From 8921a9dd60c3e00d4ba676cdaad4e5141f9d7d04 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Fri, 10 Apr 2026 20:11:34 -1000
Subject: [PATCH] [core] fast_select stats: log details on load-bearing hit
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Adds a non-inline helper note_fast_select_load_bearing_() invoked only
when the scan found data and the task notification counter was 0.
Logs: hit sequence number, lwip_sock pointer, index in
monitored_sockets_, raw rcvevent value, delay_ms, and how many other
sockets also had data at that instant.

Hot path is unchanged — the helper is out-of-line so yield_with_select_
still inlines to the same code as before for the zero-hit path.
---
 esphome/core/application.cpp | 28 ++++++++++++++++++++++++++++
 esphome/core/application.h   |  5 ++++-
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/esphome/core/application.cpp b/esphome/core/application.cpp
index 8d51ddf0fd0..37aac76740b 100644
--- a/esphome/core/application.cpp
+++ b/esphome/core/application.cpp
@@ -462,6 +462,34 @@ void Application::log_fast_select_scan_stats_() {
            load_bearing);
 }
 
+void Application::note_fast_select_load_bearing_(struct lwip_sock *sock, uint32_t delay_ms) {
+  uint32_t load_bearing = fast_select_scan_load_bearing_.fetch_add(1, std::memory_order_relaxed) + 1;
+  // Find the socket's index in monitored_sockets_ for easier correlation with registration order.
+  int index = -1;
+  for (size_t i = 0; i < this->monitored_sockets_.size(); i++) {
+    if (this->monitored_sockets_[i] == sock) {
+      index = static_cast<int>(i);
+      break;
+    }
+  }
+  // Read the rcvevent value directly. This is the same offset-based read used by
+  // esphome_lwip_socket_has_data(); value > 0 means unread data is queued.
+  int16_t rcvevent =
+      *reinterpret_cast<volatile int16_t *>(reinterpret_cast<char *>(sock) + ESPHOME_LWIP_SOCK_RCVEVENT_OFFSET);
+  // Count how many other sockets also had data at this scan (could reveal whether it's always
+  // the same socket or a burst across multiple).
+  size_t sockets_with_data = 0;
+  for (struct lwip_sock *s : this->monitored_sockets_) {
+    if (esphome_lwip_socket_has_data(s))
+      sockets_with_data++;
+  }
+  ESP_LOGW(TAG,
+           "fast_select LOAD-BEARING hit #%" PRIu32 ": sock=%p idx=%d/%u rcvevent=%d delay_ms=%" PRIu32
+           " sockets_with_data=%u",
+           load_bearing, sock, index, static_cast<unsigned>(this->monitored_sockets_.size()), rcvevent, delay_ms,
+           static_cast<unsigned>(sockets_with_data));
+}
+
 bool Application::register_socket(struct lwip_sock *sock) {
   // It modifies monitored_sockets_ without locking — must only be called from the main loop.
   if (sock == nullptr)
diff --git a/esphome/core/application.h b/esphome/core/application.h
index f0ce013224c..7072b6b4a24 100644
--- a/esphome/core/application.h
+++ b/esphome/core/application.h
@@ -664,6 +664,8 @@ class Application {
   static std::atomic<uint32_t> fast_select_scan_load_bearing_;
   uint32_t fast_select_scan_stats_last_log_{0};
   void log_fast_select_scan_stats_();
+  // Non-inline, called only on the rare load-bearing event so the hot path stays unchanged.
+  void note_fast_select_load_bearing_(struct lwip_sock *sock, uint32_t delay_ms);
 #elif defined(USE_HOST)
   std::vector<int> socket_fds_;  // Vector of all monitored socket file descriptors
 #endif
@@ -947,7 +949,8 @@ inline void ESPHOME_ALWAYS_INLINE Application::yield_with_select_(uint32_t delay
       fast_select_scan_found_data_.fetch_add(1, std::memory_order_relaxed);
       if (fast_select_notify_value_before_scan == 0) {
         // Scan was load-bearing: no notification pending, so Take would have stalled.
-        fast_select_scan_load_bearing_.fetch_add(1, std::memory_order_relaxed);
+        // Delegate to a non-inline helper so the hot path stays the same size.
+        this->note_fast_select_load_bearing_(sock, delay_ms);
       }
       yield();
       return;