) hi, I don't see the crash, but perf record/script gives me 'FAILED TO PARSE' in perf script: # cd /sys/kernel/tracing # echo 's:block_lat pid_t pid; u64 delta; unsigned long[] stack;' > dynamic_events # echo 'hist:keys=next_pid:ts=common_timestamp.usecs,st=common_stacktrace if prev_state == 2' >> events/sched/sched_switch/trigger # echo 'hist:keys=prev_pid:delta=common_timestamp.usecs-$ts,s=$st:onmax($delta).trace(block_lat,prev_pid,$delta,$s)' >> events/sched/sched_switch/trigger # echo 1 > events/synthetic/block_lat/enable # perf record -e 'synthetic:block_lat' -a ^C[ perf record: Woken up 1 times to write data ] [ perf record: Captured and wrote 0.259 MB perf.data (1 samples) ] # perf script kworker/u33:2-w 244 [000] 1707.836263: synthetic:block_lat: [FAILED TO PARSE] pid=244 delta=21 stack=ARRAY[0b, 00, 00, 00, 00, 00, 00, 00, 1d, 72, 9d, 82, ff, ff, ff, ff, 0d, 7d, 9d, 82, ff, ff, ff, ff, 3d, 3d, 9e, 82, ff, ff, ff, ff, 05, 91, 9d, 82, ff, ff, ff, ff, 40, 7a, 42, 81, ff, ff, ff, ff, 5e, f4, 0c, 82, ff, ff, ff, ff, 43, 8d, 0c, 82, ff, ff, ff, ff, 82, 2d, 89, 81, ff, ff, ff, ff, 9b, 39, 89, 81, ff, ff, ff, ff, a6, 5a, 9c, 82, ff, ff, ff, ff, 2f, 01, 00, 81, ff, ff, ff, ff] not sure it's fixed in the latest libtraceevent, mine is libtraceevent-1.8.4-3.fc42.x86_64 jirka > --- > kernel/trace/trace_events_synth.c | 121 +++++++++++++++++++++++------- > 1 file changed, 94 insertions(+), 27 deletions(-) > > diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c > index 4554c458b78c..026e06f28958 100644 > --- a/kernel/trace/trace_events_synth.c > +++ b/kernel/trace/trace_events_synth.c > @@ -493,28 +493,19 @@ static unsigned int trace_stack(struct synth_trace_event *entry, > return len; > } > > -static notrace void trace_event_raw_event_synth(void *__data, > - u64 *var_ref_vals, > - unsigned int *var_ref_idx) > +static __always_inline int get_field_size(struct synth_event *event, > + u64 *var_ref_vals, > + unsigned int *var_ref_idx) > { > - unsigned int i, n_u64, val_idx, len, data_size = 0; > - struct trace_event_file *trace_file = __data; > - struct synth_trace_event *entry; > - struct trace_event_buffer fbuffer; > - struct trace_buffer *buffer; > - struct synth_event *event; > - int fields_size = 0; > - > - event = trace_file->event_call->data; > - > - if (trace_trigger_soft_disabled(trace_file)) > - return; > + int fields_size; > > fields_size = event->n_u64 * sizeof(u64); > > - for (i = 0; i < event->n_dynamic_fields; i++) { > + for (int i = 0; i < event->n_dynamic_fields; i++) { > unsigned int field_pos = event->dynamic_fields[i]->field_pos; > char *str_val; > + int val_idx; > + int len; > > val_idx = var_ref_idx[field_pos]; > str_val = (char *)(long)var_ref_vals[val_idx]; > @@ -529,18 +520,18 @@ static notrace void trace_event_raw_event_synth(void *__data, > > fields_size += len; > } > + return fields_size; > +} > > - /* > - * Avoid ring buffer recursion detection, as this event > - * is being performed within another event. > - */ > - buffer = trace_file->tr->array_buffer.buffer; > - guard(ring_buffer_nest)(buffer); > - > - entry = trace_event_buffer_reserve(&fbuffer, trace_file, > - sizeof(*entry) + fields_size); > - if (!entry) > - return; > +static __always_inline void write_synth_entry(struct synth_event *event, > + struct synth_trace_event *entry, > + u64 *var_ref_vals, > + unsigned int *var_ref_idx) > +{ > + int data_size = 0; > + int i, n_u64; > + int val_idx; > + int len; > > for (i = 0, n_u64 = 0; i < event->n_fields; i++) { > val_idx = var_ref_idx[i]; > @@ -581,10 +572,83 @@ static notrace void trace_event_raw_event_synth(void *__data, > n_u64++; > } > } > +} > + > +static notrace void trace_event_raw_event_synth(void *__data, > + u64 *var_ref_vals, > + unsigned int *var_ref_idx) > +{ > + struct trace_event_file *trace_file = __data; > + struct synth_trace_event *entry; > + struct trace_event_buffer fbuffer; > + struct trace_buffer *buffer; > + struct synth_event *event; > + int fields_size; > + > + event = trace_file->event_call->data; > + > + if (trace_trigger_soft_disabled(trace_file)) > + return; > + > + fields_size = get_field_size(event, var_ref_vals, var_ref_idx); > + > + /* > + * Avoid ring buffer recursion detection, as this event > + * is being performed within another event. > + */ > + buffer = trace_file->tr->array_buffer.buffer; > + guard(ring_buffer_nest)(buffer); > + > + entry = trace_event_buffer_reserve(&fbuffer, trace_file, > + sizeof(*entry) + fields_size); > + if (!entry) > + return; > + > + write_synth_entry(event, entry, var_ref_vals, var_ref_idx); > > trace_event_buffer_commit(&fbuffer); > } > > +#ifdef CONFIG_PERF_EVENTS > +static notrace void perf_event_raw_event_synth(void *__data, > + u64 *var_ref_vals, > + unsigned int *var_ref_idx) > +{ > + struct trace_event_call *call = __data; > + struct synth_trace_event *entry; > + struct hlist_head *perf_head; > + struct synth_event *event; > + struct pt_regs *regs; > + int fields_size; > + size_t size; > + int context; > + > + event = call->data; > + > + perf_head = this_cpu_ptr(call->perf_events); > + > + if (!perf_head || hlist_empty(perf_head)) > + return; > + > + fields_size = get_field_size(event, var_ref_vals, var_ref_idx); > + > + size = ALIGN(sizeof(*entry) + fields_size, 8); > + > + entry = perf_trace_buf_alloc(size, ®s, &context); > + > + if (unlikely(!entry)) > + return; > + > + write_synth_entry(event, entry, var_ref_vals, var_ref_idx); > + > + perf_fetch_caller_regs(regs); > + > + perf_trace_buf_submit(entry, size, context, > + call->event.type, 1, regs, > + perf_head, NULL); > +} > +#endif > + > static void free_synth_event_print_fmt(struct trace_event_call *call) > { > if (call) { > @@ -911,6 +975,9 @@ static int register_synth_event(struct synth_event *event) > call->flags = TRACE_EVENT_FL_TRACEPOINT; > call->class->reg = synth_event_reg; > call->class->probe = trace_event_raw_event_synth; > +#ifdef CONFIG_PERF_EVENTS > + call->class->perf_probe = perf_event_raw_event_synth; > +#endif > call->data = event; > call->tp = event->tp; > > -- > 2.51.0 >