linux-kernel - Re: [PATCH net-next v7 04/10] net: psample: allow using rate as probability

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Date: Mon, 01 Jul 2024 14:10:34 -0400
From: Aaron Conole <aconole@...hat.com>
To: Adrian Moreno <amorenoz@...hat.com>
Cc: netdev@...r.kernel.org,  echaudro@...hat.com,  horms@...nel.org,
  i.maximets@....org,  dev@...nvswitch.org,  Ido Schimmel
 <idosch@...dia.com>,  Yotam Gigi <yotam.gi@...il.com>,  "David S. Miller"
 <davem@...emloft.net>,  Eric Dumazet <edumazet@...gle.com>,  Jakub
 Kicinski <kuba@...nel.org>,  Paolo Abeni <pabeni@...hat.com>,
  linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v7 04/10] net: psample: allow using rate as
 probability

Adrian Moreno <amorenoz@...hat.com> writes:

> Although not explicitly documented in the psample module itself, the
> definition of PSAMPLE_ATTR_SAMPLE_RATE seems inherited from act_sample.
>
> Quoting tc-sample(8):
> "RATE of 100 will lead to an average of one sampled packet out of every
> 100 observed."
>
> With this semantics, the rates that we can express with an unsigned
> 32-bits number are very unevenly distributed and concentrated towards
> "sampling few packets".
> For example, we can express a probability of 2.32E-8% but we
> cannot express anything between 100% and 50%.
>
> For sampling applications that are capable of sampling a decent
> amount of packets, this sampling rate semantics is not very useful.
>
> Add a new flag to the uAPI that indicates that the sampling rate is
> expressed in scaled probability, this is:
> - 0 is 0% probability, no packets get sampled.
> - U32_MAX is 100% probability, all packets get sampled.
>
> Acked-by: Eelco Chaudron <echaudro@...hat.com>
> Reviewed-by: Ido Schimmel <idosch@...dia.com>
> Signed-off-by: Adrian Moreno <amorenoz@...hat.com>
> ---

Reviewed-by: Aaron Conole <aconole@...hat.com>

>  include/net/psample.h        |  3 ++-
>  include/uapi/linux/psample.h | 10 +++++++++-
>  net/psample/psample.c        |  3 +++
>  3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/include/net/psample.h b/include/net/psample.h
> index 2ac71260a546..c52e9ebd88dd 100644
> --- a/include/net/psample.h
> +++ b/include/net/psample.h
> @@ -24,7 +24,8 @@ struct psample_metadata {
>  	u8 out_tc_valid:1,
>  	   out_tc_occ_valid:1,
>  	   latency_valid:1,
> -	   unused:5;
> +	   rate_as_probability:1,
> +	   unused:4;
>  	const u8 *user_cookie;
>  	u32 user_cookie_len;
>  };
> diff --git a/include/uapi/linux/psample.h b/include/uapi/linux/psample.h
> index e80637e1d97b..b765f0e81f20 100644
> --- a/include/uapi/linux/psample.h
> +++ b/include/uapi/linux/psample.h
> @@ -8,7 +8,11 @@ enum {
>  	PSAMPLE_ATTR_ORIGSIZE,
>  	PSAMPLE_ATTR_SAMPLE_GROUP,
>  	PSAMPLE_ATTR_GROUP_SEQ,
> -	PSAMPLE_ATTR_SAMPLE_RATE,
> +	PSAMPLE_ATTR_SAMPLE_RATE,	/* u32, ratio between observed and
> +					 * sampled packets or scaled probability
> +					 * if PSAMPLE_ATTR_SAMPLE_PROBABILITY
> +					 * is set.
> +					 */
>  	PSAMPLE_ATTR_DATA,
>  	PSAMPLE_ATTR_GROUP_REFCOUNT,
>  	PSAMPLE_ATTR_TUNNEL,
> @@ -20,6 +24,10 @@ enum {
>  	PSAMPLE_ATTR_TIMESTAMP,		/* u64, nanoseconds */
>  	PSAMPLE_ATTR_PROTO,		/* u16 */
>  	PSAMPLE_ATTR_USER_COOKIE,	/* binary, user provided data */
> +	PSAMPLE_ATTR_SAMPLE_PROBABILITY,/* no argument, interpret rate in
> +					 * PSAMPLE_ATTR_SAMPLE_RATE as a
> +					 * probability scaled 0 - U32_MAX.
> +					 */
>  
>  	__PSAMPLE_ATTR_MAX
>  };
> diff --git a/net/psample/psample.c b/net/psample/psample.c
> index 1c76f3e48dcd..f48b5b9cd409 100644
> --- a/net/psample/psample.c
> +++ b/net/psample/psample.c
> @@ -497,6 +497,9 @@ void psample_sample_packet(struct psample_group *group, struct sk_buff *skb,
>  		    md->user_cookie))
>  		goto error;
>  
> +	if (md->rate_as_probability)
> +		nla_put_flag(skb, PSAMPLE_ATTR_SAMPLE_PROBABILITY);
> +
>  	genlmsg_end(nl_skb, data);
>  	genlmsg_multicast_netns(&psample_nl_family, group->net, nl_skb, 0,
>  				PSAMPLE_NL_MCGRP_SAMPLE, GFP_ATOMIC);