Re: [Virtio-fs] [PATCH] virtiofs: reduce lock contention on fpq->lock

[piaojun <piaojun@huawei.com>]

On 2019/8/1 1:48, Liu Bo wrote:
> Hi Jun,
> 
> On Wed, Jul 31, 2019 at 08:05:03PM +0800, piaojun wrote:
>> Hi liubo,
>>
>> This change has already committed in rhvgoyal's branch:
>>
>> https://github.com/rhvgoyal/linux/tree/virtio-fs-dev-5.1
>>
> 
> Thanks for the link, the patch here does one more thing beyond Vivek's current
> code, i.e. the lock protection around req is also removed as it's not necessary.

Agreed, and it could save some CPU time.

Thanks,
Jun

> 
> thanks,
> -liubo
> 
>> Thanks,
>> Jun
>>
>> On 2019/7/30 13:51, Liu Bo wrote:
>>> Right now within virtio_fs_requests_done_work(), virtqueue is protected by
>>> both fsvq->lock and fpq->lock for processing both virtqueue and
>>> fpq->processing_list, however, it is fine for them to be protected
>>> separately.
>>>
>>> Also since %req has been removed from fpq->processing_list, no one except
>>> request_wait_answer() is looking at this %req and request_wait_answer()
>>> waits only on FINISH flag, it's OK to remove fpq->lock after %req is
>>> dropped from the list.
>>>
>>> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
>>> ---
>>>  fs/fuse/virtio_fs.c | 13 ++++++-------
>>>  1 file changed, 6 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
>>> index 9b3f2e9..d88bb6c 100644
>>> --- a/fs/fuse/virtio_fs.c
>>> +++ b/fs/fuse/virtio_fs.c
>>> @@ -347,21 +347,22 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
>>>  
>>>  	/* Collect completed requests off the virtqueue */
>>>  	spin_lock(&fsvq->lock);
>>> -	/* grab fpq->lock as req is on fpq->processing list */
>>> -	spin_lock(&fpq->lock);
>>>  	do {
>>>  		unsigned len;
>>>  
>>>  		virtqueue_disable_cb(vq);
>>>  
>>>  		while ((req = virtqueue_get_buf(vq, &len)) != NULL) {
>>> +			/* grab fpq->lock as req is on fpq->processing list */
>>> +			spin_lock(&fpq->lock);
>>> +
>>>  			/* If !connected, req is freed by fuse_abort_conn. */
>>> -			if (fpq->connected) {
>>> +			if (fpq->connected)
>>>  				list_move_tail(&req->list, &reqs);
>>> -			}
>>> +
>>> +			spin_unlock(&fpq->lock);
>>>  		}
>>>  	} while (!virtqueue_enable_cb(vq) && likely(!virtqueue_is_broken(vq)));
>>> -	spin_unlock(&fpq->lock);
>>>  	spin_unlock(&fsvq->lock);
>>>  
>>>  	/* End requests */
>>> @@ -373,10 +374,8 @@ static void virtio_fs_requests_done_work(struct work_struct *work)
>>>  
>>>  		/* TODO zeroing? */
>>>  
>>> -		spin_lock(&fpq->lock);
>>>  		clear_bit(FR_SENT, &req->flags);
>>>  		list_del_init(&req->list);
>>> -		spin_unlock(&fpq->lock);
>>>  
>>>  		fuse_request_end(fc, req);
>>>  	}
>>>
> .
>