netdev - Re: [PATCH] Break up the single NBD lock into one per NBD device

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Date:	Thu, 06 Oct 2011 21:53:43 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"H.K. Jerry Chu" <hkchu@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] Break up the single NBD lock into one per NBD device

Le lundi 26 septembre 2011 à 16:34 -0700, H.K. Jerry Chu a écrit :
> From: Jerry Chu <hkchu@...gle.com>
> 
> This patch breaks up the single NBD lock into one per
> disk. The single NBD lock has become a serious performance
> bottleneck when multiple NBD disks are being used.
> 
> The original comment on why a single lock may be ok no
> longer holds for today's much faster NICs.
> 
> Signed-off-by: H.K. Jerry Chu <hkchu@...gle.com>
> ---
>  drivers/block/nbd.c |   22 +++++++++-------------
>  1 files changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index f533f33..355e15c 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -58,20 +58,9 @@ static unsigned int debugflags;
>  
>  static unsigned int nbds_max = 16;
>  static struct nbd_device *nbd_dev;
> +static spinlock_t *nbd_locks;

static spinlock_t *nbd_locks __read_mostly;

>  static int max_part;
>  
> -/*
> - * Use just one lock (or at most 1 per NIC). Two arguments for this:
> - * 1. Each NIC is essentially a synchronization point for all servers
> - *    accessed through that NIC so there's no need to have more locks
> - *    than NICs anyway.
> - * 2. More locks lead to more "Dirty cache line bouncing" which will slow
> - *    down each lock to the point where they're actually slower than just
> - *    a single lock.
> - * Thanks go to Jens Axboe and Al Viro for their LKML emails explaining this!
> - */
> -static DEFINE_SPINLOCK(nbd_lock);
> -
>  #ifndef NDEBUG
>  static const char *ioctl_cmd_to_ascii(int cmd)
>  {
> @@ -753,6 +742,12 @@ static int __init nbd_init(void)
>  	if (!nbd_dev)
>  		return -ENOMEM;
>  
> +	nbd_locks = kcalloc(nbds_max, sizeof(*nbd_locks), GFP_KERNEL);
> +	if (!nbd_locks) {
> +		kfree(nbd_dev);
> +		return -ENOMEM;
> +	}
> +

	Please add loop to init spinlocks to help LOCKDEP...

	for (i = 0; i < nbds_max; i++)
		spin_lock_init(&nbd_locks[i]);

>  	part_shift = 0;
>  	if (max_part > 0) {
>  		part_shift = fls(max_part);
> @@ -784,7 +779,7 @@ static int __init nbd_init(void)
>  		 * every gendisk to have its very own request_queue struct.
>  		 * These structs are big so we dynamically allocate them.
>  		 */
> -		disk->queue = blk_init_queue(do_nbd_request, &nbd_lock);
> +		disk->queue = blk_init_queue(do_nbd_request, &nbd_locks[i]);
>  		if (!disk->queue) {
>  			put_disk(disk);
>  			goto out;
> @@ -832,6 +827,7 @@ out:
>  		put_disk(nbd_dev[i].disk);
>  	}
>  	kfree(nbd_dev);
> +	kfree(nbd_locks);
>  	return err;
>  }
>  


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