netdev - Re: [PATCH net-next v2 1/3] net/mlx4: Track RX allocation failures in a stat

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Date: Fri, 26 Apr 2024 17:28:03 -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 v2 1/3] net/mlx4: Track RX allocation failures
 in a stat

On Fri, Apr 26, 2024 at 04:52:13PM -0700, Jakub Kicinski wrote:
> On Fri, 26 Apr 2024 16:43:53 -0700 Joe Damato wrote:
> > > In case of mlx4 looks like the buffer refill is "async", the driver
> > > tries to refill the buffers to max, but if it fails the next NAPI poll
> > > will try again. Allocation failures are not directly tied to packet
> > > drops. In case of bnxt if "replacement" buffer can't be allocated -
> > > packet is dropped and old buffer gets returned to the ring (although 
> > > if I'm 100% honest bnxt may be off by a couple, too, as the OOM stat
> > > gets incremented on ifup pre-fill failures).  
> > 
> > Yes, I see that now. I'll drop this patch entirely from v3 and just leave
> > the other two and remove alloc_fail from the queue stats patch.
> 
> Up to you, but I'd keep alloc_fail itself.
> If mlx4 gets page pool support one day it will be useful to run this:
> https://lore.kernel.org/all/20240426232400.624864-1-kuba@kernel.org/
> 
> And I think it's useful to be able to check in case there are Rx
> discards whether the system was also under transient memory pressure 
> or not.

Ah, maybe I read what you wrote incorrectly in your previous message.

I think you were saying that I should drop just the

  dev->stats.rx_missed_errors = dropped;

due to the definition of rx_missed_errors, but that by the definition of
rx-alloc-fail:

  alloc_fail = ring->dropped;

is still valid and can stay.

Is that right or am I just totally off?