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Date: Mon, 5 Apr 2021 17:28:39 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Yordan Karadzhov (VMware)" <y.karadz@...il.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de,
peterz@...radead.org
Subject: Re: [PATCH v2 1/5] tracing: Define new ftrace event "func_repeats"
On Mon, 29 Mar 2021 16:05:29 +0300
"Yordan Karadzhov (VMware)" <y.karadz@...il.com> wrote:
> The event aims to consolidate the function tracing record in the cases
> when a single function is called number of times consecutively.
>
> while (cond)
> do_func();
>
> This may happen in various scenarios (busy waiting for example).
> The new ftrace event can be used to show repeated function events with
> a single event and save space on the ring buffer
>
> Signed-off-by: Yordan Karadzhov (VMware) <y.karadz@...il.com>
> ---
> kernel/trace/trace.h | 3 +++
> kernel/trace/trace_entries.h | 39 ++++++++++++++++++++++++++++++
> kernel/trace/trace_output.c | 47 ++++++++++++++++++++++++++++++++++++
> 3 files changed, 89 insertions(+)
>
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 5506424eae2a..6a5b4c2a0fa7 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -45,6 +45,7 @@ enum trace_type {
> TRACE_BPUTS,
> TRACE_HWLAT,
> TRACE_RAW_DATA,
> + TRACE_FUNC_REPEATS,
>
> __TRACE_LAST_TYPE,
> };
> @@ -442,6 +443,8 @@ extern void __ftrace_bad_type(void);
> TRACE_GRAPH_ENT); \
> IF_ASSIGN(var, ent, struct ftrace_graph_ret_entry, \
> TRACE_GRAPH_RET); \
> + IF_ASSIGN(var, ent, struct func_repeats_entry, \
> + TRACE_FUNC_REPEATS); \
> __ftrace_bad_type(); \
> } while (0)
>
> diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
> index 4547ac59da61..6f98c3b4e4fa 100644
> --- a/kernel/trace/trace_entries.h
> +++ b/kernel/trace/trace_entries.h
> @@ -338,3 +338,42 @@ FTRACE_ENTRY(hwlat, hwlat_entry,
> __entry->nmi_total_ts,
> __entry->nmi_count)
> );
> +
> +#define FUNC_REPEATS_GET_DELTA_TS(entry) \
> +(((u64)entry->top_delta_ts << 32) | entry->bottom_delta_ts) \
> +
> +#define FUNC_REPEATS_SET_DELTA_TS(entry, delta) \
Hmm, this isn't used anywhere. Why is it defined here?
> + do { \
> + if (likely(!((u64)delta >> 32))) { \
If statements are more expensive than shifts and masks, thus, this first if
statement isn't needed. Because:
entry->bottom_delta_ts = delta & U32_MAX;
entry->top_delta_ts = (delta >> 32);
Would produce the same faster as this conditional, but be generally faster.
> + entry->bottom_delta_ts = delta; \
> + entry->top_delta_ts = 0; \
> + } else { \
> + if (likely(!((u64)delta >> 48))) { \
> + entry->bottom_delta_ts = delta & U32_MAX; \
> + entry->top_delta_ts = (delta >> 32); \
> + } else { \
> + /* Timestamp overflow. Set to max. */ \
> + entry->bottom_delta_ts = U32_MAX; \
> + entry->top_delta_ts = U16_MAX; \
I'm almost thinking we should just ignore this as it should never happen,
because 2^48 nanoseconds is 78 hours. If we are repeating the same function
over and over again for over 78 hours, we have more issues to worry about
;-)
-- Steve
> + } \
> + } \
> + } while (0); \
> +
> +FTRACE_ENTRY(func_repeats, func_repeats_entry,
> +
> + TRACE_FUNC_REPEATS,
> +
> + F_STRUCT(
> + __field( unsigned long, ip )
> + __field( unsigned long, parent_ip )
> + __field( u16 , count )
> + __field( u16 , top_delta_ts )
> + __field( u32 , bottom_delta_ts )
> + ),
> +
> + F_printk(" %ps <-%ps\t(repeats:%u delta_ts: -%llu)",
> + (void *)__entry->ip,
> + (void *)__entry->parent_ip,
> + __entry->count,
> + FUNC_REPEATS_GET_DELTA_TS(__entry))
> +);
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index a0146e1fffdf..55b08e146afc 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -1373,6 +1373,52 @@ static struct trace_event trace_raw_data_event = {
> .funcs = &trace_raw_data_funcs,
> };
>
> +static enum print_line_t
> +trace_func_repeats_raw(struct trace_iterator *iter, int flags,
> + struct trace_event *event)
> +{
> + struct func_repeats_entry *field;
> + struct trace_seq *s = &iter->seq;
> +
> + trace_assign_type(field, iter->ent);
> +
> + trace_seq_printf(s, "%lu %lu %u %llu\n",
> + field->ip,
> + field->parent_ip,
> + field->count,
> + FUNC_REPEATS_GET_DELTA_TS(field));
> +
> + return trace_handle_return(s);
> +}
> +
> +static enum print_line_t
> +trace_func_repeats_print(struct trace_iterator *iter, int flags,
> + struct trace_event *event)
> +{
> + struct func_repeats_entry *field;
> + struct trace_seq *s = &iter->seq;
> +
> + trace_assign_type(field, iter->ent);
> +
> + seq_print_ip_sym(s, field->ip, flags);
> + trace_seq_puts(s, " <-");
> + seq_print_ip_sym(s, field->parent_ip, flags);
> + trace_seq_printf(s, " (repeats: %u, delta_ts: -%llu)\n",
> + field->count,
> + FUNC_REPEATS_GET_DELTA_TS(field));
> +
> + return trace_handle_return(s);
> +}
> +
> +static struct trace_event_functions trace_func_repeats_funcs = {
> + .trace = trace_func_repeats_print,
> + .raw = trace_func_repeats_raw,
> +};
> +
> +static struct trace_event trace_func_repeats_event = {
> + .type = TRACE_FUNC_REPEATS,
> + .funcs = &trace_func_repeats_funcs,
> +};
>
> static struct trace_event *events[] __initdata = {
> &trace_fn_event,
> @@ -1385,6 +1431,7 @@ static struct trace_event *events[] __initdata = {
> &trace_print_event,
> &trace_hwlat_event,
> &trace_raw_data_event,
> + &trace_func_repeats_event,
> NULL
> };
>
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