ACK: [PATCH][SRU][Xenial] i40e: use valid online CPU on q_vector initialization

[Kleber Souza]

On 07/11/17 20:58, Guilherme G. Piccoli wrote:
> BugLink: https://bugs.launchpad.net/bugs/1703663
> 
> Currently, the q_vector initialization routine sets the affinity_mask
> of a q_vector based on v_idx value. Meaning a loop iterates on v_idx,
> which is an incremental value, and the cpumask is created based on
> this value.
> 
> This is a problem in systems with multiple logical CPUs per core (like in
> SMT scenarios). If we disable some logical CPUs, by turning SMT off for
> example, we will end up with a sparse cpu_online_mask, i.e., only the first
> CPU in a core is online, and incremental filling in q_vector cpumask might
> lead to multiple offline CPUs being assigned to q_vectors.
> 
> Example: if we have a system with 8 cores each one containing 8 logical
> CPUs (SMT == 8 in this case), we have 64 CPUs in total. But if SMT is
> disabled, only the 1st CPU in each core remains online, so the
> cpu_online_mask in this case would have only 8 bits set, in a sparse way.
> 
> In general case, when SMT is off the cpu_online_mask has only C bits set:
> 0, 1*N, 2*N, ..., C*(N-1)  where
> C == # of cores;
> N == # of logical CPUs per core.
> In our example, only bits 0, 8, 16, 24, 32, 40, 48, 56 would be set.
> 
> This patch changes the way q_vector's affinity_mask is created: it iterates
> on v_idx, but consumes the CPU index from the cpu_online_mask instead of
> just using the v_idx incremental value.
> 
> No functional changes were introduced.
> 
> Signed-off-by: Guilherme G Piccoli <gpiccoli at linux.vnet.ibm.com>
> Tested-by: Andrew Bowers <andrewx.bowers at intel.com>
> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher at intel.com>
> (cherry picked from commit 7f6c553902bfa1c4e3f6cfa955c5ea036c7fe8e4)
> ---
>  drivers/net/ethernet/intel/i40e/i40e_main.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index 8f3b53e0dc46..f2ffa7ef74fd 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -7694,10 +7694,11 @@ static int i40e_init_msix(struct i40e_pf *pf)
>   * i40e_vsi_alloc_q_vector - Allocate memory for a single interrupt vector
>   * @vsi: the VSI being configured
>   * @v_idx: index of the vector in the vsi struct
> + * @cpu: cpu to be used on affinity_mask
>   *
>   * We allocate one q_vector.  If allocation fails we return -ENOMEM.
>   **/
> -static int i40e_vsi_alloc_q_vector(struct i40e_vsi *vsi, int v_idx)
> +static int i40e_vsi_alloc_q_vector(struct i40e_vsi *vsi, int v_idx, int cpu)
>  {
>  	struct i40e_q_vector *q_vector;
>  
> @@ -7708,7 +7709,8 @@ static int i40e_vsi_alloc_q_vector(struct i40e_vsi *vsi, int v_idx)
>  
>  	q_vector->vsi = vsi;
>  	q_vector->v_idx = v_idx;
> -	cpumask_set_cpu(v_idx, &q_vector->affinity_mask);
> +	cpumask_set_cpu(cpu, &q_vector->affinity_mask);
> +
>  	if (vsi->netdev)
>  		netif_napi_add(vsi->netdev, &q_vector->napi,
>  			       i40e_napi_poll, NAPI_POLL_WEIGHT);
> @@ -7732,8 +7734,7 @@ static int i40e_vsi_alloc_q_vector(struct i40e_vsi *vsi, int v_idx)
>  static int i40e_vsi_alloc_q_vectors(struct i40e_vsi *vsi)
>  {
>  	struct i40e_pf *pf = vsi->back;
> -	int v_idx, num_q_vectors;
> -	int err;
> +	int err, v_idx, num_q_vectors, current_cpu;
>  
>  	/* if not MSIX, give the one vector only to the LAN VSI */
>  	if (pf->flags & I40E_FLAG_MSIX_ENABLED)
> @@ -7743,10 +7744,15 @@ static int i40e_vsi_alloc_q_vectors(struct i40e_vsi *vsi)
>  	else
>  		return -EINVAL;
>  
> +	current_cpu = cpumask_first(cpu_online_mask);
> +
>  	for (v_idx = 0; v_idx < num_q_vectors; v_idx++) {
> -		err = i40e_vsi_alloc_q_vector(vsi, v_idx);
> +		err = i40e_vsi_alloc_q_vector(vsi, v_idx, current_cpu);
>  		if (err)
>  			goto err_out;
> +		current_cpu = cpumask_next(current_cpu, cpu_online_mask);
> +		if (unlikely(current_cpu >= nr_cpu_ids))
> +			current_cpu = cpumask_first(cpu_online_mask);
>  	}
>  
>  	return 0;
> 

Clean cherry-pick, good SRU justification, no functional changes and
limited to a single driver.

Acked-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>