"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote on 29/07/2014 11:06:40 AM: > From: "Michael S. Tsirkin" <mst@xxxxxxxxxx> > To: Razya Ladelsky/Haifa/IBM@IBMIL, > Cc: kvm@xxxxxxxxxxxxxxx, abel.gordon@xxxxxxxxx, Joel Nider/Haifa/ > IBM@IBMIL, Yossi Kuperman1/Haifa/IBM@IBMIL, Eran Raichstein/Haifa/ > IBM@IBMIL, Alex Glikson/Haifa/IBM@IBMIL > Date: 29/07/2014 11:06 AM > Subject: Re: [PATCH] vhost: Add polling mode > > On Mon, Jul 21, 2014 at 04:23:44PM +0300, Razya Ladelsky wrote: > > Hello All, > > > > When vhost is waiting for buffers from the guest driver (e.g., more > > packets > > to send in vhost-net's transmit queue), it normally goes to sleep and > > waits > > for the guest to "kick" it. This kick involves a PIO in the guest, and > > therefore an exit (and possibly userspace involvement in translating this > > PIO > > exit into a file descriptor event), all of which hurts performance. > > > > If the system is under-utilized (has cpu time to spare), vhost can > > continuously poll the virtqueues for new buffers, and avoid asking > > the guest to kick us. > > This patch adds an optional polling mode to vhost, that can be enabled > > via a kernel module parameter, "poll_start_rate". > > > > When polling is active for a virtqueue, the guest is asked to > > disable notification (kicks), and the worker thread continuously checks > > for > > new buffers. When it does discover new buffers, it simulates a "kick" by > > invoking the underlying backend driver (such as vhost-net), which thinks > > it > > got a real kick from the guest, and acts accordingly. If the underlying > > driver asks not to be kicked, we disable polling on this virtqueue. > > > > We start polling on a virtqueue when we notice it has > > work to do. Polling on this virtqueue is later disabled after 3 seconds of > > polling turning up no new work, as in this case we are better off > > returning > > to the exit-based notification mechanism. The default timeout of 3 seconds > > can be changed with the "poll_stop_idle" kernel module parameter. > > > > This polling approach makes lot of sense for new HW with posted-interrupts > > for which we have exitless host-to-guest notifications. But even with > > support > > for posted interrupts, guest-to-host communication still causes exits. > > Polling adds the missing part. > > > > When systems are overloaded, there won?t be enough cpu time for the > > various > > vhost threads to poll their guests' devices. For these scenarios, we plan > > to add support for vhost threads that can be shared by multiple devices, > > even of multiple vms. > > Our ultimate goal is to implement the I/O acceleration features described > > in: > > KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon) > > https://www.youtube.com/watch?v=9EyweibHfEs > > and > > https://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg98179.html > > > > > > Comments are welcome, > > Thank you, > > Razya > > > > From: Razya Ladelsky <razya@xxxxxxxxxx> > > > > Add an optional polling mode to continuously poll the virtqueues > > for new buffers, and avoid asking the guest to kick us. > > > > Signed-off-by: Razya Ladelsky <razya@xxxxxxxxxx> > > This is an optimization patch, isn't it? > Could you please include some numbers showing its > effect? > > Hi Michael, Sure. I included them in a reply to Jason Wang in this thread, Here it is: http://www.spinics.net/linux/lists/kvm/msg106049.html > > --- > > drivers/vhost/net.c | 6 +- > > drivers/vhost/scsi.c | 5 +- > > drivers/vhost/vhost.c | 247 > > +++++++++++++++++++++++++++++++++++++++++++++++-- > > drivers/vhost/vhost.h | 37 +++++++- > > 4 files changed, 277 insertions(+), 18 deletions(-) > > > Whitespace seems mangled to the point of making patch > unreadable. Can you pls repost? > Sure. > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > > index 971a760..558aecb 100644 > > --- a/drivers/vhost/net.c > > +++ b/drivers/vhost/net.c > > @@ -742,8 +742,10 @@ static int vhost_net_open(struct inode *inode, struct > > file *f) > > } > > vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX); > > > > - vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, > > dev); > > - vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, > > dev); > > + vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, > > + vqs[VHOST_NET_VQ_TX]); > > + vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, > > + vqs[VHOST_NET_VQ_RX]); > > > > f->private_data = n; > > > > diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c > > index 4f4ffa4..56f0233 100644 > > --- a/drivers/vhost/scsi.c > > +++ b/drivers/vhost/scsi.c > > @@ -1528,9 +1528,8 @@ static int vhost_scsi_open(struct inode *inode, > > struct file *f) > > if (!vqs) > > goto err_vqs; > > > > - vhost_work_init(&vs->vs_completion_work, > > vhost_scsi_complete_cmd_work); > > - vhost_work_init(&vs->vs_event_work, tcm_vhost_evt_work); > > - > > + vhost_work_init(&vs->vs_completion_work, NULL, > > vhost_scsi_complete_cmd_work); > > + vhost_work_init(&vs->vs_event_work, NULL, tcm_vhost_evt_work); > > vs->vs_events_nr = 0; > > vs->vs_events_missed = false; > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > index c90f437..678d766 100644 > > --- a/drivers/vhost/vhost.c > > +++ b/drivers/vhost/vhost.c > > @@ -24,9 +24,17 @@ > > #include <linux/slab.h> > > #include <linux/kthread.h> > > #include <linux/cgroup.h> > > +#include <linux/jiffies.h> > > #include <linux/module.h> > > > > #include "vhost.h" > > +static int poll_start_rate = 0; > > +module_param(poll_start_rate, int, S_IRUGO|S_IWUSR); > > +MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of virtqueue > > when rate of events is at least this number per jiffy. If 0, never start > > polling."); > > + > > +static int poll_stop_idle = 3*HZ; /* 3 seconds */ > > +module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR); > > +MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of virtqueue > > after this many jiffies of no work."); > > > > enum { > > VHOST_MEMORY_MAX_NREGIONS = 64, > > @@ -58,27 +66,27 @@ static int vhost_poll_wakeup(wait_queue_t *wait, > > unsigned mode, int sync, > > return 0; > > } > > > > -void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn) > > +void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq, > > vhost_work_fn_t fn) > > { > > INIT_LIST_HEAD(&work->node); > > work->fn = fn; > > init_waitqueue_head(&work->done); > > work->flushing = 0; > > work->queue_seq = work->done_seq = 0; > > + work->vq = vq; > > } > > EXPORT_SYMBOL_GPL(vhost_work_init); > > > > /* Init poll structure */ > > void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn, > > - unsigned long mask, struct vhost_dev *dev) > > + unsigned long mask, struct vhost_virtqueue *vq) > > { > > init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup); > > init_poll_funcptr(&poll->table, vhost_poll_func); > > poll->mask = mask; > > - poll->dev = dev; > > + poll->dev = vq->dev; > > poll->wqh = NULL; > > - > > - vhost_work_init(&poll->work, fn); > > + vhost_work_init(&poll->work, vq, fn); > > } > > EXPORT_SYMBOL_GPL(vhost_poll_init); > > > > @@ -174,6 +182,86 @@ void vhost_poll_queue(struct vhost_poll *poll) > > } > > EXPORT_SYMBOL_GPL(vhost_poll_queue); > > > > +/* Enable or disable virtqueue polling (vqpoll.enabled) for a virtqueue. > > + * > > + * Enabling this mode it tells the guest not to notify ("kick") us when > > its > > + * has made more work available on this virtqueue; Rather, we will > > continuously > > + * poll this virtqueue in the worker thread. If multiple virtqueues are > > polled, > > + * the worker thread polls them all, e.g., in a round-robin fashion. > > + * Note that vqpoll.enabled doesn't always mean that this virtqueue is > > + * actually being polled: The backend (e.g., net.c) may temporarily > > disable it > > + * using vhost_disable/enable_notify(), while vqpoll.enabled is > > unchanged. > > + * > > + * It is assumed that these functions are called relatively rarely, when > > vhost > > + * notices that this virtqueue's usage pattern significantly changed in a > > way > > + * that makes polling more efficient than notification, or vice versa. > > + * Also, we assume that vhost_vq_disable_vqpoll() is always called on vq > > + * cleanup, so any allocations done by vhost_vq_enable_vqpoll() can be > > + * reclaimed. > > + */ > > +static void vhost_vq_enable_vqpoll(struct vhost_virtqueue *vq) > > +{ > > + if (vq->vqpoll.enabled) > > + return; /* already enabled, nothing to do */ > > + if (!vq->handle_kick) > > + return; /* polling will be a waste of time if no callback! > > */ > > + if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY)) { > > + /* vq has guest notifications enabled. Disable them, > > + and instead add vq to the polling list */ > > + vhost_disable_notify(vq->dev, vq); > > + list_add_tail(&vq->vqpoll.link, &vq->dev->vqpoll_list); > > + } > > + vq->vqpoll.jiffies_last_kick = jiffies; > > + __get_user(vq->avail_idx, &vq->avail->idx); > > + vq->vqpoll.enabled = true; > > + > > + /* Map userspace's vq->avail to the kernel's memory space. */ > > + if (get_user_pages_fast((unsigned long)vq->avail, 1, 0, > > + &vq->vqpoll.avail_page) != 1) { > > + /* TODO: can this happen, as we check access > > + to vq->avail in advance? */ > > + BUG(); > > + } > > + vq->vqpoll.avail_mapped = (struct vring_avail *) ( > > + (unsigned long)kmap(vq->vqpoll.avail_page) | > > + ((unsigned long)vq->avail & ~PAGE_MASK)); > > +} > > + > > +/* > > + * This function doesn't always succeed in changing the mode. Sometimes > > + * a temporary race condition prevents turning on guest notifications, so > > + * vq should be polled next time again. > > + */ > > +static void vhost_vq_disable_vqpoll(struct vhost_virtqueue *vq) > > +{ > > + if (!vq->vqpoll.enabled) { > > + return; /* already disabled, nothing to do */ > > + } > > + vq->vqpoll.enabled = false; > > + > > + if (!list_empty(&vq->vqpoll.link)) { > > + /* vq is on the polling list, remove it from this list and > > + * instead enable guest notifications. */ > > + list_del_init(&vq->vqpoll.link); > > + if (unlikely(vhost_enable_notify(vq->dev,vq)) > > + && !vq->vqpoll.shutdown) { > > + /* Race condition: guest wrote before we enabled > > + * notification, so we'll never get a notification > > for > > + * this work - so continue polling mode for a > > while. */ > > + vhost_disable_notify(vq->dev, vq); > > + vq->vqpoll.enabled = true; > > + vhost_enable_notify(vq->dev, vq); > > + return; > > + } > > + } > > + > > + if (vq->vqpoll.avail_mapped) { > > + kunmap(vq->vqpoll.avail_page); > > + put_page(vq->vqpoll.avail_page); > > + vq->vqpoll.avail_mapped = 0; > > + } > > +} > > + > > static void vhost_vq_reset(struct vhost_dev *dev, > > struct vhost_virtqueue *vq) > > { > > @@ -199,6 +287,48 @@ static void vhost_vq_reset(struct vhost_dev *dev, > > vq->call = NULL; > > vq->log_ctx = NULL; > > vq->memory = NULL; > > + INIT_LIST_HEAD(&vq->vqpoll.link); > > + vq->vqpoll.enabled = false; > > + vq->vqpoll.shutdown = false; > > + vq->vqpoll.avail_mapped = NULL; > > +} > > + > > +/* roundrobin_poll() takes worker->vqpoll_list, and returns one of the > > + * virtqueues which the caller should kick, or NULL in case none should > > be > > + * kicked. roundrobin_poll() also disables polling on a virtqueue which > > has > > + * been polled for too long without success. > > + * > > + * This current implementation (the "round-robin" implementation) only > > + * polls the first vq in the list, returning it or NULL as appropriate, > > and > > + * moves this vq to the end of the list, so next time a different one is > > + * polled. > > + */ > > +static struct vhost_virtqueue *roundrobin_poll(struct list_head *list) { > > + struct vhost_virtqueue *vq; > > + u16 avail_idx; > > + > > + > > + if (list_empty(list)) > > + return NULL; > > + > > + vq = list_first_entry(list, struct vhost_virtqueue, vqpoll.link); > > + WARN_ON(!vq->vqpoll.enabled); > > + list_move_tail(&vq->vqpoll.link, list); > > + > > + /* See if there is any new work available from the guest. */ > > + /* TODO: can check the optional idx feature, and if we haven't > > + * reached that idx yet, don't kick... */ > > + avail_idx = vq->vqpoll.avail_mapped->idx; > > + if (avail_idx != vq->last_avail_idx) { > > + return vq; > > + } > > + if (jiffies > vq->vqpoll.jiffies_last_kick + poll_stop_idle) { > > + /* We've been polling this virtqueue for a long time with > > no > > + * results, so switch back to guest notification > > + */ > > + vhost_vq_disable_vqpoll(vq); > > + } > > + return NULL; > > } > > > > static int vhost_worker(void *data) > > @@ -237,12 +367,66 @@ static int vhost_worker(void *data) > > spin_unlock_irq(&dev->work_lock); > > > > if (work) { > > + struct vhost_virtqueue *vq = work->vq; > > __set_current_state(TASK_RUNNING); > > work->fn(work); > > + /* Keep track of the work rate, for deciding when > > to > > + * enable polling */ > > + if (vq) { > > + if (vq->vqpoll.jiffies_last_work != > > jiffies) { > > + vq->vqpoll.jiffies_last_work = > > jiffies; > > + vq->vqpoll.work_this_jiffy = 0; > > + } > > + vq->vqpoll.work_this_jiffy++; > > + } > > + /* If vq is in the round-robin list of virtqueues > > being > > + * constantly checked by this thread, move vq the > > end > > + * of the queue, because it had its fair chance > > now. > > + */ > > + if (vq && !list_empty(&vq->vqpoll.link)) { > > + list_move_tail(&vq->vqpoll.link, > > + &dev->vqpoll_list); > > + } > > + /* Otherwise, if this vq is looking for > > notifications > > + * but vq polling is not enabled for it, do it > > now. > > + */ > > + else if (poll_start_rate && vq && vq->handle_kick > > && > > + !vq->vqpoll.enabled && > > + !vq->vqpoll.shutdown && > > + !(vq->used_flags & VRING_USED_F_NO_NOTIFY) > > && > > + vq->vqpoll.work_this_jiffy >= > > + poll_start_rate) { > > + vhost_vq_enable_vqpoll(vq); > > + } > > + } > > + /* Check one virtqueue from the round-robin list */ > > + if (!list_empty(&dev->vqpoll_list)) { > > + struct vhost_virtqueue *vq; > > + > > + vq = roundrobin_poll(&dev->vqpoll_list); > > + > > + if (vq) { > > + vq->handle_kick(&vq->poll.work); > > + vq->vqpoll.jiffies_last_kick=jiffies; > > + } > > + > > + /* If our polling list isn't empty, ask to > > continue > > + * running this thread, don't yield. > > + */ > > + __set_current_state(TASK_RUNNING); > > if (need_resched()) > > + schedule(); > > + > > + } > > + else { > > + if (work) > > + { > > + if (need_resched()) > > + schedule(); > > + } > > + else > > schedule(); > > - } else > > - schedule(); > > + } > > > > } > > unuse_mm(dev->mm); > > @@ -306,6 +490,7 @@ void vhost_dev_init(struct vhost_dev *dev, > > dev->mm = NULL; > > spin_lock_init(&dev->work_lock); > > INIT_LIST_HEAD(&dev->work_list); > > + INIT_LIST_HEAD(&dev->vqpoll_list); > > dev->worker = NULL; > > > > for (i = 0; i < dev->nvqs; ++i) { > > @@ -318,7 +503,7 @@ void vhost_dev_init(struct vhost_dev *dev, > > vhost_vq_reset(dev, vq); > > if (vq->handle_kick) > > vhost_poll_init(&vq->poll, vq->handle_kick, > > - POLLIN, dev); > > + POLLIN, vq); > > } > > } > > EXPORT_SYMBOL_GPL(vhost_dev_init); > > @@ -350,7 +535,7 @@ static int vhost_attach_cgroups(struct vhost_dev *dev) > > struct vhost_attach_cgroups_struct attach; > > > > attach.owner = current; > > - vhost_work_init(&attach.work, vhost_attach_cgroups_work); > > + vhost_work_init(&attach.work, NULL, vhost_attach_cgroups_work); > > vhost_work_queue(dev, &attach.work); > > vhost_work_flush(dev, &attach.work); > > return attach.ret; > > @@ -444,6 +629,25 @@ void vhost_dev_stop(struct vhost_dev *dev) > > } > > EXPORT_SYMBOL_GPL(vhost_dev_stop); > > > > +/* shutdown_vqpoll() asks the worker thread to shut down virtqueue > > polling > > + * mode for a given virtqueue which is itself being shut down. We ask the > > + * worker thread to do this rather than doing it directly, so that we > > don't > > + * race with the worker thread's use of the queue. > > + */ > > +static void shutdown_vqpoll_work(struct vhost_work *work) > > +{ > > + work->vq->vqpoll.shutdown = true; > > + vhost_vq_disable_vqpoll(work->vq); > > + WARN_ON(work->vq->vqpoll.avail_mapped); > > +} > > + > > +static void shutdown_vqpoll(struct vhost_virtqueue *vq) > > +{ > > + struct vhost_work work; > > + vhost_work_init(&work, vq, shutdown_vqpoll_work); > > + vhost_work_queue(vq->dev, &work); > > + vhost_work_flush(vq->dev, &work); > > +} > > /* Caller should have device mutex if and only if locked is set */ > > void vhost_dev_cleanup(struct vhost_dev *dev, bool locked) > > { > > @@ -460,6 +664,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool > > locked) > > eventfd_ctx_put(dev->vqs[i]->call_ctx); > > if (dev->vqs[i]->call) > > fput(dev->vqs[i]->call); > > + shutdown_vqpoll(dev->vqs[i]); > > vhost_vq_reset(dev, dev->vqs[i]); > > } > > vhost_dev_free_iovecs(dev); > > @@ -1491,6 +1696,19 @@ bool vhost_enable_notify(struct vhost_dev *dev, > > struct vhost_virtqueue *vq) > > u16 avail_idx; > > int r; > > > > + /* In polling mode, when the backend (e.g., net.c) asks to enable > > + * notifications, we don't enable guest notifications. Instead, > > start > > + * polling on this vq by adding it to the round-robin list. > > + */ > > + if (vq->vqpoll.enabled) { > > + if (list_empty(&vq->vqpoll.link)) { > > + list_add_tail(&vq->vqpoll.link, > > + &vq->dev->vqpoll_list); > > + vq->vqpoll.jiffies_last_kick = jiffies; > > + } > > + return false; > > + } > > + > > if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY)) > > return false; > > vq->used_flags &= ~VRING_USED_F_NO_NOTIFY; > > @@ -1528,6 +1746,17 @@ void vhost_disable_notify(struct vhost_dev *dev, > > struct vhost_virtqueue *vq) > > { > > int r; > > > > + /* If this virtqueue is vqpoll.enabled, and on the polling list, > > it > > + * will generate notifications even if the guest is asked not to > > send > > + * them. So we must remove it from the round-robin polling list. > > + * Note that vqpoll.enabled remains set. > > + */ > > + if (vq->vqpoll.enabled) { > > + if(!list_empty(&vq->vqpoll.link)) > > + list_del_init(&vq->vqpoll.link); > > + return; > > + } > > + > > if (vq->used_flags & VRING_USED_F_NO_NOTIFY) > > return; > > vq->used_flags |= VRING_USED_F_NO_NOTIFY; > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h > > index 3eda654..feb16d6 100644 > > --- a/drivers/vhost/vhost.h > > +++ b/drivers/vhost/vhost.h > > @@ -24,6 +24,7 @@ struct vhost_work { > > int flushing; > > unsigned queue_seq; > > unsigned done_seq; > > + struct vhost_virtqueue *vq; > > }; > > > > /* Poll a file (eventfd or socket) */ > > @@ -37,11 +38,11 @@ struct vhost_poll { > > struct vhost_dev *dev; > > }; > > > > -void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn); > > +void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq, > > vhost_work_fn_t fn); > > void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work); > > > > void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn, > > - unsigned long mask, struct vhost_dev *dev); > > + unsigned long mask, struct vhost_virtqueue *vq); > > int vhost_poll_start(struct vhost_poll *poll, struct file *file); > > void vhost_poll_stop(struct vhost_poll *poll); > > void vhost_poll_flush(struct vhost_poll *poll); > > @@ -54,8 +55,6 @@ struct vhost_log { > > u64 len; > > }; > > > > -struct vhost_virtqueue; > > - > > /* The virtqueue structure describes a queue attached to a device. */ > > struct vhost_virtqueue { > > struct vhost_dev *dev; > > @@ -110,6 +109,35 @@ struct vhost_virtqueue { > > /* Log write descriptors */ > > void __user *log_base; > > struct vhost_log *log; > > + struct { > > + /* When a virtqueue is in vqpoll.enabled mode, it declares > > + * that instead of using guest notifications (kicks) to > > + * discover new work, we prefer to continuously poll this > > + * virtqueue in the worker thread. > > + * If !enabled, the rest of the fields below are undefined. > > + */ > > + bool enabled; > > + /* vqpoll.enabled doesn't always mean that this virtqueue is > > + * actually being polled: The backend (e.g., net.c) may > > + * temporarily disable it using vhost_disable/enable_notify(). > > + * vqpoll.link is used to maintain the thread's round-robin > > + * list of virtqueues that actually need to be polled. > > + * Note list_empty(link) means this virtqueue isn't polled. > > + */ > > + struct list_head link; > > + /* If this flag is true, the virtqueue is being shut down, > > + * so vqpoll should not be re-enabled. > > + */ > > + bool shutdown; > > + /* Various counters used to decide when to enter polling mode > > + * or leave it and return to notification mode. > > + */ > > + unsigned long jiffies_last_kick; > > + unsigned long jiffies_last_work; > > + int work_this_jiffy; > > + struct page *avail_page; > > + volatile struct vring_avail *avail_mapped; > > + } vqpoll; > > }; > > > > struct vhost_dev { > > @@ -123,6 +151,7 @@ struct vhost_dev { > > spinlock_t work_lock; > > struct list_head work_list; > > struct task_struct *worker; > > + struct list_head vqpoll_list; > > }; > > > > void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int > > nvqs); > > -- > > 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
