[ovs-dev] [dpdk-latest PATCH] netdev-dpdk: add custom vhost statistics to count IRQs

[Ilya Maximets]

On 25.10.2019 15:51, Eelco Chaudron wrote:
> 
> 
> On 16 Oct 2019, at 15:03, Ilya Maximets wrote:
> 
>> On 15.10.2019 13:20, Eelco Chaudron wrote:
>>> When the dpdk vhost library executes an eventfd_write() call,
>>> i.e. waking up the guest, a new callback will be called.
>>>
>>> This patch adds the callback to count the number of
>>> interrupts sent to the VM to track the number of times
>>> interrupts where generated.
>>>
>>> This might be of interest to find out system-calls were
>>> called in the DPDK fast path.
>>>
>>> The custom statistics can be seen with the following command:
>>>
>>>    ovs-ofctl -O OpenFlow14 dump-ports <bridge> {<port>}
>>>
>>> Signed-off-by: Eelco Chaudron <echaudro at redhat.com>
>>> ---
>>>   lib/netdev-dpdk.c |   27 ++++++++++++++++++++++++++-
>>>   1 file changed, 26 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/lib/netdev-dpdk.c b/lib/netdev-dpdk.c
>>> index ba92e89..7ebbcdc 100644
>>> --- a/lib/netdev-dpdk.c
>>> +++ b/lib/netdev-dpdk.c
>>> @@ -165,6 +165,8 @@ static int new_device(int vid);
>>>   static void destroy_device(int vid);
>>>   static int vring_state_changed(int vid, uint16_t queue_id, int enable);
>>>   static void destroy_connection(int vid);
>>> +static void vhost_guest_notified(int vid);
>>> +
>>>   static const struct vhost_device_ops virtio_net_device_ops =
>>>   {
>>>       .new_device =  new_device,
>>> @@ -173,6 +175,7 @@ static const struct vhost_device_ops virtio_net_device_ops =
>>>       .features_changed = NULL,
>>>       .new_connection = NULL,
>>>       .destroy_connection = destroy_connection,
>>> +    .guest_notified = vhost_guest_notified,
>>>   };
>>>    enum { DPDK_RING_SIZE = 256 };
>>> @@ -416,6 +419,8 @@ struct netdev_dpdk {
>>>           struct netdev_stats stats;
>>>           /* Custom stat for retries when unable to transmit. */
>>>           uint64_t tx_retries;
>>> +        /* Custom stat counting number of times a vhost remote was woken up */
>>> +        uint64_t vhost_irqs;
>>>           /* Protects stats */
>>>           rte_spinlock_t stats_lock;
>>>           /* 4 pad bytes here. */
>>> @@ -2826,7 +2831,8 @@ netdev_dpdk_vhost_get_custom_stats(const struct netdev *netdev,
>>>       int i;
>>>    #define VHOST_CSTATS \
>>> -    VHOST_CSTAT(tx_retries)
>>> +    VHOST_CSTAT(tx_retries) \
>>> +    VHOST_CSTAT(vhost_irqs)
>>>    #define VHOST_CSTAT(NAME) + 1
>>>       custom_stats->size = VHOST_CSTATS;
>>> @@ -3746,6 +3752,25 @@ destroy_connection(int vid)
>>>       }
>>>   }
>>>  +static
>>> +void vhost_guest_notified(int vid)
>>> +{
>>> +    struct netdev_dpdk *dev;
>>> +
>>> +    ovs_mutex_lock(&dpdk_mutex);
>>> +    LIST_FOR_EACH (dev, list_node, &dpdk_list) {
>>> +        if (netdev_dpdk_get_vid(dev) == vid) {
>>> +            ovs_mutex_lock(&dev->mutex);
>>> +            rte_spinlock_lock(&dev->stats_lock);
>>> +            dev->vhost_irqs++;
>>> +            rte_spinlock_unlock(&dev->stats_lock);
>>> +            ovs_mutex_unlock(&dev->mutex);
>>> +            break;
>>> +        }
>>> +    }
>>> +    ovs_mutex_unlock(&dpdk_mutex);
>>
>> So, for every single eventfd_write() we're taking the global mutex,
>> traversing list of all the devices, taking one more mutex and finally
>> taking the spinlock just to increment a single counter.
>> I think, it's too much.
> 
> Yes you are right this might be a bit of overkill…
> 
>> Wouldn't it significantly affect interrupt based virtio drivers in
>> terms of performance?  How frequently 2 vhost-user ports will lock
>> each other on the global device mutex?  How frequently 2 PMD threads
>> will lock each other on rx/tx to different vrings of the same vhost
>> port?
> 
> I used iperf3 and did not show any negative effects (maybe I should add more than 4 physical queues, 1 had one VM queue), but also the results show large deviations.

 From my experience, I'd say that iperf via kernel virtio driver is not able
to saturate a PMD thread.  They are likely relaxed and have time to wait
on the mutex.  Also, You, probably, have only couple of ports, so the critical
section is not large enough to produce frequent interlocking.

I'm just pointing that to count number of eventfd syscalls we're probably
making 2 other syscalls and probably sleeping in them.

For testing purposes, I'd suggest to add 10-20(100?) dummy pmd ports (e.g.
vhost without VMs) and assign them to different thread. Probably, more threads
in iperf for traffic generation, switching to small udp packets.
You could also replace lock() with trylock() for dpdk_mutex and count
contentions in a same way as it done for tx_lock in David's patch.

> 
>> I see that 'guest_notified' patch is already in dpdk master, but
>> wouldn't it be better to accumulate this statistics inside DPDK
>> and return it with some new API like rte_vhost_get_vring_xstats()
>> if required?  I don't see any usecase for this callback beside
>> counting some stats.
> 
> I agree, however, the vhost library has no internal counters (only the PMD implementation which we do not use), so hence they liked the callback.

One more possible issue is that this not-so-useful callback hijacked
the reserved space in the structure and we could suffer in the
future while adding new callbacks due to ABI breakage.
(In context of DPDK API/ABI stability discussions)

> 
>> For the current patch I could only suggest to convert it to simple
>> coverage counter like it done in 'vhost_tx_contention' patch here:
>> https://patchwork.ozlabs.org/patch/1175849/
> 
> This is what I’ll do (and will send a patch soon). Although from a user perspective a per vhost user would make more sense as it could easily indicate which VM is configured wrongly…

Sure.  I think that we need to re-imagine concept and implementation of
pmd-perf-stats in a "coverage"-like way, as I suggested previously while
discussing initial patches for pmd-perf, to count stats like vhost qlen,
tx contentions and this one.  At least, this should be not so hard to do
on a per-PMD basis.  per-netdev might be more tricky, but I believe that
it is possible.

For now we have architectural limitations that we have to live with.

Best regards, Ilya Maximets.