Re: [PATCH] vhost: use try_lock in rte_vhost_vring_call

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 9/20/22 09:23, Maxime Coquelin wrote:
> 
> 
> On 9/20/22 04:53, Xia, Chenbo wrote:
>>> -----Original Message-----
>>> From: Liu, Changpeng <changpeng.liu@intel.com>
>>> Sent: Tuesday, September 20, 2022 10:34 AM
>>> To: Xia, Chenbo <chenbo.xia@intel.com>; dev@dpdk.org
>>> Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
>>> Subject: RE: [PATCH] vhost: use try_lock in rte_vhost_vring_call
>>>
>>> Hi Bo,
>>>
>>>> -----Original Message-----
>>>> From: Xia, Chenbo <chenbo.xia@intel.com>
>>>> Sent: Tuesday, September 20, 2022 10:25 AM
>>>> To: Liu, Changpeng <changpeng.liu@intel.com>; dev@dpdk.org
>>>> Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
>>>> Subject: RE: [PATCH] vhost: use try_lock in rte_vhost_vring_call
>>>>
>>>> Hi Changpeng,
>>>>
>>>>> -----Original Message-----
>>>>> From: Liu, Changpeng <changpeng.liu@intel.com>
>>>>> Sent: Tuesday, September 6, 2022 10:22 AM
>>>>> To: dev@dpdk.org
>>>>> Cc: Liu, Changpeng <changpeng.liu@intel.com>; Maxime Coquelin
>>>>> <maxime.coquelin@redhat.com>; Xia, Chenbo <chenbo.xia@intel.com>
>>>>> Subject: [PATCH] vhost: use try_lock in rte_vhost_vring_call
>>>>>
>>>>> Note that this function is in data path, so the thread context
>>>>> may not same as socket messages processing context, by using
>>>>> try_lock here, users can have another try in case of VQ's access
>>>>> lock is held by `vhost-events` thread.
>>>>
>>>> Better to describe the issue this patch wants to fix and how does
>>>> it fix.
>>>>
>>>> I remember it's a bz issue, do you want to backport? And it has
>>>> some bz ID, we need to add it in commit message.
>>> Actually it's my intention not to add bz ID, as I think for this bz ID,
>>> It's better not to lock all VQ's access lock for KICK/CALLFD messages,
>>
>> Do you plan to add this change? I think that may be an improvement to 
>> current
>> locking implementation.
>>
>> Maxime, what do you think of this idea about only locking specific 
>> queue when
>> handling vring related message (not global config like mem table)?
> 
> I think this is not a good idea.
> For example SET_VRING_KICK can currently call
> translate_ring_addresses(), which itself can call numa_realloc().
> 
> numa_realloc() may reallocate the dev, so you don't want it to be used
> by other queues while it happens.

Hmm, actually that may be possible because numa_realloc() reallocs the 
dev only if it is not running.

So maybe you can propose something, but you will have to test it
carefully with use-cases involving NUMA reallocation.

>>> What do you think? If this is identified as a fix, I can backport it to
>>> 22.05.
>>
>> You can decide, if this is planned to be the fix, just backport. I am 
>> just
>> thinking if this is not the fix for the bz, do we still need this?
>>
>> Thanks,
>> Chenbo
>>
>>>>
>>>>>
>>>>> Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
>>>>> ---
>>>>>   lib/vhost/vhost.c | 6 +++++-
>>>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/lib/vhost/vhost.c b/lib/vhost/vhost.c
>>>>> index 60cb05a0ff..072d2acb7b 100644
>>>>> --- a/lib/vhost/vhost.c
>>>>> +++ b/lib/vhost/vhost.c
>>>>> @@ -1329,7 +1329,11 @@ rte_vhost_vring_call(int vid, uint16_t
>>> vring_idx)
>>>>>       if (!vq)
>>>>>           return -1;
>>>>>
>>>>> -    rte_spinlock_lock(&vq->access_lock);
>>>>> +    if (!rte_spinlock_trylock(&vq->access_lock)) {
>>>>> +        VHOST_LOG_CONFIG(dev->ifname, DEBUG,
>>>>
>>>> Should use VHOST_LOG_DATA
>>> OK.
>>>>
>>>> Thanks,
>>>> Chenbo
>>>>
>>>>> +            "failed to kick guest, virtqueue busy.\n");
>>>>> +        return -1;
>>>>> +    }
>>>>>
>>>>>       if (vq_is_packed(dev))
>>>>>           vhost_vring_call_packed(dev, vq);
>>>>> -- 
>>>>> 2.21.3
>>