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

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Message-ID: <20210426073906.GB327854@leoy-ThinkPad-X240s>
Date:   Mon, 26 Apr 2021 15:39:06 +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

On Mon, Apr 26, 2021 at 10:26:12AM +0300, Adrian Hunter wrote:
> On 26/04/21 9:44 am, Leo Yan wrote:
> > 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.
> 
> If it is 1 byte, I'd have thought __u8 i.e.
> 
> struct perf_record_time_conv {
> 	struct perf_event_header header;
> 	__u64			 time_shift;
> 	__u64			 time_mult;
> 	__u64			 time_zero;
> 	__u64			 time_cycles;
> 	__u64			 time_mask;
> 	__u8			 cap_user_time_zero;
> 	__u8			 cap_user_time_short;
> 	__u8			 unused[6];
> };
> 

Ah, yes, thanks a lot for reminding.