[PATCH mptcp-next v3 1/4] mptcp: sched: avoid truncating the pacing rate in the scheduler

mptcp_subflow_get_send() derives each subflow's linger time and a running
pacing-rate average through a u32 "pace", while sk_pacing_rate and
avg_pacing_rate are unsigned long.

On 64-bit systems the u32 truncates pacing rates above ~4.29 GB/s, skewing
the linger-time based subflow selection on very fast paths. On 32-bit
systems, where unsigned long is also 32-bit, the sk_pacing_rate * burst
product overflows: with burst up to ~65428 it wraps once the pacing rate
exceeds ~526 kbit/s, misestimating the running average at ordinary rates.

Widen "pace" to unsigned long, divide the linger time with a 64-bit
divisor, and cast the pacing-rate average product to u64.

Fixes: 3ce0852c86b9 ("mptcp: enforce HoL-blocking estimation")
Suggested-by: Matthieu Baerts (NGI0) <[email protected]>
Signed-off-by: Shardul Bankar <[email protected]>
---
 net/mptcp/protocol.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c2762d74f29d..46a49ec75e78 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1626,7 +1626,8 @@ struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 	struct subflow_send_info send_info[SSK_MODE_MAX];
 	struct mptcp_subflow_context *subflow;
 	struct sock *sk = (struct sock *)msk;
-	u32 pace, burst, wmem;
+	unsigned long pace;
+	u32 burst, wmem;
 	int i, nr_active = 0;
 	struct sock *ssk;
 	u64 linger_time;
@@ -1657,7 +1658,7 @@ struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 				continue;
 		}
 
-		linger_time = div_u64((u64)READ_ONCE(ssk->sk_wmem_queued) << 32, pace);
+		linger_time = div64_u64((u64)READ_ONCE(ssk->sk_wmem_queued) << 32, pace);
 		if (linger_time < send_info[backup].linger_time) {
 			send_info[backup].ssk = ssk;
 			send_info[backup].linger_time = linger_time;
@@ -1691,7 +1692,7 @@ struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 
 	subflow = mptcp_subflow_ctx(ssk);
 	subflow->avg_pacing_rate = div_u64((u64)subflow->avg_pacing_rate * wmem +
-					   READ_ONCE(ssk->sk_pacing_rate) * burst,
+					   (u64)READ_ONCE(ssk->sk_pacing_rate) * burst,
 					   burst + wmem);
 	msk->snd_burst = burst;
 	return ssk;

-- 
2.34.1