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

Leo Yan
2019-05-16T15:01:28Z

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])

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