Re: [PATCH V2 6/6] perf tools: Properly set the value of 'old' and 'head' in snapshot mode

Mathieu Poirier
2019-05-20T19:53:56Z

On Thu, 16 May 2019 at 09:00, Leo Yan <leo.yan@linaro.org> wrote:
>
> On Tue, May 14, 2019 at 01:40:18PM -0600, Mathieu Poirier wrote:
> > 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.
> >
> > Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> > ---
> > tools/perf/arch/arm/util/cs-etm.c | 124 ++++++++++++++++++++++++++++--
> > 1 file changed, 119 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/perf/arch/arm/util/cs-etm.c b/tools/perf/arch/arm/util/cs-etm.c
> > index 911426721170..3734e3fd18f8 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 +538,126 @@ 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() */
> > + wrapped = reallocarray(ptr->wrapped, cnt, sizeof(bool));
> > + if (!wrapped)
> > + return -ENOMEM;
> > +
> > + 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])
>
> I think here should be:
> if (buf[i << 3])
>

It would be if buf[] was a char *, but it is a u64 *.

> Otherwise, this patch looks good to me.
>
> > + 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)
> > {
> > - pr_debug3("%s: mmap index %d old head %zu new head %zu size %zu\n",
> > + 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 %lx new head %lx size %lx\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 +698,8 @@ 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
> >