linux-kernel - Re: [PATCH 1/2] genirq/affinity: improve __irq_build_affinity

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Date:   Fri, 9 Aug 2019 08:42:04 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@....de>, linux-nvme@...ts.infradead.org,
        Jon Derrick <jonathan.derrick@...el.com>
Subject: Re: [PATCH 1/2] genirq/affinity: improve __irq_build_affinity_masks()

On Fri, Aug 09, 2019 at 06:23:09PM +0800, Ming Lei wrote:
> One invariant of __irq_build_affinity_masks() is that all CPUs in the
> specified masks( cpu_mask AND node_to_cpumask for each node) should be
> covered during the spread. Even though all requested vectors have been
> reached, we still need to spread vectors among left CPUs. The similar
> policy has been taken in case of 'numvecs <= nodes'.
> 
> So remove the following check inside the loop:
> 
> 	if (done >= numvecs)
> 		break;
> 
> Meantime assign at least 1 vector for left nodes if 'numvecs' vectors
> have been spread.
> 
> Also, if the specified cpumask for one numa node is empty, simply not
> spread vectors on this node.
> 
> Cc: Christoph Hellwig <hch@....de>
> Cc: Keith Busch <kbusch@...nel.org>
> Cc: linux-nvme@...ts.infradead.org,
> Cc: Jon Derrick <jonathan.derrick@...el.com>
> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
>  kernel/irq/affinity.c | 33 +++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
> index 6fef48033f96..bc3652a2c61b 100644
> --- a/kernel/irq/affinity.c
> +++ b/kernel/irq/affinity.c
> @@ -129,21 +129,32 @@ static int __irq_build_affinity_masks(unsigned int startvec,
>  	for_each_node_mask(n, nodemsk) {
>  		unsigned int ncpus, v, vecs_to_assign, vecs_per_node;
>  
> -		/* Spread the vectors per node */
> -		vecs_per_node = (numvecs - (curvec - firstvec)) / nodes;
> -
>  		/* Get the cpus on this node which are in the mask */
>  		cpumask_and(nmsk, cpu_mask, node_to_cpumask[n]);
> -
> -		/* Calculate the number of cpus per vector */
>  		ncpus = cpumask_weight(nmsk);
> +		if (!ncpus)
> +			continue;

This shouldn't be possible, right? The nodemsk we're looping  wouldn't
have had that node set if no CPUs intersect the node_to_cpu_mask for
that node, so the resulting cpumask should always have a non-zero weight.

> @@ -153,16 +164,14 @@ static int __irq_build_affinity_masks(unsigned int startvec,
>  			}
>  			irq_spread_init_one(&masks[curvec].mask, nmsk,
>  						cpus_per_vec);
> +			if (++curvec >= last_affv)
> +				curvec = firstvec;

I'm not so sure about wrapping the vector to share it across nodes. We
have enough vectors in this path to ensure each compute node can have
a unique one, and it's much cheaper to share these within nodes than
across them.

>  		}
>  
>  		done += v;
> -		if (done >= numvecs)
> -			break;
> -		if (curvec >= last_affv)
> -			curvec = firstvec;
>  		--nodes;
>  	}
> -	return done;
> +	return done < numvecs ? done : numvecs;
>  }