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Date:   Wed, 14 Dec 2022 09:25:50 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Linyu Yuan <quic_linyyuan@quicinc.com>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        <linux-kernel@vger.kernel.org>, <bpf@vger.kernel.org>
Subject: Re: [RFC PATCH 2/2] trace: allocate temparary buffer for trace
 output usage
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In-Reply-To: <1671027102-21403-2-git-send-email-quic_linyyuan@quicinc.com>
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        <1671027102-21403-2-git-send-email-quic_linyyuan@quicinc.com>
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On Wed, 14 Dec 2022 22:11:42 +0800
Linyu Yuan <quic_linyyuan@quicinc.com> wrote:

> there is one dwc3 trace event declare as below,
> DECLARE_EVENT_CLASS(dwc3_log_event,
> 	TP_PROTO(u32 event, struct dwc3 *dwc),
> 	TP_ARGS(event, dwc),
> 	TP_STRUCT__entry(
> 		__field(u32, event)
> 		__field(u32, ep0state)
> 		__dynamic_array(char, str, DWC3_MSG_MAX)
> 	),
> 	TP_fast_assign(
> 		__entry->event = event;
> 		__entry->ep0state = dwc->ep0state;
> 	),
> 	TP_printk("event (%08x): %s", __entry->event,
> 			dwc3_decode_event(__get_str(str), DWC3_MSG_MAX,
> 				__entry->event, __entry->ep0state))
> );
> the problem is when trace function called, it will allocate up to
> DWC3_MSG_MAX bytes from trace event buffer, but never fill the buffer
> during fast assignment, it only fill the buffer when output function are
> called, so this means if output function are not called, the buffer will
> never used.
> 
> add __alloc_buf() and __get_buf() which will not allocate event buffer
> when trace function called, but when trace output function called, it will
> kmalloc buffer with size DWC3_MSG_MAX for temprary usage and free it
> before trace output function return.

This looks exactly like what the trace_seq *p is to be used for.

static notrace enum print_line_t					\
trace_raw_output_##call(struct trace_iterator *iter, int flags,		\
			struct trace_event *trace_event)		\
{									\
	struct trace_seq *s = &iter->seq;				\
	struct trace_seq __maybe_unused *p = &iter->tmp_seq;		\
                                        ^^^^^^^^^^^^^^^^^^^^

	struct trace_event_raw_##call *field;				\
	int ret;							\
									\
	field = (typeof(field))iter->ent;				\
									\
	ret = trace_raw_output_prep(iter, trace_event);			\
	if (ret != TRACE_TYPE_HANDLED)					\
		return ret;						\
									\
	trace_event_printf(iter, print);				\
									\
	return trace_handle_return(s);					\
}									\

That is a trace_seq buffer that is for temporary usage during the output.

See:
  include/trace/events/libata.h
  include/trace/events/scsi.h

As well as the macros trace_print_bitmask_seq(), trace_print_flags_seq(),
trace_print_symbols_seq(), etc.

-- Steve