netdev - Re: [PATCH net-next] net: mvneta: add support for page_pool_get

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Message-ID: <20220406230136.GA96269@fastly.com>
Date:   Wed, 6 Apr 2022 16:01:37 -0700
From:   Joe Damato <jdamato@...tly.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     Andrew Lunn <andrew@...n.ch>, netdev@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, thomas.petazzoni@...tlin.com,
        linux@...linux.org.uk, jbrouer@...hat.com,
        ilias.apalodimas@...aro.org
Subject: Re: [PATCH net-next] net: mvneta: add support for page_pool_get_stats

On Wed, Apr 06, 2022 at 04:02:44PM +0200, Lorenzo Bianconi wrote:
> > > +static void mvneta_ethtool_update_pp_stats(struct mvneta_port *pp,
> > > +					   struct page_pool_stats *stats)
> > > +{
> > > +	int i;
> > > +
> > > +	memset(stats, 0, sizeof(*stats));
> > > +	for (i = 0; i < rxq_number; i++) {
> > > +		struct page_pool *page_pool = pp->rxqs[i].page_pool;
> > > +		struct page_pool_stats pp_stats = {};
> > > +
> > > +		if (!page_pool_get_stats(page_pool, &pp_stats))
> > > +			continue;
> > > +
> > > +		stats->alloc_stats.fast += pp_stats.alloc_stats.fast;
> > > +		stats->alloc_stats.slow += pp_stats.alloc_stats.slow;
> > > +		stats->alloc_stats.slow_high_order +=
> > > +			pp_stats.alloc_stats.slow_high_order;
> > > +		stats->alloc_stats.empty += pp_stats.alloc_stats.empty;
> > > +		stats->alloc_stats.refill += pp_stats.alloc_stats.refill;
> > > +		stats->alloc_stats.waive += pp_stats.alloc_stats.waive;
> > > +		stats->recycle_stats.cached += pp_stats.recycle_stats.cached;
> > > +		stats->recycle_stats.cache_full +=
> > > +			pp_stats.recycle_stats.cache_full;
> > > +		stats->recycle_stats.ring += pp_stats.recycle_stats.ring;
> > > +		stats->recycle_stats.ring_full +=
> > > +			pp_stats.recycle_stats.ring_full;
> > > +		stats->recycle_stats.released_refcnt +=
> > > +			pp_stats.recycle_stats.released_refcnt;
> > 
> > Am i right in saying, these are all software stats? They are also
> > generic for any receive queue using the page pool?
> 
> yes, these stats are accounted by the kernel so they are sw stats, but I guess
> xdp ones are sw as well, right?
> 
> > 
> > It seems odd the driver is doing the addition here. Why not pass stats
> > into page_pool_get_stats()? That will make it easier when you add
> > additional statistics?
> > 
> > I'm also wondering if ethtool -S is even the correct API. It should be
> > for hardware dependent statistics, those which change between
> > implementations. Where as these statistics should be generic. Maybe
> > they should be in /sys/class/net/ethX/statistics/ and the driver
> > itself is not even involved, the page pool code implements it?
> 
> I do not have a strong opinion on it, but I can see an issue for some drivers
> (e.g. mvpp2 iirc) where page_pools are not specific for each net_device but are shared
> between multiple ports, so maybe it is better to allow the driver to decide how
> to report them. What do you think?

When I did the implementation of this API the feedback was essentially
that the drivers should be responsible for reporting the stats of their
active page_pool structures; this is why the first driver to use this
(mlx5) uses the API and outputs the stats via ethtool -S.

I have no strong preference, either, but I think that exposing the stats
via an API for the drivers to consume is less tricky; the driver knows
which page_pools are active and which pool is associated with which
RX-queue, and so on.

If there is general consensus for a different approach amongst the
page_pool maintainers, I am happy to implement it.

Thanks,
Joe