Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback

Leo Yan
2021-09-23T13:50:27Z

Hi German,

On Thu, Sep 16, 2021 at 04:46:34PM +0100, German Gomez wrote:
> The head pointer of the AUX buffer managed by the arm_spe_pmu.c driver
> is not monotonically increasing, therefore the find_snapshot callback is
> needed in order to find the trace data within the AUX buffer and avoid
> wasting space in the perf.data file.
>
> The pointer is assumed to have wrapped if the buffer contains non-zero
> data at the end. If it has wrapped, the entire contents of the AUX
> buffer are stored in the perf.data file. Otherwise only the data up to
> the head pointer is stored.
>
> Reviewed-by: James Clark <james.clark@arm.com>
> Signed-off-by: German Gomez <german.gomez@arm.com>
> ---
> tools/perf/arch/arm64/util/arm-spe.c | 145 +++++++++++++++++++++++++++
> 1 file changed, 145 insertions(+)
>
> diff --git a/tools/perf/arch/arm64/util/arm-spe.c b/tools/perf/arch/arm64/util/arm-spe.c
> index f8b03d164b42..56785034fc84 100644
> --- a/tools/perf/arch/arm64/util/arm-spe.c
> +++ b/tools/perf/arch/arm64/util/arm-spe.c
> @@ -23,6 +23,7 @@
> #include "../../../util/auxtrace.h"
> #include "../../../util/record.h"
> #include "../../../util/arm-spe.h"
> +#include <tools/libc_compat.h> // reallocarray
>
> #define KiB(x) ((x) * 1024)
> #define MiB(x) ((x) * 1024 * 1024)
> @@ -31,6 +32,8 @@ struct arm_spe_recording {
> struct auxtrace_record itr;
> struct perf_pmu *arm_spe_pmu;
> struct evlist *evlist;
> + int wrapped_cnt;
> + bool *wrapped;
> };
>
> static void arm_spe_set_timestamp(struct auxtrace_record *itr,
> @@ -299,6 +302,146 @@ static int arm_spe_snapshot_finish(struct auxtrace_record *itr)
> return -EINVAL;
> }
>
> +static int arm_spe_alloc_wrapped_array(struct arm_spe_recording *ptr, int idx)
> +{
> + bool *wrapped;
> + int cnt = ptr->wrapped_cnt, new_cnt, i;
> +
> + /*
> + * No need to allocate, so return early.
> + */
> + if (idx < cnt)
> + return 0;
> +
> + /*
> + * Make ptr->wrapped as big as idx.
> + */
> + new_cnt = idx + 1;
> +
> + /*
> + * Free'ed in arm_spe_recording_free().
> + */
> + wrapped = reallocarray(ptr->wrapped, new_cnt, sizeof(bool));
> + if (!wrapped)
> + return -ENOMEM;
> +
> + /*
> + * init new allocated values.
> + */
> + for (i = cnt; i < new_cnt; i++)
> + wrapped[i] = false;
> +
> + ptr->wrapped_cnt = new_cnt;
> + ptr->wrapped = wrapped;
> +
> + return 0;
> +}
> +
> +static bool arm_spe_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;
> +
> + /*
> + * Defensively handle the case where head might be continually increasing - if its value is
> + * equal or greater than the size of the ring buffer, then we can safely determine it has
> + * wrapped around. Otherwise, continue to detect if head might have wrapped.
> + */
> + if (head >= buffer_size)
> + return true;
> +
> + /*
> + * We want to look the very last 512 byte (chosen arbitrarily) in the ring buffer.
> + */
> + watermark = buf_size - 512;
> +
> + /*
> + * 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 /= sizeof(u64);
> + buf_size /= sizeof(u64);
> +
> + /*
> + * 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 arm_spe_find_snapshot(struct auxtrace_record *itr, int idx,
> + struct auxtrace_mmap *mm, unsigned char *data,
> + u64 *head, u64 *old)
> +{
> + int err;
> + bool wrapped;
> + struct arm_spe_recording *ptr =
> + container_of(itr, struct arm_spe_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 = arm_spe_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 && arm_spe_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);
> +
> + /*
> + * 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;
> +}
> +
> static u64 arm_spe_reference(struct auxtrace_record *itr __maybe_unused)
> {
> struct timespec ts;
> @@ -313,6 +456,7 @@ static void arm_spe_recording_free(struct auxtrace_record *itr)
> struct arm_spe_recording *sper =
> container_of(itr, struct arm_spe_recording, itr);
>
> + free(sper->wrapped);
> free(sper);
> }
>
> @@ -336,6 +480,7 @@ struct auxtrace_record *arm_spe_recording_init(int *err,
> sper->itr.pmu = arm_spe_pmu;
> sper->itr.snapshot_start = arm_spe_snapshot_start;
> sper->itr.snapshot_finish = arm_spe_snapshot_finish;
> + sper->itr.find_snapshot = arm_spe_find_snapshot;

If I understand correctly, this patch copies the code from cs-etm for
snapshot handling. About 2 months ago, we removed the Arm cs-etm's
specific snapshot callback function and directly use perf's function
__auxtrace_mmap__read() to handle 'head' and 'tail' pointers. Please
see the commit for details:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2f01c200d4405c4562e45e8bb4de44a5ce37b217

Before I review more details for snapshot enabling in patches 03 and
04, could you confirm if Arm SPE can use the same way with cs-etm for
snapshot handling? From my understanding, this is a better way to
handle AUX buffer's 'head' and 'tail'.

Thanks,
Leo

> sper->itr.parse_snapshot_options = arm_spe_parse_snapshot_options;
> sper->itr.recording_options = arm_spe_recording_options;
> sper->itr.info_priv_size = arm_spe_info_priv_size;
> --
> 2.17.1
>