Re: [PATCH] perf tools: Properly set the value of 'old' and 'head' in snapshot mode

[Leo Yan]

On Thu, Jun 06, 2019 at 05:10:56PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Jun 05, 2019 at 10:16:33AM -0600, Mathieu Poirier escreveu:
> > This patch adds the necessay intelligence to properly compute the value
> > of 'old' and 'head' when operating in snapshot mode.  That way we can get
> > the latest information in the AUX buffer and be compatible with the
> > generic AUX ring buffer mechanic.
> 
> Leo, have you had the chance to test/review this one? Suzuki?

Sure.  I applied this patch on the perf/core branch (with latest
commit 3e4fbf36c1e3 'perf augmented_raw_syscalls: Move reading
filename to the loop') and passed testing with below steps:

# perf record -e cs_etm/@tmc_etr0/ -S -m,64 --per-thread ./sort &
[1] 19097
Bubble sorting array of 30000 elements

# kill -USR2 19097
# kill -USR2 19097
# kill -USR2 19097
[ perf record: Woken up 4 times to write data ]
[ perf record: Captured and wrote 0.753 MB perf.data ]

FWIW:

Tested-by: Leo Yan <leo.yan@xxxxxxxxxx>

> I also changed the subject to:
> 
>   [PATCH] perf cs-etm: Properly set the value of 'old' and 'head' in snapshot mode
> 
> So that when looking at a 'git log --oneline' one can have the proper
> context and know that its about cs-etm.
> 
> - Arnaldo
>  
> > Signed-off-by: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> > ---
> >  tools/perf/arch/arm/util/cs-etm.c | 127 +++++++++++++++++++++++++++++-
> >  1 file changed, 123 insertions(+), 4 deletions(-)
> > 
> > diff --git a/tools/perf/arch/arm/util/cs-etm.c b/tools/perf/arch/arm/util/cs-etm.c
> > index 911426721170..0a278bbcaba6 100644
> > --- a/tools/perf/arch/arm/util/cs-etm.c
> > +++ b/tools/perf/arch/arm/util/cs-etm.c
> > @@ -31,6 +31,8 @@ struct cs_etm_recording {
> >  	struct auxtrace_record	itr;
> >  	struct perf_pmu		*cs_etm_pmu;
> >  	struct perf_evlist	*evlist;
> > +	int			wrapped_cnt;
> > +	bool			*wrapped;
> >  	bool			snapshot_mode;
> >  	size_t			snapshot_size;
> >  };
> > @@ -536,16 +538,131 @@ static int cs_etm_info_fill(struct auxtrace_record *itr,
> >  	return 0;
> >  }
> >  
> > -static int cs_etm_find_snapshot(struct auxtrace_record *itr __maybe_unused,
> > +static int cs_etm_alloc_wrapped_array(struct cs_etm_recording *ptr, int idx)
> > +{
> > +	bool *wrapped;
> > +	int cnt = ptr->wrapped_cnt;
> > +
> > +	/* Make @ptr->wrapped as big as @idx */
> > +	while (cnt <= idx)
> > +		cnt++;
> > +
> > +	/*
> > +	 * Free'ed in cs_etm_recording_free().  Using realloc() to avoid
> > +	 * cross compilation problems where the host's system supports
> > +	 * reallocarray() but not the target.
> > +	 */
> > +	wrapped = realloc(ptr->wrapped, cnt * sizeof(bool));
> > +	if (!wrapped)
> > +		return -ENOMEM;
> > +
> > +	wrapped[cnt - 1] = false;
> > +	ptr->wrapped_cnt = cnt;
> > +	ptr->wrapped = wrapped;
> > +
> > +	return 0;
> > +}
> > +
> > +static bool cs_etm_buffer_has_wrapped(unsigned char *buffer,
> > +				      size_t buffer_size, u64 head)
> > +{
> > +	u64 i, watermark;
> > +	u64 *buf = (u64 *)buffer;
> > +	size_t buf_size = buffer_size;
> > +
> > +	/*
> > +	 * We want to look the very last 512 byte (chosen arbitrarily) in
> > +	 * the ring buffer.
> > +	 */
> > +	watermark = buf_size - 512;
> > +
> > +	/*
> > +	 * @head is continuously increasing - if its value is equal or greater
> > +	 * than the size of the ring buffer, it has wrapped around.
> > +	 */
> > +	if (head >= buffer_size)
> > +		return true;
> > +
> > +	/*
> > +	 * The value of @head is somewhere within the size of the ring buffer.
> > +	 * This can be that there hasn't been enough data to fill the ring
> > +	 * buffer yet or the trace time was so long that @head has numerically
> > +	 * wrapped around.  To find we need to check if we have data at the very
> > +	 * end of the ring buffer.  We can reliably do this because mmap'ed
> > +	 * pages are zeroed out and there is a fresh mapping with every new
> > +	 * session.
> > +	 */
> > +
> > +	/* @head is less than 512 byte from the end of the ring buffer */
> > +	if (head > watermark)
> > +		watermark = head;
> > +
> > +	/*
> > +	 * Speed things up by using 64 bit transactions (see "u64 *buf" above)
> > +	 */
> > +	watermark >>= 3;
> > +	buf_size >>= 3;
> > +
> > +	/*
> > +	 * If we find trace data at the end of the ring buffer, @head has
> > +	 * been there and has numerically wrapped around at least once.
> > +	 */
> > +	for (i = watermark; i < buf_size; i++)
> > +		if (buf[i])
> > +			return true;
> > +
> > +	return false;
> > +}
> > +
> > +static int cs_etm_find_snapshot(struct auxtrace_record *itr,
> >  				int idx, struct auxtrace_mmap *mm,
> > -				unsigned char *data __maybe_unused,
> > +				unsigned char *data,
> >  				u64 *head, u64 *old)
> >  {
> > +	int err;
> > +	bool wrapped;
> > +	struct cs_etm_recording *ptr =
> > +			container_of(itr, struct cs_etm_recording, itr);
> > +
> > +	/*
> > +	 * Allocate memory to keep track of wrapping if this is the first
> > +	 * time we deal with this *mm.
> > +	 */
> > +	if (idx >= ptr->wrapped_cnt) {
> > +		err = cs_etm_alloc_wrapped_array(ptr, idx);
> > +		if (err)
> > +			return err;
> > +	}
> > +
> > +	/*
> > +	 * Check to see if *head has wrapped around.  If it hasn't only the
> > +	 * amount of data between *head and *old is snapshot'ed to avoid
> > +	 * bloating the perf.data file with zeros.  But as soon as *head has
> > +	 * wrapped around the entire size of the AUX ring buffer it taken.
> > +	 */
> > +	wrapped = ptr->wrapped[idx];
> > +	if (!wrapped && cs_etm_buffer_has_wrapped(data, mm->len, *head)) {
> > +		wrapped = true;
> > +		ptr->wrapped[idx] = true;
> > +	}
> > +
> >  	pr_debug3("%s: mmap index %d old head %zu new head %zu size %zu\n",
> >  		  __func__, idx, (size_t)*old, (size_t)*head, mm->len);
> >  
> > -	*old = *head;
> > -	*head += mm->len;
> > +	/* No wrap has occurred, we can just use *head and *old. */
> > +	if (!wrapped)
> > +		return 0;
> > +
> > +	/*
> > +	 * *head has wrapped around - adjust *head and *old to pickup the
> > +	 * entire content of the AUX buffer.
> > +	 */
> > +	if (*head >= mm->len) {
> > +		*old = *head - mm->len;
> > +	} else {
> > +		*head += mm->len;
> > +		*old = *head - mm->len;
> > +	}
> >  
> >  	return 0;
> >  }
> > @@ -586,6 +703,8 @@ static void cs_etm_recording_free(struct auxtrace_record *itr)
> >  {
> >  	struct cs_etm_recording *ptr =
> >  			container_of(itr, struct cs_etm_recording, itr);
> > +
> > +	zfree(&ptr->wrapped);
> >  	free(ptr);
> >  }
> >  
> > -- 
> > 2.17.1
> 
> -- 
> 
> - Arnaldo