[libvirt] [PATCH v1] libvirtd: Increase NL buffer size for lots of interface

[Laine Stump]

Now that everyone is back from holidays, I wanted to revisit this patch 
(since the message I'm replying to contains most of the discussion, but 
not the original patch, I've attached it at the end) and hopefully get 
input from Tomas Graf about whether or not the problems with 
netlink/libnl noted in the commit message that prevent proper operation 
of message peeking are eliminated in recent versions (if so, then I 
think the "128k buffer + enable peeking" you propose will be a proper 
permanent fix.)

Also, in the message referenced below I had asked a question about the 
"LTC-Bugzilla" references, and pointed out that your patch only works 
with libnl3 (not libnl-1.1) and only fixes the problem in one of several 
places that libvirt uses libnl to create a netlink socket in this 
message, then suggested a method of making a general purpose fix for all 
uses an all versions of libnl:

https://www.redhat.com/archives/libvir-list/2015-December/msg00697.html

Can you create a new version of the patch that provides a general 
solution as suggested (or similar) and either gives a clickable link to 
the referenced LTC-Bugzilla issues or removes the reference (former is 
preferred)?

Thanks!

On 12/20/2015 05:06 PM, Laine Stump wrote:
> On 12/18/2015 02:30 AM, Leno Hou wrote:
>>
>> On 2015年12月17日 02:33, Laine Stump wrote:
>>> On 12/16/2015 10:24 AM, Michal Privoznik wrote:
>>>> On 10.12.2015 07:34, Leno Hou wrote:
>>>>> 1. When switching CPUs to offline/online in a system more than 128 
>>>>> cpus
>>>>> 2. When using virsh to destroy domain in a system with more interface
>>>>>
>>>>> All of above happens nl_recv returned with error: No buffer space 
>>>>> available.
>>>>> This patch set socket buffer size to 128K and turn on message 
>>>>> peeking for nl_recv,
>>>>> as this would solve this problem totally and permanetly.
>>>>>
>>>
>>>>> Signed-off-by: Leno Hou <houqy at linux.vnet.ibm.com>
>>>>> Cc: Wenyi Gao <wenyi at linux.vnet.ibm.com>
>>>>> ---
>>>>>   src/util/virnetlink.c | 8 ++++++++
>>>>>   1 file changed, 8 insertions(+)
>>>>>
>>>>> diff --git a/src/util/virnetlink.c b/src/util/virnetlink.c
>>>>> index 679b48e..c8c9fe0 100644
>>>>> --- a/src/util/virnetlink.c
>>>>> +++ b/src/util/virnetlink.c
>>>>> @@ -696,6 +696,14 @@ virNetlinkEventServiceStart(unsigned int 
>>>>> protocol, unsigned int groups)
>>>>>           goto error_server;
>>>>>       }
>>>>>   +    if (nl_socket_set_buffer_size(srv->netlinknh, 131702, 0) < 
>>>>> 0) {
>>>
>>> The above function doesn't exist in libnl 1.1 (still used in 
>>> RHEL6/CentOS6, for example), so that would cause a build failure on 
>>> some systems. In libnl 1.1 the function is called nl_set_buffer_size().
>>>
>>> Also, how did you arrive at 128k for the default buffer size? What 
>>> kind of sizes are you seeing?
>> This buffer size is the receive socket buffer size. When I switching 
>> CPUs to offline/online, it's receives 107K.
>
> Wow! What is in this message, and where/how is it used in the code?
>
>>
>>>
>>>
>>>>> +        virReportSystemError(errno,
>>>>> +                "%s",_("cannot set netlink socket buffer size to 
>>>>> 128k"));
>>>>> +        goto error_server;
>>>>> +    }
>>>>> +
>>>>> +    nl_socket_enable_msg_peek(srv->netlinknh);
>>>>> +
>>>
>>> According to a link I followed from another message on this topic 
>>> last week, libnl's message peeking can't be guaranteed to always 
>>> work, because netlink doesn't always return the proper buffer size 
>>> (depending on version).
>> I would not use message peeking  due to I agree with you on that 
>> message peeking can't be guaranteed to always work.
>
> I'm hoping that is only for "older" versions of kernel/netlink/libnl. 
> Thomas?
>
>>>
>>>>>       if ((srv->eventwatch = virEventAddHandle(fd,
>>>>> VIR_EVENT_HANDLE_READABLE,
>>>>> virNetlinkEventCallback,
>>>>>
>>>> I believe this patch appears over and over again. Usually, the problem
>>>> was in libnl library we use and this was just a workaround. Can you 
>>>> test
>>>> with the latest libnl version (probably even GIT HEAD) and see if that
>>>> helps?
>>>
>>> I had the same memory. So I just looked back through the history of 
>>> bug reports about this issue, and found the following:
>>>
>>> * libnl-1.1 and libnl-3 both originally set the default message 
>>> buffersize to 4096 bytes, with MSG_PEEK turned off.
>>>
>>> * when this problem came up in RHEL6, it was unfortunately reported 
>>> as a private BZ (a pet peeve of mine), and the result of the 
>>> discussion about it was that libnl-1.1 (the version used in RHEL6) 
>>> was patched *upstream* to set the default message buffersize to 
>>> 16384 bytes (getpagesize() * 4), which would solve the problem for 
>>> even very large numbers of VFs. That was in 2013 and there have been 
>>> no further reports against RHEL6.
>>>
>>> * Although I had assumed the problem was solved, it again came up in 
>>> RHEL7 (which uses libnl-3 - a slightly different API, and maintained 
>>> in a separate git repo), this time in a public BZ:
>>>
>>>   https://bugzilla.redhat.com/show_bug.cgi?id=1040626
>> I'm not familar with this bug :-(    I think this maybe different 
>> from my case.
>
> Well, I wasn't even aware that libvirt's netlink service was used for 
> CPU hotplug (and still don't see how/where that happens) :-)
>
> The above bug reports were due to 4096 bytes not being enough to read 
> an entire  IFLA_VF_PORTS array.
>
>
>>
>>>
>>> I asked if perhaps the change that had been made upstream in 
>>> libnl-1.1 hadn't also been made to libnl-3 (this is what I assumed 
>>> during the previous incident). It hadn't. So the same change was 
>>> made for libnl-3, both upstream and as a backport to RHEL7, and 
>>> everyone was happy.
>>
>>> I have very little detailed memory of that time (the above was all 
>>> recalled by looking at archives of discussions) but what had stuck 
>>> in my mind was "This problem has been fixed in libnl, so libvirt 
>>> should NOT put in "workarounds" for broken versions of libnl."
>>>
>>> But if you are using a version of libnl3 with this patch (which was 
>>> in libnl-3.2.22 upstream, and is in the libnl-3.2.21 that's in 
>>> RHEL7.0+), :
>>>
>>> https://github.com/tgraf/libnl/commit/807fddc4cd9ecb12ba64e1b7fa26d86b6c2f19b0 
>>>
>> This patch sets *message* buffer size to 4 pages.  But I'm prefer to 
>> say I wanna to set *socket* buffer size to 128k
>>
>>>
>>> then the change to quadruple the buffer size in libnl was 
>>> insufficient, (and also, when I looked back at the discussion now, > 
>>> I see that the libnl maintainer had said "The permanent fix would be 
>>> for libvirt to enable message peeking", so I suppose it's time to 
>>> "bite the bullet" and enable netlink message 
>> Yes, I would to set socket buffer size in libvirt because If we can 
>> set socket buffer size in libnl, it's can give impact on the app that 
>> do not need 128K socket buffer size.
>
> Yes. 16K is one thing, but 128K is quite a lot more, and could more 
> easily cause a problem with other applications.
>
>>
>>> peeking in libvirt (but, since there are apparently versions of 
>>> netlink that don't properly inform libnl when a re-read is 
>>> necessary, we also need to increase the default buffer size).
>>>
>>> However, your patch is only fixing the problem in one place. There 
>>> are several places that we allocate netlink sockets, and they should 
>>> all get the same fix, implying that there should be a common 
>>> function called by all three. Fortunately, we already have a macro 
>>> called "virNetlinkAlloc" that is #defined differently depending on 
>>> the libnl version - this macro can simply be made into a static 
>>> function that is defined differently depending on libnl version. It 
>>> can call nl_handle_alloc or nl_socket_alloc depending on libnl 
>>> version, then call nl_socket_set_buffer_size/nl_set_buffer_size 
>>> depending on version, and finally call nl_socket_enable_msg_peek.
>>>
>>> A bit of due diligence about the default buffer size is in order 
>>> though - just to make sure that we don't open a ton of netlink 
>>> sockets at the same time, each with an unnecessarily huge buffer.
>>>
>> Does anyone has an good idea to solve this problem permanently? i.e.  
>> Dynamically allocate the socket buffer size accordingly. Thanks
>>
>
> I *think* (based on the things I've read, not on personal experience) 
> that sufficiently new versions of libnl and kernel *do* properly 
> implement message peeking, so we can just start off with a buffer size 
> larger than what we think will ever be needed on a current system, and 
> also enable message peeking, which should future-proof the code (so, 
> basically what you've done). I'm Cc'ing Thomas Graf (from libnl) for 
> his opinion though.

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Subject: [libvirt] [PATCH v1] libvirtd: Increase NL buffer size for lots of	interface
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