|
| From: | Si-Wei Liu |
| Subject: | Re: [External] : Re: [PATCH v4 2/2] vhost: Perform memory section dirty scans once per iteration |
| Date: | Mon, 25 Mar 2024 16:20:47 -0700 |
| User-agent: | Mozilla Thunderbird |
On 3/24/2024 11:13 PM, Jason Wang wrote:
To make it independent of the specific vhost_dev, it would require the framework (migration dirty logger) to pass down "dirty_sync_count" like information to the vhost layer through memory listener .log_sync interface. I'm not sure if this change is worth the effort, as this patch is meant to fix a long standing bug I suppose we need to find out a way that is applicable to back-port to past -stable's.On Sat, Mar 23, 2024 at 5:14 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On 3/21/2024 10:08 PM, Jason Wang wrote:On Fri, Mar 22, 2024 at 5:43 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On 3/20/2024 8:56 PM, Jason Wang wrote:On Thu, Mar 21, 2024 at 5:03 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On 3/19/2024 8:27 PM, Jason Wang wrote:On Tue, Mar 19, 2024 at 6:16 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On 3/17/2024 8:22 PM, Jason Wang wrote:On Sat, Mar 16, 2024 at 2:45 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On 3/14/2024 9:03 PM, Jason Wang wrote:On Fri, Mar 15, 2024 at 5:39 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:On setups with one or more virtio-net devices with vhost on, dirty tracking iteration increases cost the bigger the number amount of queues are set up e.g. on idle guests migration the following is observed with virtio-net with vhost=on: 48 queues -> 78.11% [.] vhost_dev_sync_region.isra.13 8 queues -> 40.50% [.] vhost_dev_sync_region.isra.13 1 queue -> 6.89% [.] vhost_dev_sync_region.isra.13 2 devices, 1 queue -> 18.60% [.] vhost_dev_sync_region.isra.14 With high memory rates the symptom is lack of convergence as soon as it has a vhost device with a sufficiently high number of queues, the sufficient number of vhost devices. On every migration iteration (every 100msecs) it will redundantly query the *shared log* the number of queues configured with vhost that exist in the guest. For the virtqueue data, this is necessary, but not for the memory sections which are the same. So essentially we end up scanning the dirty log too often. To fix that, select a vhost device responsible for scanning the log with regards to memory sections dirty tracking. It is selected when we enable the logger (during migration) and cleared when we disable the logger. If the vhost logger device goes away for some reason, the logger will be re-selected from the rest of vhost devices. After making mem-section logger a singleton instance, constant cost of 7%-9% (like the 1 queue report) will be seen, no matter how many queues or how many vhost devices are configured: 48 queues -> 8.71% [.] vhost_dev_sync_region.isra.13 2 devices, 8 queues -> 7.97% [.] vhost_dev_sync_region.isra.14 Co-developed-by: Joao Martins <joao.m.martins@oracle.com> Signed-off-by: Joao Martins <joao.m.martins@oracle.com> Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com> --- v3 -> v4: - add comment to clarify effect on cache locality and performance v2 -> v3: - add after-fix benchmark to commit log - rename vhost_log_dev_enabled to vhost_dev_should_log - remove unneeded comparisons for backend_type - use QLIST array instead of single flat list to store vhost logger devices - simplify logger election logic --- hw/virtio/vhost.c | 67 ++++++++++++++++++++++++++++++++++++++++++----- include/hw/virtio/vhost.h | 1 + 2 files changed, 62 insertions(+), 6 deletions(-) diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c index 612f4db..58522f1 100644 --- a/hw/virtio/vhost.c +++ b/hw/virtio/vhost.c @@ -45,6 +45,7 @@ static struct vhost_log *vhost_log[VHOST_BACKEND_TYPE_MAX]; static struct vhost_log *vhost_log_shm[VHOST_BACKEND_TYPE_MAX]; +static QLIST_HEAD(, vhost_dev) vhost_log_devs[VHOST_BACKEND_TYPE_MAX]; /* Memslots used by backends that support private memslots (without an fd). */ static unsigned int used_memslots; @@ -149,6 +150,47 @@ bool vhost_dev_has_iommu(struct vhost_dev *dev) } } +static inline bool vhost_dev_should_log(struct vhost_dev *dev) +{ + assert(dev->vhost_ops); + assert(dev->vhost_ops->backend_type > VHOST_BACKEND_TYPE_NONE); + assert(dev->vhost_ops->backend_type < VHOST_BACKEND_TYPE_MAX); + + return dev == QLIST_FIRST(&vhost_log_devs[dev->vhost_ops->backend_type]);A dumb question, why not simple check dev->log == vhost_log_shm[dev->vhost_ops->backend_type]Because we are not sure if the logger comes from vhost_log_shm[] or vhost_log[]. Don't want to complicate the check here by calling into vhost_dev_log_is_shared() everytime when the .log_sync() is called.It has very low overhead, isn't it?Whether this has low overhead will have to depend on the specific backend's implementation for .vhost_requires_shm_log(), which the common vhost layer should not assume upon or rely on the current implementation.static bool vhost_dev_log_is_shared(struct vhost_dev *dev) { return dev->vhost_ops->vhost_requires_shm_log && dev->vhost_ops->vhost_requires_shm_log(dev); }For example, if I understand the code correctly, the log type won't be changed during runtime, so we can endup with a boolean to record that instead of a query ops?Right now the log type won't change during runtime, but I am not sure if this may prohibit future revisit to allow change at the runtime,We can be bothered when we have such a request then.then there'll be complex code involvled to maintain the state. Other than this, I think it's insufficient to just check the shm log v.s. normal log. The logger device requires to identify a leading logger device that gets elected in vhost_dev_elect_mem_logger(), as all the dev->log points to the same logger that is refenerce counted, that we have to add extra field and complex logic to maintain the election status.One thing I don't understand here (and in the changelog) is why do we need an election here?vhost_sync_dirty_bitmap() not just scans the guest memory sections but the specific one for virtqueues (used rings) also. To save more CPU cycles to the best extend, the guest memory must be scanned only once in each log iteration, though the logging for used rings would still have to use the specific vhost instance, so all vhost_device instance still keeps the dev->log pointer to the shared log as-is. Generally the shared memory logger can be picked from an arbitrary vhost_device instance, but to keep the code simple, performant and predictableThis is the point, I don't see why election is simpler than picking an arbitrary shared log in this case.Maybe I missed your point, but I am confused and fail to understand why electing a fixed vhost_dev is not as simple? Regardless of the specifics, I think the point is one _fixed_ vhost_dev has to be picked amongst all the instances per backend type in charge of logging guest memory, no matter if it's at the start on the list or not.See below., logger selection is made on the control path at the vhost add/remove time rather than be determined at the dirty log collection runtime, the latter of which is in the hotpath.I thought that Eugenio's previous suggestion tried to simplify the logic in vhost_dev_elect_mem_logger(), as the QLIST_FIRST macro that gets expanded to use the lh_first field for the QLIST would simply satisfy the basic need. Why extra logic to make the check ever more complex, is there any benefit by adding more fields to the vhost_dev?I don't get here, the idea is to just pick one shared log which should be much more simpler than what is proposed here.The code you showed earlier won't work as all vhost_device instance points to the same dev->log device...This part I don't understand.vhost_log_get() has the following: log = share ? vhost_log_shm[backend_type] : vhost_log[backend_type]; if (!log || log->size != size) { log = vhost_log_alloc(size, share); if (share) { vhost_log_shm[backend_type] = log; } else { vhost_log[backend_type] = log; } } else { ++log->refcnt; } So for a specific backend type, vhost_log_get() would return the same logger device (stored at either vhost_log_shm[] or vhost_log[]) to all vhost_dev instances, and the check you suggested earlier: dev->log == vhost_log_shm[dev->vhost_ops->backend_type] will always hold true if the vhost_dev instance (representing the specific virtqueue) is active.Right, so the point is see if we can find something simpler to avoid the QLIST as it involves vhost_dev which seems unnecessary.
It won't. It seems the "rounds" counter is still per vhost_dev instance, but we need it per migration log_sync iteration across all vhost_dev instance of same backend type. Maybe I miss something, but I don't see how easily it would be to come up with proper accounting for "rounds", if not going through the list of vhost_dev instances at the run time (which is what I tried to avoid).Does something like a counter work?
Thanks, -Siwei
vhost_sync_dirty_bitmap() { rounds ++; vhost_dev_sync_region(rounds); } vhost_dev_sync_region(rounds) { if (dev->log->rounds == rounds) return; else dev->log->rounds; } (pseudo codes, just used to demonstrate the idea). ThanksRegards, -SiweiThanksRegards, -SiweiThanksThanks, -SiweiAnd it helps to simplify the logic.Generally yes, but when it comes to hot path operations the performance consideration could override this principle. I think there's no harm to check against logger device cached in vhost layer itself, and the current patch does not create a lot of complexity or performance side effect (actually I think the conditional should be very straightforward to turn into just a couple of assembly compare and branch instructions rather than indirection through another jmp call).Thanks-SiweiThanks-Siwei? Thanks
| [Prev in Thread] | Current Thread | [Next in Thread] |