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Re: [PATCH v4 2/2] vhost: Perform memory section dirty scans once per it

From: Si-Wei Liu
Subject: Re: [PATCH v4 2/2] vhost: Perform memory section dirty scans once per iteration
Date: Thu, 21 Mar 2024 14:42:52 -0700
User-agent: Mozilla Thunderbird 


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 predictable, 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... 

Regards,
-Siwei


Thanks



Thanks,
-Siwei


And 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


-Siwei


Thanks


-Siwei

?

Thanks
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