Subject: Re: [PATCH net-next v3 1/7] enic: Create enic_wq/rq structures to bundle per wq/rq data

From: Vadim Fedorenko <[email-masked]>
Date: Sat, 9 Nov 2024 20:27:50 +0000

On 08/11/2024 21:47, Nelson Escobar wrote:
> Bundling the wq/rq specific data into dedicated enic_wq/rq structures
> cleans up the enic structure and simplifies future changes related to
> wq/rq.
> 
> Co-developed-by: John Daley <[email-masked]>
> Signed-off-by: John Daley <[email-masked]>
> Co-developed-by: Satish Kharat <[email-masked]>
> Signed-off-by: Satish Kharat <[email-masked]>
> Signed-off-by: Nelson Escobar <[email-masked]>
> Reviewed-by: Simon Horman <[email-masked]>
> ---
>   drivers/net/ethernet/cisco/enic/enic.h         |  18 ++--
>   drivers/net/ethernet/cisco/enic/enic_ethtool.c |   4 +-
>   drivers/net/ethernet/cisco/enic/enic_main.c    | 120 ++++++++++++-------------
>   drivers/net/ethernet/cisco/enic/enic_res.c     |  12 +--
>   4 files changed, 81 insertions(+), 73 deletions(-)
> 
> diff --git a/drivers/net/ethernet/cisco/enic/enic.h b/drivers/net/ethernet/cisco/enic/enic.h
> index 0cc3644ee8554f52401a0be7f44a1475ab2ea2b9..e6edb43515b97feeb21a9b55a1eeaa9b9381183f 100644
> --- a/drivers/net/ethernet/cisco/enic/enic.h
> +++ b/drivers/net/ethernet/cisco/enic/enic.h
> @@ -162,6 +162,17 @@ struct enic_rq_stats {
>   	u64 desc_skip;			/* Rx pkt went into later buffer */
>   };
>   
> +struct enic_wq {
> +	struct vnic_wq vwq;
> +	struct enic_wq_stats stats;
> +	spinlock_t lock;		/* spinlock for wq */
> +};
> +

This change will make vnic_wq spinlock share the latest cache line of
queue statitics while protecting struct vnic_wq.

struct enic_wq {
         struct vnic_wq             vwq;                  /*     0   632 */

         /* XXX last struct has 4 bytes of padding, 1 hole */

         /* --- cacheline 9 boundary (576 bytes) was 56 bytes ago --- */
         struct enic_wq_stats       stats;                /*   632   120 */
         /* --- cacheline 11 boundary (704 bytes) was 48 bytes ago --- */
         spinlock_t                 lock;                 /*   752     4 */

         /* size: 760, cachelines: 12, members: 3 */
         /* padding: 4 */
         /* member types with holes: 1, total: 1 */
         /* paddings: 1, sum paddings: 4 */
         /* last cacheline: 56 bytes */
};

That is not good for performance. Simple re-arrange of the struct makes
lock and vnic_wq structure properly aligned:

struct enic_wq {
         spinlock_t                 lock;                 /*     0     4 */

         /* XXX 4 bytes hole, try to pack */

         struct vnic_wq             vwq;                  /*     8   632 */

         /* XXX last struct has 4 bytes of padding, 1 hole */

         /* --- cacheline 10 boundary (640 bytes) --- */
         struct enic_wq_stats       stats;                /*   640   120 */

         /* size: 760, cachelines: 12, members: 3 */
         /* sum members: 756, holes: 1, sum holes: 4 */
         /* member types with holes: 1, total: 1 */
         /* paddings: 1, sum paddings: 4 */
         /* last cacheline: 56 bytes */
};

Adding one u64 pad field to enic_wq_stats makes whole enic_wq properly
aligned to 12 cache lines on x86.

It might be worth adding ____cacheline_aligned to struct enic_wq to keep
the same performance expectations after converting to dynamic allocation
later in the patchset.

> +struct enic_rq {
> +	struct vnic_rq vrq;
> +	struct enic_rq_stats stats;
> +};
> +
>   /* Per-instance private data structure */
>   struct enic {
>   	struct net_device *netdev;