Re: [PATCH v3] virtio_blk: unlock vblk->lock during kick

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On Fri, Jun 01, 2012 at 10:13:06AM +0100, Stefan Hajnoczi wrote:
> Holding the vblk->lock across kick causes poor scalability in SMP
> guests.  If one CPU is doing virtqueue kick and another CPU touches the
> vblk->lock it will have to spin until virtqueue kick completes.
> 
> This patch reduces system% CPU utilization in SMP guests that are
> running multithreaded I/O-bound workloads.  The improvements are small
> but show as iops and SMP are increased.
> 
> Khoa Huynh <khoa@xxxxxxxxxx> provided initial performance data that
> indicates this optimization is worthwhile at high iops.
> 
> Asias He <asias@xxxxxxxxxx> reports the following fio results:
> 
> Host: Linux 3.4.0+ #302 SMP x86_64 GNU/Linux
> Guest: same as host kernel
> 
> Average 3 runs:
> with locked kick
> read    iops=119907.50 bw=59954.00 runt=35018.50 io=2048.00
> write   iops=217187.00 bw=108594.00 runt=19312.00 io=2048.00
> read    iops=33948.00 bw=16974.50 runt=186820.50 io=3095.70
> write   iops=35014.00 bw=17507.50 runt=181151.00 io=3095.70
> clat (usec)     max=3484.10 avg=121085.38 stdev=174416.11 min=0.00
> clat (usec)     max=3438.30 avg=59863.35 stdev=116607.69 min=0.00
> clat (usec)     max=3745.65 avg=454501.30 stdev=332699.00 min=0.00
> clat (usec)     max=4089.75 avg=442374.99 stdev=304874.62 min=0.00
> cpu     sys=615.12 majf=24080.50 ctx=64253616.50 usr=68.08 minf=17907363.00
> cpu     sys=1235.95 majf=23389.00 ctx=59788148.00 usr=98.34 minf=20020008.50
> cpu     sys=764.96 majf=28414.00 ctx=848279274.00 usr=36.39 minf=19737254.00
> cpu     sys=714.13 majf=21853.50 ctx=854608972.00 usr=33.56 minf=18256760.50
> 
> with unlocked kick
> read    iops=118559.00 bw=59279.66 runt=35400.66 io=2048.00
> write   iops=227560.00 bw=113780.33 runt=18440.00 io=2048.00
> read    iops=34567.66 bw=17284.00 runt=183497.33 io=3095.70
> write   iops=34589.33 bw=17295.00 runt=183355.00 io=3095.70
> clat (usec)     max=3485.56 avg=121989.58 stdev=197355.15 min=0.00
> clat (usec)     max=3222.33 avg=57784.11 stdev=141002.89 min=0.00
> clat (usec)     max=4060.93 avg=447098.65 stdev=315734.33 min=0.00
> clat (usec)     max=3656.30 avg=447281.70 stdev=314051.33 min=0.00
> cpu     sys=683.78 majf=24501.33 ctx=64435364.66 usr=68.91 minf=17907893.33
> cpu     sys=1218.24 majf=25000.33 ctx=60451475.00 usr=101.04 minf=19757720.00
> cpu     sys=740.39 majf=24809.00 ctx=845290443.66 usr=37.25 minf=19349958.33
> cpu     sys=723.63 majf=27597.33 ctx=850199927.33 usr=35.35 minf=19092343.00
> 
> FIO config file:
> 
> [global]
> exec_prerun="echo 3 > /proc/sys/vm/drop_caches"
> group_reporting
> norandommap
> ioscheduler=noop
> thread
> bs=512
> size=4MB
> direct=1
> filename=/dev/vdb
> numjobs=256
> ioengine=aio
> iodepth=64
> loops=3
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxxxxxxxxxx>
> ---
> Other block drivers (cciss, rbd, nbd) use spin_unlock_irq() so I followed that.
> To me this seems wrong: blk_run_queue() uses spin_lock_irqsave() but we enable
> irqs with spin_unlock_irq().  If the caller of blk_run_queue() had irqs
> disabled and we enable them again this could be a problem, right?  Can someone
> more familiar with kernel locking comment?
> 
>  drivers/block/virtio_blk.c |   10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 774c31d..d674977 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -199,8 +199,14 @@ static void do_virtblk_request(struct request_queue *q)
>  		issued++;
>  	}
>  
> -	if (issued)
> -		virtqueue_kick(vblk->vq);
> +	if (!issued)
> +		return;
> +
> +	if (virtqueue_kick_prepare(vblk->vq)) {
> +		spin_unlock_irq(vblk->disk->queue->queue_lock);
> +		virtqueue_notify(vblk->vq);

If blk_done runs and completes the request at this point,
can hot unplug then remove the queue?
If yes will we get a use after free?

> +		spin_lock_irq(vblk->disk->queue->queue_lock);
> +	}
>  }
>  
>  /* return id (s/n) string for *disk to *id_str
> -- 
> 1.7.10
> 
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