Subject: Re: [PATCH][net-next] net/mlx5e: avoid to call net_dim and dim_update_sample

From: Vadim Fedorenko <[email-masked]>
Date: Mon, 16 Dec 2024 21:11:43 +0000

On 12/12/2024 11:47, Li RongQing wrote:
> High cpu usage for net_dim is seen still after commit 61bf0009a765
> ("dim: pass dim_sample to net_dim() by reference"), the calling
> net_dim can be avoid under network low throughput or pingpong mode by
> checking the event counter, even under high throughput, it maybe only
> rx or tx direction
> 
> And don't initialize dim_sample variable, since it will gets
> overwritten by dim_update_sample

dim_update_sample doesn't init dim_sample::comp_ctr, which is later used 
in net_dim()->dim_calc_stats(). This change can bring uninitialized
memory to the whole calculation. Keep it initialized.

> 
> Signed-off-by: Li RongQing <[email-masked]>
> Signed-off-by: Shuo Li <[email-masked]>
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/en_txrx.c | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_txrx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_txrx.c
> index 7610829..7c525e9 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_txrx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_txrx.c
> @@ -49,11 +49,19 @@ static inline bool mlx5e_channel_no_affinity_change(struct mlx5e_channel *c)
>   static void mlx5e_handle_tx_dim(struct mlx5e_txqsq *sq)
>   {
>   	struct mlx5e_sq_stats *stats = sq->stats;
> -	struct dim_sample dim_sample = {};
> +	struct dim_sample dim_sample;
> +	u16 nevents;
>   
>   	if (unlikely(!test_bit(MLX5E_SQ_STATE_DIM, &sq->state)))
>   		return;
>   
> +	if (sq->dim->state == DIM_MEASURE_IN_PROGRESS) {
> +		nevents = BIT_GAP(BITS_PER_TYPE(u16), sq->cq.event_ctr,
> +						  sq->dim->start_sample.event_ctr);
> +		if (nevents < DIM_NEVENTS)
> +			return;
> +	}
> +
>   	dim_update_sample(sq->cq.event_ctr, stats->packets, stats->bytes, &dim_sample);
>   	net_dim(sq->dim, &dim_sample);
>   }
> @@ -61,11 +69,19 @@ static void mlx5e_handle_tx_dim(struct mlx5e_txqsq *sq)
>   static void mlx5e_handle_rx_dim(struct mlx5e_rq *rq)
>   {
>   	struct mlx5e_rq_stats *stats = rq->stats;
> -	struct dim_sample dim_sample = {};
> +	struct dim_sample dim_sample;
> +	u16 nevents;
>   
>   	if (unlikely(!test_bit(MLX5E_RQ_STATE_DIM, &rq->state)))
>   		return;
>   
> +	if (rq->dim->state == DIM_MEASURE_IN_PROGRESS) {
> +		nevents = BIT_GAP(BITS_PER_TYPE(u16), rq->cq.event_ctr,
> +						  rq->dim->start_sample.event_ctr);
> +		if (nevents < DIM_NEVENTS)
> +			return;
> +	}
> +
>   	dim_update_sample(rq->cq.event_ctr, stats->packets, stats->bytes, &dim_sample);
>   	net_dim(rq->dim, &dim_sample);
>   }