[dpdk-dev] [PATCH] net/failsafe: improve stats accuracy

[Gaëtan Rivet]

Hey Matan,

Thanks for the patch. Here are some remarks:

On Mon, Oct 16, 2017 at 07:41:50AM +0000, Matan Azrad wrote:
> The stats_get API was changed to return error in case of failure at
> stats getting process time.
> By this way, failsafe can get stats snapshot in removal process for
> each PMD which can give stats after removal event.
> 
> This patch implements ultimate stats snapshot on removal time by
> trying to get the removed device stats before calling to dev_close.
> 

I would reformulate the commit log as such:

The stats_get API has changed to signal a potential failure to read
stats. Furthermore, some PMDs are able to provide statistics even
after a removal event occured.

Considering this, the fail-safe can try to access the latest statistics
of a PMD to improve statistics readings.

Attempt an ultimate statistics read on removal time;
if that fails, use the latest recorded snapshot.

> Signed-off-by: Matan Azrad <matan at mellanox.com>
> ---
>  drivers/net/failsafe/failsafe_ether.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/failsafe/failsafe_ether.c b/drivers/net/failsafe/failsafe_ether.c
> index f4db423..2758d4c 100644
> --- a/drivers/net/failsafe/failsafe_ether.c
> +++ b/drivers/net/failsafe/failsafe_ether.c
> @@ -312,8 +312,12 @@
>  static void
>  fs_dev_stats_save(struct sub_device *sdev)
>  {
> +	struct rte_eth_stats stats;
> +
> +	/* Get stats now or take it from last snapshot. */
>  	failsafe_stats_increment(&PRIV(sdev->fs_dev)->stats_accumulator,
> -			&sdev->stats_snapshot);
> +		rte_eth_stats_get(PORT_ID(sdev), &stats) ?
> +		&sdev->stats_snapshot : &stats);

The conditional is awkward within the parameter list here.
Something like:

    struct rte_eth_stats stats;
    int err;

    /* Attempt to read current stats. */
    err = rte_eth_stats_get(PORT_ID(sdev), &stats);
    if (err)
        WARN("Could not access latest statistics from sub-device %d, using latest snapshot",
             SUB_ID(sdev));
    /* Use either current stats or latest snapshot. */
    failsafe_stats_increment(&PRIV(sdev->fs_dev)->stats_accumulator,
                             err ? &sdev->stats_snapshot : &stats);

Seems easier to read and allows warning the user of potential inaccuracies.
On that matter, it may be interesting to measure the time since the last snapshot
and display it as well.

>  	memset(&sdev->stats_snapshot, 0, sizeof(struct rte_eth_stats));
>  }
>  
> -- 
> 1.8.3.1
> 

-- 
Gaëtan Rivet
6WIND