Date:   Thu, 25 Aug 2016 13:44:50 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Rick Jones <rick.jones2@....com>
Cc:     Netdev <netdev@...r.kernel.org>, sathya.perla@...adcom.com,
        ajit.khaparde@...adcom.com, sriharsha.basavapatna@...adcom.com,
        somnath.kotur@...adcom.com
Subject: Re: A second case of XPS considerably reducing single-stream performance

On Thu, Aug 25, 2016 at 1:18 PM, Rick Jones <rick.jones2@....com> wrote:
> On 08/25/2016 12:19 PM, Alexander Duyck wrote:
>>
>> The problem is that there is no socket associated with the guest from
>> the host's perspective.  This is resulting in the traffic bouncing
>> between queues because there is no saved socket  to lock the interface
>> onto.
>>
>> I was looking into this recently as well and had considered a couple
>> of options.  The first is to fall back to just using skb_tx_hash()
>> when skb->sk is null for a given buffer.  I have a patch I have been
>> toying around with but I haven't submitted it yet.  If you would like
>> I can submit it as an RFC to get your thoughts.  The second option is
>> to enforce the use of RPS for any interfaces that do not perform Rx in
>> NAPI context.  The correct solution for this is probably some
>> combination of the two as you have to have all queueing done in order
>> at every stage of the packet processing.
>
>
> I don't know with interfaces would be hit, but just in general, I'm not sure
> that requiring RPS be enabled is a good solution - picking where traffic is
> processed based on its addressing is fine in a benchmarking situation, but I
> think it is better to have the process/thread scheduler decide where
> something should run and not the addressing of the connections that
> thread/process is servicing.
>
> I would be interested in seeing the RFC patch you propose.
>
> Apart from that, given the prevalence of VMs these days I wonder if perhaps
> simply not enabling XPS by default isn't a viable alternative. I've not
> played with containers to know if they would exhibit this too.
>
> Drifting ever so slightly, if drivers are going to continue to enable XPS by
> default, Documentation/networking/scaling.txt might use a tweak:
>
> diff --git a/Documentation/networking/scaling.txt
> b/Documentation/networking/sca
> index 59f4db2..8b5537c 100644
> --- a/Documentation/networking/scaling.txt
> +++ b/Documentation/networking/scaling.txt
> @@ -402,10 +402,12 @@ acknowledged.
>
>  ==== XPS Configuration
>
> -XPS is only available if the kconfig symbol CONFIG_XPS is enabled (on by
> -default for SMP). The functionality remains disabled until explicitly
> -configured. To enable XPS, the bitmap of CPUs that may use a transmit
> -queue is configured using the sysfs file entry:
> +XPS is available only when the kconfig symbol CONFIG_XPS is enabled
> +(on by default for SMP). The drivers for some NICs will enable the
> +functionality by default.  For others the functionality remains
> +disabled until explicitly configured. To enable XPS, the bitmap of
> +CPUs that may use a transmit queue is configured using the sysfs file
> +entry:
>
>  /sys/class/net/<dev>/queues/tx-<n>/xps_cpus
>
>
> The original wording leaves the impression that XPS is not enabled by
> default.
>
> rick

That's true. The original documentation probably wasn't updated after
I added netif_set_xps_queue giving drivers the ability to specify the
XPS map themselves.  That was a workaround to get the drivers to stop
bypassing all of this entirely and using ndo_select_queue.

We might want to tweak the documentation to state that XPS is disabled
unless either the user enables it via the syfs or the device's driver
enables it via netif_set_xps_queue.  If you want to submit something
like that as an official patch I would have no problem with it.  Then
if nothing else it becomes much easier to identify which drivers
enable this by default as all you have to do is plug the function into
LXR and you have your list.

- Alex

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