netdev - Re: [PATCH net-next 3/3] net/mlx4: support per-queue statistics via netlink

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Message-ID: <ZiqFUs-z5We2--5n@LQ3V64L9R2>
Date: Thu, 25 Apr 2024 09:31:14 -0700
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, tariqt@...dia.com,
	saeedm@...dia.com, mkarsten@...terloo.ca, gal@...dia.com,
	nalramli@...tly.com, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	"open list:MELLANOX MLX4 core VPI driver" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH net-next 3/3] net/mlx4: support per-queue statistics via
 netlink

On Wed, Apr 24, 2024 at 09:39:43AM -0700, Joe Damato wrote:
> On Wed, Apr 24, 2024 at 07:28:18AM -0700, Jakub Kicinski wrote:
> > On Tue, 23 Apr 2024 22:54:50 -0700 Joe Damato wrote:
> > > On Tue, Apr 23, 2024 at 05:57:18PM -0700, Jakub Kicinski wrote:
> > > > On Tue, 23 Apr 2024 12:42:13 -1000 Joe Damato wrote:  
> > > > > I realized in this case, I'll need to set the fields initialized to 0xff
> > > > > above to 0 before doing the increments below.  
> > > > 
> > > > I don't know mlx4 very well, but glancing at the code - are you sure we
> > > > need to loop over the queues is the "base" callbacks?
> > > > 
> > > > The base callbacks are for getting "historical" data, i.e. info which
> > > > was associated with queues which are no longer present. You seem to
> > > > sweep all queues, so I'd have expected "base" to just set the values 
> > > > to 0. And the real values to come from the per-queue callbacks.  
> > > 
> > > Hmm. Sorry I must have totally misunderstood what the purpose of "base"
> > > was. I've just now more closely looked at bnxt which (maybe?) is the only
> > > driver that implements base and I think maybe I kind of get it now.
> > > 
> > > For some reason, I thought it meant "the total stats of all queues"; I didn't
> > > know it was intended to provide "historical" data as you say.
> > > 
> > > Making it set everything to 0 makes sense to me. I suppose I could also simply
> > > omit it? What do you think?
> > 
> > The base is used to figure out which stats are reported when we dump 
> > a summary for the whole device. So you gotta set them to 0.
> 
> OK, thanks for your patience and the explanation. Will do.
> 
> > > > The init to 0xff looks quite sus.  
> > > 
> > > Yes the init to 0xff is wrong, too. I noticed that, as well.
> > > 
> > > Here's what I have listed so far in my changelog for the v2 (which I haven't
> > > sent yet), but perhaps the maintainers of mlx4 can weigh in?
> > > 
> > > v1 -> v2:
> > >  - Patch 1/3 now initializes dropped to 0.
> > >  - Patch 3/3 includes several changes:
> > >    - mlx4_get_queue_stats_rx and mlx4_get_queue_stats_tx check if i is
> > >      valid before proceeding.
> > >    - All initialization to 0xff for stats fields has been omit. The
> > >      network stack does this before calling into the driver functions, so
> > >      I've adjusted the driver functions to only set values if there is
> > >      data to set, leaving the network stack's 0xff in place if not.
> > >    - mlx4_get_base_stats sets all stats to 0 (no locking etc needed).
> > 
> > All the ones you report right? Not just zero the struct.
> > Any day now (tm) someone will add a lot more stats to the struct
> > so the init should be selective only to the stats that are actually
> > supported.
> 
> Yes, not just zero the struct. Since I am reporting bytes packets for both
> RX and TX and alloc_fail for RX I'll be setting those fields to 0
> explicitly.
> 
> And there's also a warning about unused qtype (oops) in patch 2/3.
> 
> So, the revised v2 list (pending anything Mellanox wants) is:
> 
>   v1 -> v2:
>    - Patch 1/3 now initializes dropped to 0.
>    - Patch 2/3 fix use of unitialized qtype warning.
>    - Patch 3/3 includes several changes:
>      - mlx4_get_queue_stats_rx and mlx4_get_queue_stats_tx check if i is
>        valid before proceeding.
>      - All initialization to 0xff for stats fields has been omit. The
>        network stack does this before calling into the driver functions, so
>        I've adjusted the driver functions to only set values if there is
>        data to set, leaving the network stack's 0xff in place if not.
>      - mlx4_get_base_stats set all stat fields to 0 individually (no locking etc needed).
> 
> I'll hold off on sending this v2 until we hear back from Mellanox to be
> sure there isn't anything else I'm missing.

It's been a few days and I haven't heard back from the mlx4 folks, so I
think I'll probably send my v2 later today which, hopefully, will fix most
of the above issues.