linux-kernel - Re: [PATCH v2 02/13] virtio_net: simplify virtnet_set

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Message-ID: <Z6PthznH5Tp-ZdHw@li-4c4c4544-0047-5210-804b-b8c04f323634.ibm.com>
Date: Wed, 5 Feb 2025 17:00:23 -0600
From: Nick Child <nnac123@...ux.ibm.com>
To: Yury Norov <yury.norov@...il.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        virtualization@...ts.linux.dev, "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Eugenio Pérez <eperezma@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Christoph Hellwig <hch@....de>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v2 02/13] virtio_net: simplify virtnet_set_affinity()

On Tue, Jan 28, 2025 at 11:46:31AM -0500, Yury Norov wrote:
> The inner loop may be replaced with the dedicated for_each_online_cpu_wrap.
> It helps to avoid setting the same bits in the @mask more than once, in
> case of group_size is greater than number of online CPUs.

nit: Looking at the previous logic of how group_stride is calculated, I don't
think there is possibility of "setting the same bits in the @mask more
than once". group_stride = n_cpu / n_queues

nit: I see this more as 2 patches. The introduction of a new core
helper function is a bit buried.

> 
> CC: Nick Child <nnac123@...ux.ibm.com>
> Signed-off-by: Yury Norov <yury.norov@...il.com>

Don't know if my comments alone merit a v3 and I think the patch
does simplify the codebase so:
Reviewed-by: Nick Child <nnac123@...ux.ibm.com>

> ---
>  drivers/net/virtio_net.c | 12 +++++++-----
>  include/linux/cpumask.h  |  4 ++++
>  2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7646ddd9bef7..9d7c37e968b5 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3826,7 +3826,7 @@ static void virtnet_set_affinity(struct virtnet_info *vi)
>  	cpumask_var_t mask;
>  	int stragglers;
>  	int group_size;
> -	int i, j, cpu;
> +	int i, start = 0, cpu;
>  	int num_cpu;
>  	int stride;
>  
> @@ -3840,16 +3840,18 @@ static void virtnet_set_affinity(struct virtnet_info *vi)
>  	stragglers = num_cpu >= vi->curr_queue_pairs ?
>  			num_cpu % vi->curr_queue_pairs :
>  			0;
> -	cpu = cpumask_first(cpu_online_mask);
>  
>  	for (i = 0; i < vi->curr_queue_pairs; i++) {
>  		group_size = stride + (i < stragglers ? 1 : 0);
>  
> -		for (j = 0; j < group_size; j++) {
> +		for_each_online_cpu_wrap(cpu, start) {
> +			if (!group_size--) {
> +				start = cpu;
> +				break;
> +			}
>  			cpumask_set_cpu(cpu, mask);
> -			cpu = cpumask_next_wrap(cpu, cpu_online_mask,
> -						nr_cpu_ids, false);
>  		}
> +
>  		virtqueue_set_affinity(vi->rq[i].vq, mask);
>  		virtqueue_set_affinity(vi->sq[i].vq, mask);
>  		__netif_set_xps_queue(vi->dev, cpumask_bits(mask), i, XPS_CPUS);
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 5cf69a110c1c..30042351f15f 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -1036,6 +1036,8 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
>  
>  #define for_each_possible_cpu_wrap(cpu, start)	\
>  	for ((void)(start), (cpu) = 0; (cpu) < 1; (cpu)++)
> +#define for_each_online_cpu_wrap(cpu, start)	\
> +	for ((void)(start), (cpu) = 0; (cpu) < 1; (cpu)++)
>  #else
>  #define for_each_possible_cpu(cpu) for_each_cpu((cpu), cpu_possible_mask)
>  #define for_each_online_cpu(cpu)   for_each_cpu((cpu), cpu_online_mask)
> @@ -1044,6 +1046,8 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
>  
>  #define for_each_possible_cpu_wrap(cpu, start)	\
>  	for_each_cpu_wrap((cpu), cpu_possible_mask, (start))
> +#define for_each_online_cpu_wrap(cpu, start)	\
> +	for_each_cpu_wrap((cpu), cpu_online_mask, (start))
>  #endif
>  
>  /* Wrappers for arch boot code to manipulate normally-constant masks */
> -- 
> 2.43.0
>