[lttng-dev] [PATCH lttng-tools] Fix: use "flush empty" ioctl for snapshots

[Jérémie Galarneau]

Merged in master, stable-2.10, stable-2.9, and stable-2.8 (slightly adapted).

Thanks!
Jérémie

On 11 May 2017 at 17:53, Mathieu Desnoyers
<mathieu.desnoyers at efficios.com> wrote:
> When the flush empty ioctl is available, use it to produce an empty
> packet at the end of the snapshot, which ensures the stream intersection
> feature works.
>
> If this specific ioctl is not available, fallback on the "flush" ioctl,
> which does not produce empty packets.
>
> In that situation, there were two prior behaviors possible for
> lttng-modules: earlier versions implement a "snapshot" command which
> does not perform an implicit "flush_empty". In that case, the stream
> intersection feature may not be reliable. In more recent lttng-modules
> versions (included stable branch) which did not implement the
> flush_empty ioctl, the snapshot ioctl implicitly performed a
> flush_empty, which makes the stream intersection feature work, but has
> side-effects on the snapshot ioctl performed by the live timer (produces
> a stream of empty packets in live mode).
>
> [ Please apply to master, 2.10, 2.9, 2.8 branches. ]
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> ---
>  src/common/kernel-consumer/kernel-consumer.c | 15 +++++++++++++--
>  src/common/kernel-ctl/kernel-ctl.c           |  5 +++++
>  src/common/kernel-ctl/kernel-ctl.h           |  1 +
>  src/common/kernel-ctl/kernel-ioctl.h         |  4 +++-
>  4 files changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/src/common/kernel-consumer/kernel-consumer.c b/src/common/kernel-consumer/kernel-consumer.c
> index 16aeae1..0a8256f 100644
> --- a/src/common/kernel-consumer/kernel-consumer.c
> +++ b/src/common/kernel-consumer/kernel-consumer.c
> @@ -199,9 +199,20 @@ int lttng_kconsumer_snapshot_channel(uint64_t key, char *path,
>                         channel->streams_sent_to_relayd = true;
>                 }
>
> -               ret = kernctl_buffer_flush(stream->wait_fd);
> +               ret = kernctl_buffer_flush_empty(stream->wait_fd);
>                 if (ret < 0) {
> -                       ERR("Failed to flush kernel stream");
> +                       /*
> +                        * Doing a buffer flush which does not take into
> +                        * account empty packets. This is not perfect
> +                        * for stream intersection, but required as a
> +                        * fall-back when "flush_empty" is not
> +                        * implemented by lttng-modules.
> +                        */
> +                       ret = kernctl_buffer_flush(stream->wait_fd);
> +                       if (ret < 0) {
> +                               ERR("Failed to flush kernel stream");
> +                               goto end_unlock;
> +                       }
>                         goto end_unlock;
>                 }
>
> diff --git a/src/common/kernel-ctl/kernel-ctl.c b/src/common/kernel-ctl/kernel-ctl.c
> index 4911d86..6a256fe 100644
> --- a/src/common/kernel-ctl/kernel-ctl.c
> +++ b/src/common/kernel-ctl/kernel-ctl.c
> @@ -408,6 +408,11 @@ int kernctl_buffer_flush(int fd)
>         return LTTNG_IOCTL_CHECK(fd, RING_BUFFER_FLUSH);
>  }
>
> +int kernctl_buffer_flush_empty(int fd)
> +{
> +       return LTTNG_IOCTL_CHECK(fd, RING_BUFFER_FLUSH_EMPTY);
> +}
> +
>  /* returns the version of the metadata. */
>  int kernctl_get_metadata_version(int fd, uint64_t *version)
>  {
> diff --git a/src/common/kernel-ctl/kernel-ctl.h b/src/common/kernel-ctl/kernel-ctl.h
> index 952fb6f..d88f5e7 100644
> --- a/src/common/kernel-ctl/kernel-ctl.h
> +++ b/src/common/kernel-ctl/kernel-ctl.h
> @@ -93,6 +93,7 @@ int kernctl_get_subbuf(int fd, unsigned long *pos);
>  int kernctl_put_subbuf(int fd);
>
>  int kernctl_buffer_flush(int fd);
> +int kernctl_buffer_flush_empty(int fd);
>  int kernctl_get_metadata_version(int fd, uint64_t *version);
>
>  /* index */
> diff --git a/src/common/kernel-ctl/kernel-ioctl.h b/src/common/kernel-ctl/kernel-ioctl.h
> index cb23620..3c4f412 100644
> --- a/src/common/kernel-ctl/kernel-ioctl.h
> +++ b/src/common/kernel-ctl/kernel-ioctl.h
> @@ -47,7 +47,7 @@
>  #define RING_BUFFER_GET_MMAP_LEN            _IOR(0xF6, 0x0A, unsigned long)
>  /* returns the offset of the subbuffer belonging to the mmap reader. */
>  #define RING_BUFFER_GET_MMAP_READ_OFFSET    _IOR(0xF6, 0x0B, unsigned long)
> -/* flush the current sub-buffer */
> +/* Flush the current sub-buffer, if non-empty. */
>  #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)
> @@ -57,6 +57,8 @@
>   * sub-buffer.
>   */
>  #define RING_BUFFER_SNAPSHOT_SAMPLE_POSITIONS  _IO(0xF6, 0x0E)
> +/* Flush the current sub-buffer, even if empty. */
> +#define RING_BUFFER_FLUSH_EMPTY                        _IO(0xF6, 0x0F)
>
>  /* returns the timestamp begin of the current sub-buffer */
>  #define LTTNG_RING_BUFFER_GET_TIMESTAMP_BEGIN     _IOR(0xF6, 0x20, uint64_t)
> --
> 2.1.4
>



-- 
Jérémie Galarneau
EfficiOS Inc.
http://www.efficios.com