linux-kernel - Re: [PATCH v1 2/3] perf session: Add swap operation for event TIME

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Date:   Mon, 26 Apr 2021 14:44:21 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Steve MacLean <Steve.MacLean@...rosoft.com>,
        Yonatan Goldschmidt <yonatan.goldschmidt@...nulate.io>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/3] perf session: Add swap operation for event
 TIME_CONV

Hi Adrian,

On Mon, Apr 26, 2021 at 08:40:00AM +0300, Adrian Hunter wrote:

[...]

> > +static void perf_event__time_conv_swap(union perf_event *event,
> > +				       bool sample_id_all __maybe_unused)
> > +{
> > +	size_t time_zero_size;
> > +
> > +	event->time_conv.time_shift = bswap_64(event->time_conv.time_shift);
> > +	event->time_conv.time_mult  = bswap_64(event->time_conv.time_mult);
> > +	event->time_conv.time_zero  = bswap_64(event->time_conv.time_zero);
> > +
> > +	time_zero_size = (void *)&event->time_conv.time_cycles - (void *)event;
> > +	if (event->header.size > time_zero_size) {
> 
> I wonder if we could have a helper for this e.g. (untested)
> 
> #define event_contains(obj, mem) (obj.header.size > offsetof(typeof(obj), mem))
> 
> 	if (event_contains(event->time_conv, time_cycles)) {

Yeah, this is a better implementation.  Will refine patch for this.

> > +		event->time_conv.time_cycles = bswap_64(event->time_conv.time_cycles);
> > +		event->time_conv.time_mask = bswap_64(event->time_conv.time_mask);
> > +		event->time_conv.cap_user_time_zero =
> > +			bswap_32(event->time_conv.cap_user_time_zero);
> > +		event->time_conv.cap_user_time_short =
> > +			bswap_32(event->time_conv.cap_user_time_short);
> 
> 'struct perf_record_time_conv' contains bool, the sizeof which, AFAIK, is not defined.
> Is it really 4 bytes on your implementation?  It is only 1 byte with gcc on x86.

On Arm64, bool is also 1 byte with GCC.

When working on this patch, I checked the size for structure
perf_record_time_conv, it gave out the structure size is 56; I wrongly
thought the bool size is 4 bytes and there have no padding.  In fact,
bool size is 1 byte and GCC pads 6 bytes at the end of structure.

> Either way, you should change 'struct perf_record_time_conv' so it uses a type of known size.
> Since you are the only one using it, it should match your implementation.

Will change to "unsigned int" value.  Thank you for reviewing and
suggestions.

Leo

> > +	}
> > +}
> > +
> >  typedef void (*perf_event__swap_op)(union perf_event *event,
> >  				    bool sample_id_all);
> >  
> > @@ -985,7 +1005,7 @@ static perf_event__swap_op perf_event__swap_ops[] = {
> >  	[PERF_RECORD_STAT]		  = perf_event__stat_swap,
> >  	[PERF_RECORD_STAT_ROUND]	  = perf_event__stat_round_swap,
> >  	[PERF_RECORD_EVENT_UPDATE]	  = perf_event__event_update_swap,
> > -	[PERF_RECORD_TIME_CONV]		  = perf_event__all64_swap,
> > +	[PERF_RECORD_TIME_CONV]		  = perf_event__time_conv_swap,
> >  	[PERF_RECORD_HEADER_MAX]	  = NULL,
> >  };
> >  
> > 
>