On 2018/7/14 20:47, Dominique Martinet wrote: > jiangyiwen wrote on Sat, Jul 14, 2018: >> On 2018/7/14 17:05, Dominique Martinet wrote: >>> jiangyiwen wrote on Sat, Jul 14, 2018: >>>> When client has multiple threads that issue io requests all the >>>> time, and the server has a very good performance, it may cause >>>> cpu is running in the irq context for a long time because it can >>>> check virtqueue has buf in the *while* loop. >>>> >>>> So we should keep chan->lock in the whole loop. >>> >>> Hmm, this is generally bad practice to hold a spin lock for long. >>> In general, spin locks are meant to protect data, not code. >>> >>> I'd want some numbers to decide on this one, even if I think this >>> particular case is safe (e.g. this cannot dead-lock) >>> >> >> Actually, the loop will not hold a spin lock for long, because other >> threads will not issue new requests in this case. In addition, >> virtio-blk or virtio-scsi also use this solution, I guess it may also >> encounter this problem before. > > Fair enough. If you do have some numbers to give though (throughput > and/or iops before/after) I'd still be really curious. > >>>> chan->ring_bufs_avail = 1; >>>> - spin_unlock_irqrestore(&chan->lock, flags); >>>> /* Wakeup if anyone waiting for VirtIO ring space. */ >>>> wake_up(chan->vc_wq); >>> >>> In particular, the wake up here echoes to wait events that will >>> immediately try to grab the lock, and will needlessly spin on it until >>> this thread is done. >>> If we do go this way I'd want setting chan->ring_bufs_avail to be done >>> just before unlocking and the wakeup to be done just after unlocking out >>> of the loop iff we processed at least one iteration here. >> >> I can move the wakeup operation after the unlocking. Like what I said >> above, I think this loop will not execute for long. > > Please do, you listed virtio_blk as doing this and they have the same > kind of pattern with a req_done bool and only restarting stopped queues > if they processed something >
You're right, this wake up operation should be put after the unlocking, I will resend it. In addition, whether I should resend this patch based on your 9p-next branch? Thanks, Yiwen.
