Re: [PATCH net-next v2 4/4] net: hns3: support skb's frag page recycling based on page pool

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 08/09/2021 17.57, Jakub Kicinski wrote:
> On Wed, 8 Sep 2021 18:26:35 +0300 Ilias Apalodimas wrote:
>>> Normally I'd say put the stats in ethtool -S and the rest in debugfs
>>> but I'm not sure if exposing pages_state_hold_cnt and
>>> pages_state_release_cnt directly. Those are short counters, and will
>>> very likely wrap. They are primarily meaningful for calculating
>>> page_pool_inflight(). Given this I think their semantics may be too
>>> confusing for an average ethtool -S user.
>>>
>>> Putting all the information in debugfs seems like a better idea.
>>
>> I can't really disagree on the aforementioned stats being confusing.
>> However at some point we'll want to add more useful page_pool stats (e.g the
>> percentage of the page/page fragments that are hitting the recycling path).
>> Would it still be 'ok' to have info split across ethtool and debugfs?
> 
> Possibly. We'll also see what Alex L comes up with for XDP stats. Maybe
> we can arrive at a netlink API for standard things (broken record).
> 
> You said percentage - even tho I personally don't like it - there is a
> small precedent of ethtool -S containing non-counter information (IOW
> not monotonically increasing event counters), e.g. some vendors rammed
> PCI link quality in there. So if all else fails ethtool -S should be
> fine.

I agree with Ilias, that we ought-to add some page_pool stats.
*BUT* ONLY if this doesn't hurt performance!!!

We have explained before, how this is possible, e.g. by keeping consumer 
vs. producer counters on separate cache-lines (internally in page_pool 
struct and likely on per CPU for returning pages).  Then the drivers 
ethtool functions can request the page_pool to fillout a driver provided 
stats area, such that the collection and aggregation of counters are not 
on the fast-path.

I definitely don't want to see pages_state_hold_cnt and 
pages_state_release_cnt being exposed directly.  These were carefully 
designed to not hurt performance. An inflight counter can be deducted by 
above ethtool-driver step and presented to userspace.


Notice that while developing page_pool, I've been using tracepoints and 
bpftrace scripts to inspect the behavior and internals of page_pool.
See[1] and I've even written a page leak detector[2].

In principle you could write a bpftrace tool that extract stats, the 
same way. But I would only recommend doing this for devel phase, because 
these tracepoints do add some overhead.
Originally I wanted to push people to use this for stats, but I've 
realized that not having these stats easy available is annoying ;-)

-Jesper

[1] 
https://github.com/xdp-project/xdp-project/tree/master/areas/mem/bpftrace
[2] 
https://github.com/xdp-project/xdp-project/blob/master/areas/mem/bpftrace/page_pool_track_leaks02.bt