[lttng-dev] [RFC PATCH lttng-ust] Add ustctl_snapshot_sample_positions ustctl command

[Mathieu Desnoyers]

It should be a minor bump of the ABI only, and you should investigate bumping
the right number for the soname:

configure.ac:
AC_SUBST([LTTNG_UST_CTL_LIBRARY_VERSION], [2:0:0])

Following the link in the comment above, as we are adding a new symbol.

Thanks,

Mathieu

----- On May 4, 2017, at 5:26 PM, Jeremie Galarneau jeremie.galarneau at efficios.com wrote:

> The major version is bumped as a new API entry point is
> introduced.
> 
> Signed-off-by: Jérémie Galarneau <jeremie.galarneau at efficios.com>
> ---
> include/lttng/ust-abi.h              |  2 +-
> include/lttng/ust-ctl.h              |  1 +
> liblttng-ust-ctl/ustctl.c            | 19 ++++++++++++++++++
> libringbuffer/frontend.h             |  5 +++++
> libringbuffer/ring_buffer_frontend.c | 38 ++++++++++++++++++++++++++++++++++++
> 5 files changed, 64 insertions(+), 1 deletion(-)
> 
> diff --git a/include/lttng/ust-abi.h b/include/lttng/ust-abi.h
> index 6e8f8d7b..35b8da00 100644
> --- a/include/lttng/ust-abi.h
> +++ b/include/lttng/ust-abi.h
> @@ -42,7 +42,7 @@
> #define LTTNG_UST_COMM_MAGIC			0xC57C57C5
> 
> /* Version for ABI between liblttng-ust, sessiond, consumerd */
> -#define LTTNG_UST_ABI_MAJOR_VERSION		7
> +#define LTTNG_UST_ABI_MAJOR_VERSION		8
> #define LTTNG_UST_ABI_MINOR_VERSION		1
> 
> enum lttng_ust_instrumentation {
> diff --git a/include/lttng/ust-ctl.h b/include/lttng/ust-ctl.h
> index 7b750fd4..f6c1dc45 100644
> --- a/include/lttng/ust-ctl.h
> +++ b/include/lttng/ust-ctl.h
> @@ -226,6 +226,7 @@ int ustctl_put_next_subbuf(struct ustctl_consumer_stream
> *stream);
> /* snapshot */
> 
> int ustctl_snapshot(struct ustctl_consumer_stream *stream);
> +int ustctl_snapshot_sample_positions(struct ustctl_consumer_stream *stream);
> int ustctl_snapshot_get_consumed(struct ustctl_consumer_stream *stream,
> 		unsigned long *pos);
> int ustctl_snapshot_get_produced(struct ustctl_consumer_stream *stream,
> diff --git a/liblttng-ust-ctl/ustctl.c b/liblttng-ust-ctl/ustctl.c
> index 2af79147..a317de96 100644
> --- a/liblttng-ust-ctl/ustctl.c
> +++ b/liblttng-ust-ctl/ustctl.c
> @@ -1500,6 +1500,25 @@ int ustctl_snapshot(struct ustctl_consumer_stream
> *stream)
> 			&buf->prod_snapshot, consumer_chan->chan->handle);
> }
> 
> +/*
> + * Get a snapshot of the current ring buffer producer and consumer positions
> + * even if the consumed and produced positions are contained withinin the same
> + * subbuffer.
> + */
> +int ustctl_snapshot_sample_positions(struct ustctl_consumer_stream *stream)
> +{
> +	struct lttng_ust_lib_ring_buffer *buf;
> +	struct ustctl_consumer_channel *consumer_chan;
> +
> +	if (!stream)
> +		return -EINVAL;
> +	buf = stream->buf;
> +	consumer_chan = stream->chan;
> +	return lib_ring_buffer_snapshot_sample_positions(buf,
> +			&buf->cons_snapshot, &buf->prod_snapshot,
> +			consumer_chan->chan->handle);
> +}
> +
> /* Get the consumer position (iteration start) */
> int ustctl_snapshot_get_consumed(struct ustctl_consumer_stream *stream,
> 		unsigned long *pos)
> diff --git a/libringbuffer/frontend.h b/libringbuffer/frontend.h
> index 160fd455..a2f74596 100644
> --- a/libringbuffer/frontend.h
> +++ b/libringbuffer/frontend.h
> @@ -131,6 +131,11 @@ extern int lib_ring_buffer_snapshot(struct
> lttng_ust_lib_ring_buffer *buf,
> 				    unsigned long *consumed,
> 				    unsigned long *produced,
> 				    struct lttng_ust_shm_handle *handle);
> +extern int lib_ring_buffer_snapshot_sample_positions(
> +				    struct lttng_ust_lib_ring_buffer *buf,
> +				    unsigned long *consumed,
> +				    unsigned long *produced,
> +				    struct lttng_ust_shm_handle *handle);
> extern void lib_ring_buffer_move_consumer(struct lttng_ust_lib_ring_buffer *buf,
> 					  unsigned long consumed_new,
> 					  struct lttng_ust_shm_handle *handle);
> diff --git a/libringbuffer/ring_buffer_frontend.c
> b/libringbuffer/ring_buffer_frontend.c
> index 6dd81e14..4627ff2a 100644
> --- a/libringbuffer/ring_buffer_frontend.c
> +++ b/libringbuffer/ring_buffer_frontend.c
> @@ -1306,6 +1306,44 @@ nodata:
> }
> 
> /**
> + * Performs the same function as lib_ring_buffer_snapshot(), but the positions
> + * are saved regardless of whether the consumed and produced positions are
> + * in the same subbuffer.
> + * @buf: ring buffer
> + * @consumed: consumed count indicating the position where to read
> + * @produced: produced count, indicates position when to stop reading
> + *
> + * This function is meant to provide information on the exact producer and
> + * consumer positions without regard for the "snapshot" feature.
> + */
> +int lib_ring_buffer_snapshot_sample_positions(
> +			     struct lttng_ust_lib_ring_buffer *buf,
> +			     unsigned long *consumed, unsigned long *produced,
> +			     struct lttng_ust_shm_handle *handle)
> +{
> +	struct channel *chan;
> +	const struct lttng_ust_lib_ring_buffer_config *config;
> +	unsigned long consumed_cur, write_offset;
> +
> +	chan = shmp(handle, buf->backend.chan);
> +	if (!chan)
> +		return -EPERM;
> +	config = &chan->backend.config;
> +	cmm_smp_rmb();
> +	*consumed = uatomic_read(&buf->consumed);
> +	/*
> +	 * No need to issue a memory barrier between consumed count read and
> +	 * write offset read, because consumed count can only change
> +	 * concurrently in overwrite mode, and we keep a sequence counter
> +	 * identifier derived from the write offset to check we are getting
> +	 * the same sub-buffer we are expecting (the sub-buffers are atomically
> +	 * "tagged" upon writes, tags are checked upon read).
> +	 */
> +	*produced = v_read(config, &buf->offset);
> +	return 0;
> +}
> +
> +/**
>  * lib_ring_buffer_move_consumer - move consumed counter forward
>  * @buf: ring buffer
>  * @consumed_new: new consumed count value
> --
> 2.12.2

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com