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Date:   Fri, 22 Mar 2019 19:53:01 +0800
From:   Dongli Zhang <dongli.zhang@...cle.com>
To:     luferry <luferry@....com>
Cc:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>,
        Christoph Hellwig <hch@....de>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block/mq: blk map queues by core id



On 03/22/2019 06:09 PM, luferry wrote:
> under virtual machine environment, cpu topology may differ from normal

Would mind share the name of virtual machine monitor, the command line if
available, and which device to reproduce.

For instance, I am not able to reproduce with qemu nvme or virtio-blk as I
assume they use pci or virtio specific mapper to establish the mapping.

E.g., with qemu and nvme: -smp 8,sockets=1,cores=4,threads=2

Indeed I use three queues instead of twp as one is reserved for admin.

# ls /sys/block/nvme0n1/mq/*
/sys/block/nvme0n1/mq/0:
cpu0  cpu1  cpu2  cpu3  cpu_list  nr_reserved_tags  nr_tags

/sys/block/nvme0n1/mq/1:
cpu4  cpu5  cpu6  cpu7  cpu_list  nr_reserved_tags  nr_tags

> physical server.
> for example (machine with 4 cores, 2 threads per core):
> 
> normal physical server:
> core-id   thread-0-id  thread-1-id
> 0         0            4
> 1         1            5
> 2         2            6
> 3         3            7
> 
> virtual machine:
> core-id   thread-0-id  thread-1-id
> 0         0            1
> 1         2            3
> 2         4            5
> 3         6            7
> 
> When attach disk with two queues, all the even numbered cpus will be
> mapped to queue 0. Under virtual machine, all the cpus is followed by
> its sibling cpu.Before this patch, all the odd numbered cpus will also
> be mapped to queue 0, can cause serious imbalance.this will lead to
> performance impact on system IO
> 
> So suggest to allocate cpu map by core id, this can be more currency
> 
> Signed-off-by: luferry <luferry@....com>
> ---
>  block/blk-mq-cpumap.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> index 03a534820271..4125e8e77679 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -35,7 +35,7 @@ int blk_mq_map_queues(struct blk_mq_queue_map *qmap)
>  {
>  	unsigned int *map = qmap->mq_map;
>  	unsigned int nr_queues = qmap->nr_queues;
> -	unsigned int cpu, first_sibling;
> +	unsigned int cpu, first_sibling, core = 0;
>  
>  	for_each_possible_cpu(cpu) {
>  		/*
> @@ -48,9 +48,10 @@ int blk_mq_map_queues(struct blk_mq_queue_map *qmap)
>  			map[cpu] = cpu_to_queue_index(qmap, nr_queues, cpu);
>  		} else {
>  			first_sibling = get_first_sibling(cpu);
> -			if (first_sibling == cpu)
> -				map[cpu] = cpu_to_queue_index(qmap, nr_queues, cpu);
> -			else
> +			if (first_sibling == cpu) {
> +				map[cpu] = cpu_to_queue_index(qmap, nr_queues, core);
> +				core++;
> +			} else
>  				map[cpu] = map[first_sibling];
>  		}
>  	}
> 

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