[PATCH net-next v2 4/9] net: psample: allow using rate as probability

[Adrian Moreno]

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.

Signed-off-by: Adrian Moreno <amorenoz@xxxxxxxxxx>
---
 include/net/psample.h                 | 3 ++-
 include/uapi/linux/psample.h          | 4 ++++
 include/uapi/linux/tc_act/tc_sample.h | 1 +
 net/psample/psample.c                 | 3 +++
 4 files changed, 10 insertions(+), 1 deletion(-)

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..8b069e75beab 100644
--- a/include/uapi/linux/psample.h
+++ b/include/uapi/linux/psample.h
@@ -20,6 +20,10 @@ 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/include/uapi/linux/tc_act/tc_sample.h b/include/uapi/linux/tc_act/tc_sample.h
index fee1bcc20793..7ee0735e7b38 100644
--- a/include/uapi/linux/tc_act/tc_sample.h
+++ b/include/uapi/linux/tc_act/tc_sample.h
@@ -18,6 +18,7 @@ enum {
 	TCA_SAMPLE_TRUNC_SIZE,
 	TCA_SAMPLE_PSAMPLE_GROUP,
 	TCA_SAMPLE_PAD,
+	TCA_SAMPLE_PROBABILITY,
 	__TCA_SAMPLE_MAX
 };
 #define TCA_SAMPLE_MAX (__TCA_SAMPLE_MAX - 1)
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);
-- 
2.45.1