Dongli Zhang
2019-Mar-12 17:22 UTC
virtio-blk: should num_vqs be limited by num_possible_cpus()?
I observed that there is one msix vector for config and one shared vector
for all queues in below qemu cmdline, when the num-queues for virtio-blk
is more than the number of possible cpus:
qemu: "-smp 4" while "-device
virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
# cat /proc/interrupts
CPU0 CPU1 CPU2 CPU3
... ...
24: 0 0 0 0 PCI-MSI 65536-edge
virtio0-config
25: 0 0 0 59 PCI-MSI 65537-edge
virtio0-virtqueues
... ...
However, when num-queues is the same as number of possible cpus:
qemu: "-smp 4" while "-device
virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
# cat /proc/interrupts
CPU0 CPU1 CPU2 CPU3
... ...
24: 0 0 0 0 PCI-MSI 65536-edge
virtio0-config
25: 2 0 0 0 PCI-MSI 65537-edge
virtio0-req.0
26: 0 35 0 0 PCI-MSI 65538-edge
virtio0-req.1
27: 0 0 32 0 PCI-MSI 65539-edge
virtio0-req.2
28: 0 0 0 0 PCI-MSI 65540-edge
virtio0-req.3
... ...
In above case, there is one msix vector per queue.
This is because the max number of queues is not limited by the number of
possible cpus.
By default, nvme (regardless about write_queues and poll_queues) and
xen-blkfront limit the number of queues with num_possible_cpus().
Is this by design on purpose, or can we fix with below?
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 4bc083b..df95ce3 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
if (err)
num_vqs = 1;
+ num_vqs = min(num_possible_cpus(), num_vqs);
+
vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
if (!vblk->vqs)
return -ENOMEM;
--
PS: The same issue is applicable to virtio-scsi as well.
Thank you very much!
Dongli Zhang
Cornelia Huck
2019-Mar-12 17:33 UTC
virtio-blk: should num_vqs be limited by num_possible_cpus()?
On Tue, 12 Mar 2019 10:22:46 -0700 (PDT) Dongli Zhang <dongli.zhang at oracle.com> wrote:> I observed that there is one msix vector for config and one shared vector > for all queues in below qemu cmdline, when the num-queues for virtio-blk > is more than the number of possible cpus: > > qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6" > > # cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 > ... ... > 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config > 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues > ... ... > > > However, when num-queues is the same as number of possible cpus: > > qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4" > > # cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 > ... ... > 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config > 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0 > 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1 > 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2 > 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3 > ... ... > > In above case, there is one msix vector per queue.Please note that this is pci-specific...> > > This is because the max number of queues is not limited by the number of > possible cpus. > > By default, nvme (regardless about write_queues and poll_queues) and > xen-blkfront limit the number of queues with num_possible_cpus()....and these are probably pci-specific as well.> > > Is this by design on purpose, or can we fix with below? > > > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c > index 4bc083b..df95ce3 100644 > --- a/drivers/block/virtio_blk.c > +++ b/drivers/block/virtio_blk.c > @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk) > if (err) > num_vqs = 1; > > + num_vqs = min(num_possible_cpus(), num_vqs); > + > vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL); > if (!vblk->vqs) > return -ENOMEM;virtio-blk, however, is not pci-specific. If we are using the ccw transport on s390, a completely different interrupt mechanism is in use ('floating' interrupts, which are not per-cpu). A check like that should therefore not go into the generic driver.
Dongli Zhang
2019-Mar-13 03:26 UTC
virtio-blk: should num_vqs be limited by num_possible_cpus()?
On 3/13/19 1:33 AM, Cornelia Huck wrote:> On Tue, 12 Mar 2019 10:22:46 -0700 (PDT) > Dongli Zhang <dongli.zhang at oracle.com> wrote: > >> I observed that there is one msix vector for config and one shared vector >> for all queues in below qemu cmdline, when the num-queues for virtio-blk >> is more than the number of possible cpus: >> >> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6" >> >> # cat /proc/interrupts >> CPU0 CPU1 CPU2 CPU3 >> ... ... >> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config >> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues >> ... ... >> >> >> However, when num-queues is the same as number of possible cpus: >> >> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4" >> >> # cat /proc/interrupts >> CPU0 CPU1 CPU2 CPU3 >> ... ... >> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config >> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0 >> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1 >> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2 >> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3 >> ... ... >> >> In above case, there is one msix vector per queue. > > Please note that this is pci-specific... > >> >> >> This is because the max number of queues is not limited by the number of >> possible cpus. >> >> By default, nvme (regardless about write_queues and poll_queues) and >> xen-blkfront limit the number of queues with num_possible_cpus(). > > ...and these are probably pci-specific as well.Not pci-specific, but per-cpu as well.> >> >> >> Is this by design on purpose, or can we fix with below? >> >> >> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c >> index 4bc083b..df95ce3 100644 >> --- a/drivers/block/virtio_blk.c >> +++ b/drivers/block/virtio_blk.c >> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk) >> if (err) >> num_vqs = 1; >> >> + num_vqs = min(num_possible_cpus(), num_vqs); >> + >> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL); >> if (!vblk->vqs) >> return -ENOMEM; > > virtio-blk, however, is not pci-specific. > > If we are using the ccw transport on s390, a completely different > interrupt mechanism is in use ('floating' interrupts, which are not > per-cpu). A check like that should therefore not go into the generic > driver. >So far there seems two options. The 1st option is to ask the qemu user to always specify "-num-queues" with the same number of vcpus when running x86 guest with pci for virtio-blk or virtio-scsi, in order to assign a vector for each queue. Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops' so that different platforms (e.g., pci or ccw) would use different ways to limit the max number of queues in guest, with something like below? So far both virtio-scsi and virtio-blk would benefit from the new hook. --- drivers/block/virtio_blk.c | 2 ++ drivers/virtio/virtio_pci_common.c | 6 ++++++ drivers/virtio/virtio_pci_common.h | 2 ++ drivers/virtio/virtio_pci_legacy.c | 1 + drivers/virtio/virtio_pci_modern.c | 2 ++ include/linux/virtio_config.h | 11 +++++++++++ 6 files changed, 24 insertions(+) diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c index 4bc083b..93cfeda 100644 --- a/drivers/block/virtio_blk.c +++ b/drivers/block/virtio_blk.c @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk) if (err) num_vqs = 1; + num_vqs = virtio_calc_num_vqs(vdev, num_vqs); + vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL); if (!vblk->vqs) return -ENOMEM; diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c index d0584c0..ce021d1 100644 --- a/drivers/virtio/virtio_pci_common.c +++ b/drivers/virtio/virtio_pci_common.c @@ -409,6 +409,12 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned nvqs, return vp_find_vqs_intx(vdev, nvqs, vqs, callbacks, names, ctx); } +/* the config->calc_num_vqs() implementation */ +unsigned short vp_calc_num_vqs(unsigned short num_vqs) +{ + return min_t(unsigned short, num_possible_cpus(), num_vqs); +} + const char *vp_bus_name(struct virtio_device *vdev) { struct virtio_pci_device *vp_dev = to_vp_device(vdev); diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h index 0227100..cc5ac80 100644 --- a/drivers/virtio/virtio_pci_common.h +++ b/drivers/virtio/virtio_pci_common.h @@ -134,6 +134,8 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned nvqs, struct virtqueue *vqs[], vq_callback_t *callbacks[], const char * const names[], const bool *ctx, struct irq_affinity *desc); +/* the config->calc_num_vqs() implementation */ +unsigned short vp_calc_num_vqs(unsigned short num_vqs); const char *vp_bus_name(struct virtio_device *vdev); /* Setup the affinity for a virtqueue: diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c index eff9ddc..69d1050 100644 --- a/drivers/virtio/virtio_pci_legacy.c +++ b/drivers/virtio/virtio_pci_legacy.c @@ -209,6 +209,7 @@ static const struct virtio_config_ops virtio_pci_config_ops = { .bus_name = vp_bus_name, .set_vq_affinity = vp_set_vq_affinity, .get_vq_affinity = vp_get_vq_affinity, + .calc_num_vqs = vp_calc_num_vqs, }; /* the PCI probing function */ diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c index 07571da..f04e44a 100644 --- a/drivers/virtio/virtio_pci_modern.c +++ b/drivers/virtio/virtio_pci_modern.c @@ -460,6 +460,7 @@ static const struct virtio_config_ops virtio_pci_config_nodev_ops = { .bus_name = vp_bus_name, .set_vq_affinity = vp_set_vq_affinity, .get_vq_affinity = vp_get_vq_affinity, + .calc_num_vqs = vp_calc_num_vqs, }; static const struct virtio_config_ops virtio_pci_config_ops = { @@ -476,6 +477,7 @@ static const struct virtio_config_ops virtio_pci_config_ops = { .bus_name = vp_bus_name, .set_vq_affinity = vp_set_vq_affinity, .get_vq_affinity = vp_get_vq_affinity, + .calc_num_vqs = vp_calc_num_vqs, }; /** diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h index bb4cc49..f32368b 100644 --- a/include/linux/virtio_config.h +++ b/include/linux/virtio_config.h @@ -65,6 +65,7 @@ struct irq_affinity; * the caller can then copy. * @set_vq_affinity: set the affinity for a virtqueue (optional). * @get_vq_affinity: get the affinity for a virtqueue (optional). + * @calc_num_vqs: calculate the number of virtqueue (optional) */ typedef void vq_callback_t(struct virtqueue *); struct virtio_config_ops { @@ -88,6 +89,7 @@ struct virtio_config_ops { const struct cpumask *cpu_mask); const struct cpumask *(*get_vq_affinity)(struct virtio_device *vdev, int index); + unsigned short (*calc_num_vqs)(unsigned short num_vqs); }; /* If driver didn't advertise the feature, it will never appear. */ @@ -207,6 +209,15 @@ int virtio_find_vqs_ctx(struct virtio_device *vdev, unsigned nvqs, desc); } +static inline +unsigned short virtio_calc_num_vqs(struct virtio_device *vdev, + unsigned short num_vqs) +{ + if (vdev->config->calc_num_vqs) + return vdev->config->calc_num_vqs(num_vqs); + return num_vqs; +} + /** * virtio_device_ready - enable vq use in probe function * @vdev: the device -- 2.7.4 Thank you very much! Dongli Zhang
Michael S. Tsirkin
2019-Mar-14 12:32 UTC
virtio-blk: should num_vqs be limited by num_possible_cpus()?
On Tue, Mar 12, 2019 at 10:22:46AM -0700, Dongli Zhang wrote:> I observed that there is one msix vector for config and one shared vector > for all queues in below qemu cmdline, when the num-queues for virtio-blk > is more than the number of possible cpus: > > qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"So why do this?> # cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 > ... ... > 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config > 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues > ... ... > > > However, when num-queues is the same as number of possible cpus: > > qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4" > > # cat /proc/interrupts > CPU0 CPU1 CPU2 CPU3 > ... ... > 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config > 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0 > 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1 > 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2 > 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3 > ... ... > > In above case, there is one msix vector per queue. > > > This is because the max number of queues is not limited by the number of > possible cpus. > > By default, nvme (regardless about write_queues and poll_queues) and > xen-blkfront limit the number of queues with num_possible_cpus(). > > > Is this by design on purpose, or can we fix with below? > > > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c > index 4bc083b..df95ce3 100644 > --- a/drivers/block/virtio_blk.c > +++ b/drivers/block/virtio_blk.c > @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk) > if (err) > num_vqs = 1; > > + num_vqs = min(num_possible_cpus(), num_vqs); > + > vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL); > if (!vblk->vqs) > return -ENOMEM; > -- > > > PS: The same issue is applicable to virtio-scsi as well. > > Thank you very much! > > Dongli ZhangI don't think this will address the issue if there's vcpu hotplug though. Because it's not about num_possible_cpus it's about the # of active VCPUs, right? Does block hangle CPU hotplug generally? We could maybe address that by switching vq to msi vector mapping in a cpu hotplug notifier... -- MST
Dongli Zhang
2019-Mar-14 15:36 UTC
virtio-blk: should num_vqs be limited by num_possible_cpus()?
On 03/14/2019 08:32 PM, Michael S. Tsirkin wrote:> On Tue, Mar 12, 2019 at 10:22:46AM -0700, Dongli Zhang wrote: >> I observed that there is one msix vector for config and one shared vector >> for all queues in below qemu cmdline, when the num-queues for virtio-blk >> is more than the number of possible cpus: >> >> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6" > > So why do this?I observed this when I was testing virtio-blk and block layer. I just assign a very large 'num-queues' to virtio-blk and keep changing the number of vcpu in order to study blk-mq. The num-queues for nvme (qemu) is by default 64, while it is 1 for virtio-blk.> >> # cat /proc/interrupts >> CPU0 CPU1 CPU2 CPU3 >> ... ... >> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config >> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues >> ... ... >> >> >> However, when num-queues is the same as number of possible cpus: >> >> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4" >> >> # cat /proc/interrupts >> CPU0 CPU1 CPU2 CPU3 >> ... ... >> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config >> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0 >> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1 >> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2 >> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3 >> ... ... >> >> In above case, there is one msix vector per queue. >> >> >> This is because the max number of queues is not limited by the number of >> possible cpus. >> >> By default, nvme (regardless about write_queues and poll_queues) and >> xen-blkfront limit the number of queues with num_possible_cpus(). >> >> >> Is this by design on purpose, or can we fix with below? >> >> >> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c >> index 4bc083b..df95ce3 100644 >> --- a/drivers/block/virtio_blk.c >> +++ b/drivers/block/virtio_blk.c >> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk) >> if (err) >> num_vqs = 1; >> >> + num_vqs = min(num_possible_cpus(), num_vqs); >> + >> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL); >> if (!vblk->vqs) >> return -ENOMEM; >> -- >> >> >> PS: The same issue is applicable to virtio-scsi as well. >> >> Thank you very much! >> >> Dongli Zhang > > I don't think this will address the issue if there's vcpu hotplug though. > Because it's not about num_possible_cpus it's about the # of active VCPUs, > right? Does block hangle CPU hotplug generally? > We could maybe address that by switching vq to msi vector mapping in > a cpu hotplug notifier... >It looks it is about num_possible_cpus/nr_cpu_ids for cpu hotplug. For instance, below is when only 2 out of 6 cpus are initialized while virtio-blk has 6 queues. "-smp 2,maxcpus=6" and "-device virtio-blk-pci,drive=drive0,id=disk0,num-queues=6,iothread=io1" # cat /sys/devices/system/cpu/present 0-1 # cat /sys/devices/system/cpu/possible 0-5 # cat /proc/interrupts | grep virtio 24: 0 0 PCI-MSI 65536-edge virtio0-config 25: 1864 0 PCI-MSI 65537-edge virtio0-req.0 26: 0 1069 PCI-MSI 65538-edge virtio0-req.1 27: 0 0 PCI-MSI 65539-edge virtio0-req.2 28: 0 0 PCI-MSI 65540-edge virtio0-req.3 29: 0 0 PCI-MSI 65541-edge virtio0-req.4 30: 0 0 PCI-MSI 65542-edge virtio0-req.5 6 + 1 irqs are assigned even 4 out of 6 cpus are still offline. Below is about the nvme emulated by qemu. While 2 out of 6 cpus are initial assigned, nvme has 64 queues by default. "-smp 2,maxcpus=6" and "-device nvme,drive=drive1,serial=deadbeaf1" # cat /sys/devices/system/cpu/present 0-1 # cat /sys/devices/system/cpu/possible 0-5 # cat /proc/interrupts | grep nvme 31: 0 16 PCI-MSI 81920-edge nvme0q0 32: 35 0 PCI-MSI 81921-edge nvme0q1 33: 0 42 PCI-MSI 81922-edge nvme0q2 34: 0 0 PCI-MSI 81923-edge nvme0q3 35: 0 0 PCI-MSI 81924-edge nvme0q4 36: 0 0 PCI-MSI 81925-edge nvme0q5 37: 0 0 PCI-MSI 81926-edge nvme0q6 6 io queues are assigned with irq, although only 2 cpus are online. When only 2 out of 48 cpus are online, there are 48 hctx created by block layer. "-smp 2,maxcpus=48" and "-device virtio-blk-pci,drive=drive0,id=disk0,num-queues=48,iothread=io1" # ls /sys/kernel/debug/block/vda/ | grep hctx | wc -l 48 The above indicates the number of hw queues/irq is related to num_possible_cpus/nr_cpu_ids. Dongli Zhang
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