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Date:   Fri, 22 Mar 2019 19:58:08 +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
> physical server.

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


Thank you very much!

Dongli Zhang

> 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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