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Date: Wed, 15 Apr 2009 21:33:15 -0400 (EDT)
From: Steven Rostedt <rostedt@...dmis.org>
To: Frederic Weisbecker <fweisbec@...il.com>
cc: Ingo Molnar <mingo@...e.hu>, Zhaolei <zhaolei@...fujitsu.com>,
Tom Zanussi <tzanussi@...il.com>,
Li Zefan <lizf@...fujitsu.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC][PATCH 1/2] tracing/events: provide string with undefined
size support
On Thu, 16 Apr 2009, Frederic Weisbecker wrote:
> Impact: less memory usage for tracing
>
> This patch provides the support for dynamic size strings on
> event tracing.
>
> The key concept is to use a structure with an ending char array field of
> undefined size and use such ability to allocate the minimal size on the ring
> buffer to make the entry fit inside as opposite to a fixed length strings with
> upper bound.
>
> This patch provides two new macros:
>
> -__ending_string(name)
>
> This one declares the string to the structure inside TP_STRUCT__entry.
> Only the name is needed.
> Two constraints: only one __ending_string() per TRACE_EVENT can be added and
> it must be the last field to be declared. Hence the __ending prefix.
>
> - open_string_assign(call, dst, src)
>
> This one does the copy inside TP__fast_assign() of the source
> string to the destination. The name of the tracepoint (call) must be provided
> for now. Hopefully I will find a solution to avoid it later.
>
> Two constraint: can be used only once and always on the beginning because
> it needs to manage the ring buffer reservation by itself. Hence the open prefix.
>
> How does it works?
>
> A new has_ending_string field has been added to struct ftrace_event_call and is
> false by default.
> Once a TRACE_EVENT uses an __ending_string field, it is set to 1.
>
> Until now, the ring buffer reservation was done in ftrace_raw_event_##call().
> It is still the case if we don't have an __ending_string() field. If we have one,
> open_string_assign() will manage it itself to allocate the appropriate size,
> depending of the size of the string to be copied for each trace.
>
> The choice between the usual static allocation and the new dynamic one depends
> on the "has_ending_string" value.
>
> It also support filtering because these strings behave essentially
> like usual fixed length string.
And I thought I was the only one that could master CPP wackiness ;-)
BTW, I was confused on IRC, because I was thinking this was printf like,
but no, we only need to be concerned about a string.
But I have another idea:
>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
> include/linux/ftrace_event.h | 1 +
> include/trace/ftrace.h | 62 ++++++++++++++++++++++++++++++++++++-----
> 2 files changed, 55 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> index 75f3ac0..b49bfbf 100644
> --- a/include/linux/ftrace_event.h
> +++ b/include/linux/ftrace_event.h
> @@ -100,6 +100,7 @@ struct ftrace_event_call {
> int n_preds;
> struct filter_pred **preds;
> void *mod;
> + bool has_ending_string;
>
> #ifdef CONFIG_EVENT_PROFILE
> atomic_t profile_count;
> diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
> index 60c5323..8ea750e 100644
> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -27,6 +27,9 @@
> #undef __field
> #define __field(type, item) type item;
>
> +#undef __ending_string
> +#define __ending_string(item) char item[];
> +
> #undef TP_STRUCT__entry
> #define TP_STRUCT__entry(args...) args
>
> @@ -146,12 +149,23 @@ ftrace_raw_output_##call(struct trace_iterator *iter, int flags) \
> if (!ret) \
> return 0;
>
> +#undef __ending_string
> +#define __ending_string(item) \
> + ret = trace_seq_printf(s, "\tfield: char " #item "[];\t" \
> + "offset:%u;\n", \
> + (unsigned int)offsetof(typeof(field), item)); \
> + if (!ret) \
> + return 0;
> +
> #undef __entry
> #define __entry REC
>
> #undef TP_printk
> #define TP_printk(fmt, args...) "%s, %s\n", #fmt, __stringify(args)
>
> +#undef open_string_assign
> +#define open_string_assign(call, dst, src) strcpy(__entry->dst, src)
> +
> #undef TP_fast_assign
> #define TP_fast_assign(args...) args
>
> @@ -189,6 +203,19 @@ ftrace_format_##call(struct trace_seq *s) \
> if (ret) \
> return ret;
>
> +/*
> + * We choose a size of MAX_FILTER_STR_VAL, then we behave like
> + * a usual string with the maximum size to keep being filterable.
> + */
> +#undef __ending_string
> +#define __ending_string(item) \
> + ret = trace_define_field(event_call, "char []", #item, \
> + offsetof(typeof(field), item), \
> + MAX_FILTER_STR_VAL); \
> + if (ret) \
> + return ret; \
> + event_call->has_ending_string = true;
> +
> #undef TRACE_EVENT
> #define TRACE_EVENT(call, proto, args, tstruct, func, print) \
> int \
> @@ -409,6 +436,22 @@ __attribute__((section("_ftrace_events"))) event_##call = { \
> #undef __entry
> #define __entry entry
>
> +/*
> + * If we have and ending undefined string size, then the size of
> + * the entry is dynamic. In such case we override the ring buffer
> + * reservation to manage it ourselves with our dynamic string size.
> + */
> +#undef open_string_assign
> +#define open_string_assign(call, dst, src) \
> + event = trace_current_buffer_lock_reserve( \
> + event_##call.id, \
> + sizeof(struct ftrace_raw_##call) + strlen(src) + 1, \
> + irq_flags, pc); \
> + if (!event) \
> + return; \
> + entry = ring_buffer_event_data(event); \
> + strcpy(entry->dst, src);
> +
> #undef TRACE_EVENT
> #define TRACE_EVENT(call, proto, args, tstruct, assign, print) \
> _TRACE_PROFILE(call, PARAMS(proto), PARAMS(args)) \
> @@ -418,20 +461,23 @@ static struct ftrace_event_call event_##call; \
> static void ftrace_raw_event_##call(proto) \
> { \
> struct ftrace_event_call *call = &event_##call; \
> - struct ring_buffer_event *event; \
> - struct ftrace_raw_##call *entry; \
> + struct ring_buffer_event *event = NULL; \
> + struct ftrace_raw_##call *entry = NULL; \
> unsigned long irq_flags; \
> int pc; \
> \
> local_save_flags(irq_flags); \
> pc = preempt_count(); \
> \
> - event = trace_current_buffer_lock_reserve(event_##call.id, \
> - sizeof(struct ftrace_raw_##call), \
> - irq_flags, pc); \
> - if (!event) \
> - return; \
> - entry = ring_buffer_event_data(event); \
> + if (!call->has_ending_string) { \
> + event = trace_current_buffer_lock_reserve( \
> + event_##call.id, \
> + sizeof(struct ftrace_raw_##call), \
> + irq_flags, pc); \
> + if (!event) \
> + return; \
> + entry = ring_buffer_event_data(event); \
> + } \
> \
> assign; \
> \
We can get rid of the has_ending_string and open_string_assign and have
this:
TP_STRUCT__entry(
__field(unsigned long, wait_usec)
__field(unsigned long, wait_nsec_rem)
__ending_string(name)
),
TP_fast_assign(
__entry->wait_nsec_rem = do_div(waittime, NSEC_PER_USEC);
__entry->wait_usec = (unsigned long) waittime;
strcpy(__entry->name, lock->name);
),
Then we could do the following in the print the assign:
#undef __field
#define __field(a,b)
#undef __array
#define __array(a,b,c)
#undef __ending_sting
#define __ending_string(name) __str_size__ += strlen(name) + 1;
#define TRACE_EVENT(call, proto, args, tstruct, assign, print)
static void ftrace_raw_event_##call(proto)
{
int __str_size__ = 0;
tstruct
event = trace_current_buffer_lock_reserve(event_##call.id,
sizeof(struct ftrace_raw_##call) +
__str_size__,
irq_flags, pc)
if (!event)
return;
entry = ring_buffer_event_data(event);
assign
}
Yes, I did not add other declarations nor the backslashes for the macro,
but you get what I'm doing, right?
-- Steve
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