[lttng-dev] [RFC PATCH lttng-modules] Add RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS command

[Mathieu Desnoyers]

Merged into master, thanks!

Mathieu

----- On May 4, 2017, at 5:25 PM, Jeremie Galarneau jeremie.galarneau at efficios.com wrote:

> There is no need to bump the LTTNG_MODULES_ABI_MINOR_VERSION
> since the multiple wildcard feature introduced as part of the 2.10
> release already bumps it from 2 to 3.
> 
> Signed-off-by: Jérémie Galarneau <jeremie.galarneau at efficios.com>
> ---
> lib/ringbuffer/frontend.h             |  4 ++++
> lib/ringbuffer/ring_buffer_frontend.c | 31 +++++++++++++++++++++++++++++++
> lib/ringbuffer/ring_buffer_vfs.c      |  3 +++
> lib/ringbuffer/vfs.h                  |  6 ++++++
> 4 files changed, 44 insertions(+)
> 
> diff --git a/lib/ringbuffer/frontend.h b/lib/ringbuffer/frontend.h
> index 6ff1545..909abc2 100644
> --- a/lib/ringbuffer/frontend.h
> +++ b/lib/ringbuffer/frontend.h
> @@ -106,6 +106,10 @@ extern void lib_ring_buffer_release_read(struct
> lib_ring_buffer *buf);
> extern int lib_ring_buffer_snapshot(struct lib_ring_buffer *buf,
> 				    unsigned long *consumed,
> 				    unsigned long *produced);
> +extern int lib_ring_buffer_snapshot_sample_positions(
> +				    struct lib_ring_buffer *buf,
> +				    unsigned long *consumed,
> +				    unsigned long *produced);
> extern void lib_ring_buffer_move_consumer(struct lib_ring_buffer *buf,
> 					  unsigned long consumed_new);
> 
> diff --git a/lib/ringbuffer/ring_buffer_frontend.c
> b/lib/ringbuffer/ring_buffer_frontend.c
> index 68b6238..a477832 100644
> --- a/lib/ringbuffer/ring_buffer_frontend.c
> +++ b/lib/ringbuffer/ring_buffer_frontend.c
> @@ -1120,6 +1120,37 @@ nodata:
> EXPORT_SYMBOL_GPL(lib_ring_buffer_snapshot);
> 
> /**
> + * 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 byte count indicating the last position read
> + * @produced: produced byte count indicating the last position written
> + *
> + * 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 lib_ring_buffer *buf,
> +		unsigned long *consumed, unsigned long *produced)
> +{
> +	struct channel *chan = buf->backend.chan;
> +	const struct lib_ring_buffer_config *config = &chan->backend.config;
> +
> +	smp_rmb();
> +	*consumed = atomic_long_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_put_snapshot - move consumed counter forward
>  *
>  * Should only be called from consumer context.
> diff --git a/lib/ringbuffer/ring_buffer_vfs.c b/lib/ringbuffer/ring_buffer_vfs.c
> index 15da212..603dcfc 100644
> --- a/lib/ringbuffer/ring_buffer_vfs.c
> +++ b/lib/ringbuffer/ring_buffer_vfs.c
> @@ -202,6 +202,9 @@ long lib_ring_buffer_ioctl(struct file *filp, unsigned int
> cmd,
> 			lib_ring_buffer_switch_remote_empty(buf);
> 		return lib_ring_buffer_snapshot(buf, &buf->cons_snapshot,
> 					    &buf->prod_snapshot);
> +	case RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS:
> +		return lib_ring_buffer_snapshot_sample_positions(buf,
> +				&buf->cons_snapshot, &buf->prod_snapshot);
> 	case RING_BUFFER_SNAPSHOT_GET_CONSUMED:
> 		return put_ulong(buf->cons_snapshot, arg);
> 	case RING_BUFFER_SNAPSHOT_GET_PRODUCED:
> diff --git a/lib/ringbuffer/vfs.h b/lib/ringbuffer/vfs.h
> index 5444a4b..8a89caf 100644
> --- a/lib/ringbuffer/vfs.h
> +++ b/lib/ringbuffer/vfs.h
> @@ -115,6 +115,12 @@ ssize_t vfs_lib_ring_buffer_splice_read(struct file *in,
> loff_t *ppos,
> #define RING_BUFFER_FLUSH			_IO(0xF6, 0x0C)
> /* Get the current version of the metadata cache (after a get_next). */
> #define RING_BUFFER_GET_METADATA_VERSION	_IOR(0xF6, 0x0D, uint64_t)
> +/*
> + * Get a snapshot of the current ring buffer producer and consumer positions,
> + * regardless of whether or not the two positions are contained within the same
> + * sub-buffer.
> + */
> +#define RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS	_IO(0xF6, 0x0E)
> 
> #ifdef CONFIG_COMPAT
> /* Get a snapshot of the current ring buffer producer and consumer positions */
> --
> 2.12.2

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