fq: Add trace event

There's only one interesting event for it -- when the queue goes
"pending" state waiting for shared capacity. No arguments here, so it
just takes 3 bytes in the buffer.

Signed-off-by: Pavel Emelyanov <[email protected]>

apps/trace/decode.cc

@@ -25,6 +25,7 @@
 #include <seastar/util/trace.hh>
 #include <seastar/core/byteorder.hh>
 #include <seastar/core/internal/io_trace.hh>
+#include <seastar/core/internal/fq_trace.hh>
 
 using namespace seastar;
 
@@ -40,6 +41,8 @@ size_t event_body_size(internal::trace_event ev) {
         return internal::event_tracer<internal::trace_event::IO_DISPATCH>::size();
     case internal::trace_event::IO_COMPLETE:
         return internal::event_tracer<internal::trace_event::IO_COMPLETE>::size();
+    case internal::trace_event::FQ_WAIT_CAPACITY:
+        return internal::event_tracer<internal::trace_event::FQ_WAIT_CAPACITY>::size();
     case internal::trace_event::BUFFER_HEAD:
     case internal::trace_event::OPENING:
     case internal::trace_event::T800:
@@ -87,6 +90,11 @@ std::string parse<internal::trace_event::IO_COMPLETE>(temporary_buffer<char> bod
     return format("IO C {:04x}", rq);
 }
 
+template<>
+std::string parse<internal::trace_event::FQ_WAIT_CAPACITY>(temporary_buffer<char> body) {
+    return "FQ WAIT";
+}
+
 std::string parse_event(internal::trace_event ev, temporary_buffer<char> body) {
     switch (ev) {
     case internal::trace_event::TICK:
@@ -99,6 +107,8 @@ std::string parse_event(internal::trace_event ev, temporary_buffer<char> body) {
         return parse<internal::trace_event::IO_DISPATCH>(std::move(body));
     case internal::trace_event::IO_COMPLETE:
         return parse<internal::trace_event::IO_COMPLETE>(std::move(body));
+    case internal::trace_event::FQ_WAIT_CAPACITY:
+        return parse<internal::trace_event::FQ_WAIT_CAPACITY>(std::move(body));
     case internal::trace_event::BUFFER_HEAD:
     case internal::trace_event::OPENING:
     case internal::trace_event::T800:

include/seastar/core/internal/fq_trace.hh

@@ -0,0 +1,37 @@
+/*
+ * This file is open source software, licensed to you under the terms
+ * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
+ * distributed with this work for additional information regarding copyright
+ * ownership.  You may not use this file except in compliance with the License.
+ *
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/*
+ * Copyright (C) 2024 ScyllaDB.
+ */
+
+
+#pragma once
+#include <seastar/util/trace.hh>
+
+namespace seastar {
+namespace internal {
+
+template<>
+struct event_tracer<trace_event::FQ_WAIT_CAPACITY> {
+    static int size() noexcept { return 0; }
+    static void put(char* buf) noexcept { }
+};
+
+} // internal namespace
+} // seastar namespace

include/seastar/util/trace.hh

@@ -43,6 +43,7 @@ enum class trace_event {
     IO_QUEUE,
     IO_DISPATCH,
     IO_COMPLETE,
+    FQ_WAIT_CAPACITY,
 };
 
 size_t tick_event_size();

src/core/fair_queue.cc

@@ -249,6 +249,7 @@ auto fair_queue::grab_capacity(const fair_queue_entry& ent) noexcept -> grab_res
     capacity_t cap = ent._capacity;
     capacity_t want_head = _group.grab_capacity(cap);
     if (_group.capacity_deficiency(want_head)) {
+        _tracer.trace<internal::trace_event::FQ_WAIT_CAPACITY>();
         _pending.emplace(want_head, cap);
         return grab_result::pending;
     }