On Sun, Aug 10, 2014 at 11:30:35AM +0300, Razya Ladelsky wrote: > From: Razya Ladelsky <razya@xxxxxxxxxx> > Date: Thu, 31 Jul 2014 09:47:20 +0300 > Subject: [PATCH] vhost: Add polling mode > > 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 > > I ran some experiments with TCP stream netperf and filebench (having 2 threads > performing random reads) benchmarks on an IBM System x3650 M4. > I have two machines, A and B. A hosts the vms, B runs the netserver. > The vms (on A) run netperf, its destination server is running on B. > All runs loaded the guests in a way that they were (cpu) saturated. For example, > I ran netperf with 64B messages, which is heavily loading the vm (which is why > its throughput is low). > The idea was to get it 100% loaded, so we can see that the polling is getting it > to produce higher throughput. And, did your tests actually produce 100% load on both host CPUs? > The system had two cores per guest, as to allow for both the vcpu and the vhost > thread to run concurrently for maximum throughput (but I didn't pin the threads > to specific cores). > My experiments were fair in a sense that for both cases, with or without > polling, I run both threads, vcpu and vhost, on 2 cores (set their affinity that > way). The only difference was whether polling was enabled/disabled. > > Results: > > Netperf, 1 vm: > The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). > Number of exits/sec decreased 6x. > The same improvement was shown when I tested with 3 vms running netperf > (4086 MB/sec -> 5545 MB/sec). > > filebench, 1 vm: > ops/sec improved by 13% with the polling patch. Number of exits was reduced by > 31%. > The same experiment with 3 vms running filebench showed similar numbers. > > Signed-off-by: Razya Ladelsky <razya@xxxxxxxxxx> > --- > drivers/vhost/net.c | 6 +- > drivers/vhost/scsi.c | 6 +- > drivers/vhost/vhost.c | 245 +++++++++++++++++++++++++++++++++++++++++++++++-- > drivers/vhost/vhost.h | 38 +++++++- > 4 files changed, 277 insertions(+), 18 deletions(-) > > 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..665eeeb 100644 > --- a/drivers/vhost/scsi.c > +++ b/drivers/vhost/scsi.c > @@ -1528,9 +1528,9 @@ 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..fbe8174 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 +66,28 @@ 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 +183,86 @@ 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 +288,48 @@ 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 +368,62 @@ 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 > - schedule(); > + } else { > + if (work) { > + if (need_resched()) > + schedule(); > + } else > + schedule(); > + } > > } > unuse_mm(dev->mm); > @@ -306,6 +487,7 @@ 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 +500,7 @@ 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 +532,7 @@ 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 +626,26 @@ 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 +662,7 @@ 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 +1694,19 @@ 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 +1744,17 @@ 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..11aaaf4 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 +38,12 @@ 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 +56,6 @@ 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 +110,35 @@ 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 +152,7 @@ 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
