linux-kernel - Re: [PATCH net-next v3 1/3] net/mlx4: Track RX allocation failures in a stat

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Date: Tue, 7 May 2024 15:53:07 +0000
From: Joe Damato <jdamato@...tly.com>
To: Larysa Zaremba <larysa.zaremba@...el.com>
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>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	"open list:MELLANOX MLX4 core VPI driver" <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH net-next v3 1/3] net/mlx4: Track RX allocation failures
 in a stat

On Tue, May 07, 2024 at 05:44:27PM +0200, Larysa Zaremba wrote:
> On Thu, May 02, 2024 at 09:26:25PM +0000, Joe Damato wrote:
> > mlx4_en_alloc_frags currently returns -ENOMEM when mlx4_alloc_page
> > fails but does not increment a stat field when this occurs.
> > 
> > struct mlx4_en_rx_ring has a dropped field which is tabulated in
> > mlx4_en_DUMP_ETH_STATS, but never incremented by the driver.
> > 
> > This change modifies mlx4_en_alloc_frags to increment mlx4_en_rx_ring's
> > dropped field for the -ENOMEM case.
> > 
> > Signed-off-by: Joe Damato <jdamato@...tly.com>
> > Tested-by: Martin Karsten <mkarsten@...terloo.ca>
> > ---
> >  drivers/net/ethernet/mellanox/mlx4/en_rx.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > index 8328df8645d5..573ae10300c7 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > @@ -82,8 +82,10 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
> >  
> >  	for (i = 0; i < priv->num_frags; i++, frags++) {
> >  		if (!frags->page) {
> > -			if (mlx4_alloc_page(priv, frags, gfp))
> > +			if (mlx4_alloc_page(priv, frags, gfp)) {
> > +				ring->dropped++;
> >  				return -ENOMEM;
> > +			}
> 
> Correct me if I'm wrong, but ring->dropped is added to rx_dropped stats in 
> mlx4_en_DUMP_ETH_STATS(). You have already established with Jakub that 
> allocation error does not mean dropped packet, but the counter contributes to 
> dropped packets stats.
> 
> Also, I do not think that using a `dropped` counter for something that does not 
> neccessarily result in a dropped packet is plain confusing.

Fair enough; I could add a new field called "alloc_fail" to
mlx4_en_rx_ring and increment that instead as it is, according to the
earlier thread, an alloc_fail as far as netdev-genl is concerned, I
think.

FWIW: I had spoken to Mellanox about this off list many weeks ago and
they had agreed at that time to this general approach. I haven't heard
from them in some time, but I am open to something else if that's the
blocker here.