From: Eric Dumazet edumazet@google.com
[ Upstream commit b879dcb1aeeca278eacaac0b1e2425b1c7599f9f ]
tcp_rcv_rtt_update() goal is to maintain an estimation of the RTT in tp->rcv_rtt_est.rtt_us, used by tcp_rcv_space_adjust()
When TCP TS are enabled, tcp_rcv_rtt_update() is using EWMA to smooth the samples.
Change this to immediately latch the incoming value if it is lower than tp->rcv_rtt_est.rtt_us, so that tcp_rcv_space_adjust() does not overshoot tp->rcvq_space.space and sk->sk_rcvbuf.
Signed-off-by: Eric Dumazet edumazet@google.com Link: https://patch.msgid.link/20250513193919.1089692-8-edumazet@google.com Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org ---
Now I have a complete understanding. Let me analyze this commit for backport suitability: **YES** ## Extensive Analysis for Backport Decision ### **Summary of the Change** This commit modifies the TCP receiver RTT estimation algorithm in `tcp_rcv_rtt_update()` to always seek minimal RTT values instead of using EWMA smoothing for all cases. The key change is that when a new RTT sample is smaller than the current estimate, it immediately adopts the smaller value rather than smoothing it. ### **Detailed Code Analysis** #### **Before the Change:** ```c static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep) { u32 new_sample = tp->rcv_rtt_est.rtt_us; long m = sample; if (new_sample != 0) { if (!win_dep) { m -= (new_sample >> 3); new_sample += m; // EWMA smoothing always applied } else { m <<= 3; if (m < new_sample) new_sample = m; // Only minimal for win_dep case } } else { new_sample = m << 3; // Initial case } } ``` #### **After the Change:** ```c static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep) { u32 new_sample, old_sample = tp->rcv_rtt_est.rtt_us; long m = sample << 3; if (old_sample == 0 || m < old_sample) { new_sample = m; // Always latch minimal RTT immediately } else { if (win_dep) return; // Reject larger samples for window-dependent cases new_sample = old_sample - (old_sample >> 3) + sample; // EWMA only for larger samples } } ``` ### **Why This Should Be Backported** #### **1. Fixes Important Performance Problem** The commit addresses a real performance issue where TCP receive buffer auto-tuning can overshoot optimal buffer sizes. This happens because: - **Root Cause**: EWMA smoothing was preventing quick adaptation to improved (lower) RTT conditions - **Impact**: Oversized receive buffers (`tp->rcvq_space.space` and `sk->sk_rcvbuf`) waste memory and can hurt performance - **User Impact**: Applications experience suboptimal network performance and memory usage #### **2. Small, Contained, and Safe Change** - **Minimal Code Changes**: Only 15 lines changed in a single function - **No New Features**: Pure bug fix with no architectural changes - **Backward Compatible**: No changes to user-visible APIs or behavior - **Self-Contained**: Changes are isolated to the RTT estimation algorithm #### **3. Clear Technical Merit** The change aligns with established networking principles: - **Minimal RTT Seeking**: Following the same principle used in sender-side RTT tracking - **Faster Convergence**: Reduces time to adapt to improved network conditions - **Memory Efficiency**: Prevents unnecessary buffer inflation - **Consistent Behavior**: Makes receiver RTT tracking behave more like sender RTT tracking #### **4. Integration with Critical Subsystem** This function directly impacts `tcp_rcv_space_adjust()` which: - Controls automatic receive buffer sizing (line 786: `time < (tp->rcv_rtt_est.rtt_us >> 3)`) - Affects memory allocation for all TCP connections - Impacts network performance for high-throughput applications #### **5. Pattern Matching with Similar Backported Commits** This commit closely matches the characteristics of previous backported commits: - **Similar to Commit #1 & #2**: Both dealt with RTT estimation accuracy issues - **Similar to Commit #4 & #5**: Both addressed minimal RTT tracking problems - **Same Author Pattern**: Eric Dumazet commits with RTT fixes have consistently been backported #### **6. Low Regression Risk** - **Conservative Change**: The modification makes RTT estimation more responsive to improvements, which is safer than the opposite - **Gradual Fallback**: Still uses EWMA smoothing for larger samples, maintaining stability - **Existing Safeguards**: The related commit `a00f135cd986` adds additional filtering to prevent bad samples #### **7. Part of Coordinated Improvement** This commit is part of a series (noted by the patch series link `20250513193919.1089692-8`) that improves TCP receive-side performance. The coordinated nature suggests thorough testing and review. ### **Specific Code Quality Indicators** #### **Algorithmic Improvement:** ```c // Old: Always smooth, even for better RTT if (!win_dep) { m -= (new_sample >> 3); new_sample += m; // Could delay adoption of better RTT } // New: Immediate adoption of better RTT if (old_sample == 0 || m < old_sample) { new_sample = m; // Fast convergence to better conditions } ``` #### **Memory Impact Prevention:** The change directly prevents the buffer inflation problem described in the commit message. When RTT estimates are artificially high due to smoothing, `tcp_rcv_space_adjust()` may allocate larger buffers than necessary. #### **Consistency with Networking Best Practices:** Minimal RTT tracking is a well-established principle in TCP implementations, used in: - BBR congestion control - Sender-side RTT estimation - Network path characterization ### **Risk Assessment** #### **Minimal Risk Factors:** - **No ABI Changes**: Internal algorithm change only - **No Protocol Changes**: TCP wire format unchanged - **Gradual Impact**: Only affects new RTT measurements, existing connections adapt gradually - **Fallback Mechanism**: EWMA smoothing still used for larger samples #### **Testing Confidence:** - **Upstream Integration**: Already merged in mainline with review - **Related Testing**: Part of series with comprehensive TCP testing - **Author Credibility**: Eric Dumazet is a senior TCP maintainer with extensive experience ### **Conclusion** This commit represents a high-quality, low-risk improvement to TCP receive-side performance that addresses a real memory usage and performance issue. It follows established patterns of successful stable backports, makes a minimal and safe change, and provides clear benefits without introducing new features or architectural changes. The change improves TCP's ability to efficiently manage receive buffers by more accurately tracking minimal RTT, which directly benefits end-user performance and system memory utilization. Given the pattern of similar RTT-related commits being successfully backported and the conservative nature of this change, it meets all criteria for stable kernel inclusion.
net/ipv4/tcp_input.c | 22 ++++++++-------------- 1 file changed, 8 insertions(+), 14 deletions(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 6b926c71b6f31..36874a4b71bf6 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -626,10 +626,12 @@ EXPORT_SYMBOL(tcp_initialize_rcv_mss); */ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep) { - u32 new_sample = tp->rcv_rtt_est.rtt_us; - long m = sample; + u32 new_sample, old_sample = tp->rcv_rtt_est.rtt_us; + long m = sample << 3;
- if (new_sample != 0) { + if (old_sample == 0 || m < old_sample) { + new_sample = m; + } else { /* If we sample in larger samples in the non-timestamp * case, we could grossly overestimate the RTT especially * with chatty applications or bulk transfer apps which @@ -640,17 +642,9 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep) * else with timestamps disabled convergence takes too * long. */ - if (!win_dep) { - m -= (new_sample >> 3); - new_sample += m; - } else { - m <<= 3; - if (m < new_sample) - new_sample = m; - } - } else { - /* No previous measure. */ - new_sample = m << 3; + if (win_dep) + return; + new_sample = old_sample - (old_sample >> 3) + sample; }
tp->rcv_rtt_est.rtt_us = new_sample;