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

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Message-ID: <Z6tW1VleHcUQ_9p3@thinkpad>
Date: Tue, 11 Feb 2025 08:55:41 -0500
From: Yury Norov <yury.norov@...il.com>
To: Nick Child <nnac123@...ux.ibm.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()

Thanks for review and testing, Nick!

On Wed, Feb 05, 2025 at 05:00:23PM -0600, Nick Child wrote:
> 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>

I fixed the comments to #2 and #3 as you suggested and split-out new
for_each() loops to the new patch.

I also think those are trivial changes not worth v3. So it's in
bitmap-for-next:

https://github.com/norov/linux/tree/bitmap-for-next

Thanks for review, Nick!

Thanks,
Yury
 
> > ---
> >  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
> >