[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 3/3] xen-blkfront: dynamic configuration of per-vbd resources
On 07/25/2016 05:20 PM, Roger Pau Monné wrote: > On Sat, Jul 23, 2016 at 06:18:23AM +0800, Bob Liu wrote: >> >> On 07/22/2016 07:45 PM, Roger Pau Monné wrote: >>> On Fri, Jul 22, 2016 at 05:43:32PM +0800, Bob Liu wrote: >>>> >>>> On 07/22/2016 05:34 PM, Roger Pau Monné wrote: >>>>> On Fri, Jul 22, 2016 at 04:17:48PM +0800, Bob Liu wrote: >>>>>> >>>>>> On 07/22/2016 03:45 PM, Roger Pau Monné wrote: >>>>>>> On Thu, Jul 21, 2016 at 06:08:05PM +0800, Bob Liu wrote: >>>>>>>> >>>>>>>> On 07/21/2016 04:57 PM, Roger Pau Monné wrote: >>>>>> ..[snip].. >>>>>>>>>> + >>>>>>>>>> +static ssize_t dynamic_reconfig_device(struct blkfront_info *info, >>>>>>>>>> ssize_t count) >>>>>>>>>> +{ >>>>>>>>>> + unsigned int i; >>>>>>>>>> + int err = -EBUSY; >>>>>>>>>> + >>>>>>>>>> + /* >>>>>>>>>> + * Make sure no migration in parallel, device lock is actually a >>>>>>>>>> + * mutex. >>>>>>>>>> + */ >>>>>>>>>> + if (!device_trylock(&info->xbdev->dev)) { >>>>>>>>>> + pr_err("Fail to acquire dev:%s lock, may be in >>>>>>>>>> migration.\n", >>>>>>>>>> + dev_name(&info->xbdev->dev)); >>>>>>>>>> + return err; >>>>>>>>>> + } >>>>>>>>>> + >>>>>>>>>> + /* >>>>>>>>>> + * Prevent new requests and guarantee no uncompleted reqs. >>>>>>>>>> + */ >>>>>>>>>> + blk_mq_freeze_queue(info->rq); >>>>>>>>>> + if (part_in_flight(&info->gd->part0)) >>>>>>>>>> + goto out; >>>>>>>>>> + >>>>>>>>>> + /* >>>>>>>>>> + * Front Backend >>>>>>>>>> + * Switch to XenbusStateClosed >>>>>>>>>> + * frontend_changed(): >>>>>>>>>> + * case XenbusStateClosed: >>>>>>>>>> + * >>>>>>>>>> xen_blkif_disconnect() >>>>>>>>>> + * Switch to >>>>>>>>>> XenbusStateClosed >>>>>>>>>> + * blkfront_resume(): >>>>>>>>>> + * frontend_changed(): >>>>>>>>>> + * reconnect >>>>>>>>>> + * Wait until XenbusStateConnected >>>>>>>>>> + */ >>>>>>>>>> + info->reconfiguring = true; >>>>>>>>>> + xenbus_switch_state(info->xbdev, XenbusStateClosed); >>>>>>>>>> + >>>>>>>>>> + /* Poll every 100ms, 1 minute timeout. */ >>>>>>>>>> + for (i = 0; i < 600; i++) { >>>>>>>>>> + /* >>>>>>>>>> + * Wait backend enter XenbusStateClosed, >>>>>>>>>> blkback_changed() >>>>>>>>>> + * will clear reconfiguring. >>>>>>>>>> + */ >>>>>>>>>> + if (!info->reconfiguring) >>>>>>>>>> + goto resume; >>>>>>>>>> + schedule_timeout_interruptible(msecs_to_jiffies(100)); >>>>>>>>>> + } >>>>>>>>> >>>>>>>>> Instead of having this wait, could you just set info->reconfiguring = >>>>>>>>> 1, set >>>>>>>>> the frontend state to XenbusStateClosed and mimic exactly what a >>>>>>>>> resume from >>>>>>>>> suspension does? blkback_changed would have to set the frontend state >>>>>>>>> to >>>>>>>>> InitWait when it detects that the backend has switched to Closed, and >>>>>>>>> call >>>>>>>>> blkfront_resume. >>>>>>>> >>>>>>>> >>>>>>>> I think that won't work. >>>>>>>> In the real "resume" case, the power management system will trigger >>>>>>>> all ->resume() path. >>>>>>>> But there is no place for dynamic configuration. >>>>>>> >>>>>>> Hello, >>>>>>> >>>>>>> I think it should be possible to set info->reconfiguring and wait for >>>>>>> the >>>>>>> backend to switch to state Closed, at that point we should call >>>>>>> blkif_resume >>>>>>> (from blkback_changed) and the backend will follow the reconection. >>>>>>> >>>>>> >>>>>> Okay, I get your point. Yes, that's an option. >>>>>> >>>>>> But this will make 'dynamic configuration' to be async, I'm worry about >>>>>> the end-user will get panic. >>>>>> E.g >>>>>> A end-user "echo <new value> > /sys/devices/vbd-xxx/max_indirect_segs", >>>>>> but then the device will be Closed and disappeared, the user have to >>>>>> wait for a random time so that the device can resume. >>>>> >>>>> That should not happen, AFAICT on migration the device never dissapears. >>>> >>>> Oh, yes. >>>> >>>>> alloc_disk and friends should not be called on resume from migration (see >>>>> the switch in blkfront_connect, you should take the BLKIF_STATE_SUSPENDED >>>>> path for the reconfiguration). >>>>> >>>> >>>> What about if the end-user starts I/O immediately after writing new value >>>> to /sys? >>>> But the resume is still in progress. >>> >>> blkif_free already stops the queues by calling blk_mq_stop_hw_queues, and >>> blkif_queue_request will refuse to put anything on the ring if the state >>> is different than connected, which in turn makes blkif_queue_rq call >>> blk_mq_stop_hw_queue to stop the queue, so it should be safe. >>> >> >> But this will surprise the end-user, our user script is like this: >> 1) echo <new value> > /sys/xxxx >> 2) Start I/O immediately. >> ^^^ Fail because requests would be refused(even software queue was >> still freezed). > > Which error do you get? AFAICT reads/writes should either block when the You are right, the reads/writes will just block which is fine. > queue is full, or if the fd is in non-blocking mode it should return EAGAIN > (which a properly coded application should be prepared to deal with > gracefully if it's using non-blocking fds anyway). > >> It's not good for the end user have to wait for a random time before restart >> I/O. >> >> >> There are two more concerns I have: >> * blkif_resume() may fail, how the end-user can aware that if "echo <new >> value> > /sys/xxx" already returned success. > > If you really think this is needed, can't you use a monitor or some kind of > condition with a timeout instead of open-coding it? Although I'm still not > convinced that blocking here is TRTTD. > Let me ask in another way. If moving blkfront_resume() to blkback_changed, should we check the return value of blkfront_resume()? And what to do if it returns error. >> * We get the device lock and blk_mq_freeze_queue() in >> dynamic_reconfig_device(), >> and then have to release in blkif_recover() asynchronously which makes >> the code more difficult to readable. > > I'm not sure I'm following here, do you mean that you will pick the lock in > dynamic_reconfig_device and release it in blkif_recover? Why wouldn't you Yes. > release the lock in dynamic_reconfig_device after doing whatever is needed? > Both 'dynamic configuration' and migration:xenbus_dev_resume() use { blkfront_resume(); blkif_recover() } and depends on the change of xbdev->state. If they happen simultaneously, the State machine of xbdev->state is going to be a mess and very difficult to track. The lock(actually a mutex) is like a big lock to make sure no race would happen at all. Thanks, Bob Liu _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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