Re: [RFC PATCH 2/2] trace: allocate temparary buffer for trace output usage

[Linyu Yuan]

is it possible add a generic macro like #define __get_buf(offset, len)  
(p->buffer + (offset))  ?

it only use buffer filed of struct trace_seq .

On 12/15/2022 9:22 AM, Linyu Yuan wrote:
> thanks for remind, will check and use it.
>
>
> thanks
>
> On 12/14/2022 10:25 PM, Steven Rostedt wrote:
> > On Wed, 14 Dec 2022 22:11:42 +0800
> > Linyu Yuan <quic_linyyuan@xxxxxxxxxxx> 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