netdev - Re: [PATCH net-next 1/3] netdev: add per-queue statistics

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Date: Mon, 26 Feb 2024 19:37:04 -0800
From: Stanislav Fomichev <sdf@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com, 
	pabeni@...hat.com, amritha.nambiar@...el.com, danielj@...dia.com, 
	mst@...hat.com, michael.chan@...adcom.com, vadim.fedorenko@...ux.dev
Subject: Re: [PATCH net-next 1/3] netdev: add per-queue statistics

On 02/26, Jakub Kicinski wrote:
> On Mon, 26 Feb 2024 13:35:34 -0800 Stanislav Fomichev wrote:
> > > +  -
> > > +    name: stats-scope
> > > +    type: flags
> > > +    entries: [ queue ]  
> > 
> > IIUC, in order to get netdev-scoped stats in v1 (vs rfc) is to not set
> > stats-scope, right? Any reason we dropped the explicit netdev entry?
> > It seems more robust with a separate entry and removes the ambiguity about
> > which stats we're querying.
> 
> The change is because I switched from enum to flags.
> 
> I'm not 100% sure which one is going to cause fewer issues down
> the line. It's a question of whether the next scope we add will 
> be disjoint with or subdividing previous scopes.
> 
> I think only subdividing previous scopes makes sense. If we were 
> to add "stats per NAPI" (bad example) or "per buffer pool" or IDK what
> other thing -- we should expose that as a new netlink command, not mix 
> it with the queues.
> 
> The expectation is that scopes will be extended with hw vs sw, or
> per-CPU (e.g. page pool recycling). In which case we'll want flags,
> so that we can combine them -- ask for HW stats for a queue or hw
> stats for the entire netdev.
> 
> Perhaps I should rename stats -> queue-stats to make this more explicit?
> 
> The initial version I wrote could iterate both over NAPIs and
> queues. This could be helpful to some drivers - but I realized that it
> would lead to rather painful user experience (does the driver maintain
> stats per NAPI or per queue?) and tricky implementation of the device
> level sum (device stats = Sum(queue) or Sum(queue) + Sum(NAPI)??)

Yeah, same, not sure. The flags may be more flexible but a bit harder
wrt discoverability. Assuming a somewhat ignorant spec reader/user,
it might be hard to say which flags makes sense to combine and which isn't.
Or, I guess, we can try to document it?

For HW vs SW, do you think it makes sense to expose it as a scope?
Why not have something like 'rx-packets' and 'hw-rx-packets'?

Maybe, as you're suggesting, we should rename stats to queue-states
and drop the score for now? When the time comes to add hw counters,
we can revisit. For total netdev stats, we can ask the user to aggregate
the per-queue ones?