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Add tracing spans for input and output gates hold and wait #5563

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Add tracing spans for input and output gates hold and wait #5563

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1 change: 1 addition & 0 deletions src/workerd/io/BUILD.bazel
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Original file line number Diff line number Diff line change
Expand Up @@ -268,6 +268,7 @@ wd_cc_library(
hdrs = ["io-gate.h"],
visibility = ["//visibility:public"],
deps = [
":trace",
"@capnp-cpp//src/kj",
"@capnp-cpp//src/kj:kj-async",
],
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10 changes: 10 additions & 0 deletions src/workerd/io/io-gate.h
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Expand Up @@ -20,6 +20,8 @@
// to the application are still being flushed to disk. If the flush fails, these messages will
// never be sent, so that the rest of the world cannot observe a prematurely-confirmed write.

#include <workerd/io/trace.h>

#include <kj/async.h>
#include <kj/list.h>
#include <kj/one-of.h>
Expand Down Expand Up @@ -50,6 +52,10 @@ class InputGate {
virtual void inputGateWaiterRemoved() {}

static const Hooks DEFAULT;

protected:
kj::Maybe<SpanBuilder> inputGateWaitSpan;
kj::Maybe<SpanBuilder> inputGateHoldSpan;
};

// Hooks has no member variables, so const_cast is acceptable.
Expand Down Expand Up @@ -245,6 +251,10 @@ class OutputGate {
virtual void outputGateWaiterRemoved() {}

static const Hooks DEFAULT;

protected:
kj::Maybe<SpanBuilder> outputGateWaitSpan;
kj::Maybe<SpanBuilder> outputGateHoldSpan;
};

// Hooks has no member variables, so const_cast is acceptable.
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21 changes: 21 additions & 0 deletions src/workerd/io/worker.c++
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Expand Up @@ -3,6 +3,7 @@
// https://opensource.org/licenses/Apache-2.0

#include "actor-cache.h"
#include "workerd/io/io-gate.h"
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@danlapid danlapid Nov 20, 2025

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Suggested change
#include "workerd/io/io-gate.h"
#include <workerd/io/io-gate.h>


#include <workerd/api/actor-state.h>
#include <workerd/api/global-scope.h>
Expand Down Expand Up @@ -3441,15 +3442,25 @@ struct Worker::Actor::Impl {

void inputGateLocked() override {
metrics.inputGateLocked();
if (IoContext::hasCurrent()) {
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@danlapid danlapid Nov 20, 2025

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I think we should always have an active IoContext shouldn't we?
So

    auto& context = IoContext::current();
    auto inputGateHoldSpan = context.makeTraceSpan("actor_input_gate_hold"_kjc);

Should be better

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@shrima-cf shrima-cf Nov 20, 2025

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Based on my testing, we don't have IoContext when we invoke the input gate or hold them. I am trying to see if I can use the parent's context.

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@danlapid danlapid Nov 20, 2025

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That would really limit the effectiveness and reliability of this PR.

I don't understand input gates well enough but I would love to learn more about them to understand how to best approach this.
If we need to instead iterate over all of the actor's current inflight requests and open spans in all of them we could do that instead of relying on the IoContext

auto& ioContext = IoContext::current();
inputGateHoldSpan = ioContext.makeTraceSpan("actor_input_gate_hold"_kjc);
}
}
void inputGateReleased() override {
metrics.inputGateReleased();
inputGateHoldSpan = kj::none;
}
void inputGateWaiterAdded() override {
metrics.inputGateWaiterAdded();
if (IoContext::hasCurrent()) {
auto& ioContext = IoContext::current();
inputGateWaitSpan = ioContext.makeTraceSpan("actor_input_gate_wait"_kjc);
}
}
void inputGateWaiterRemoved() override {
metrics.inputGateWaiterRemoved();
inputGateWaitSpan = kj::none;
}
// Implements InputGate::Hooks.

Expand All @@ -3469,15 +3480,25 @@ struct Worker::Actor::Impl {

void outputGateLocked() override {
metrics.outputGateLocked();
if (IoContext::hasCurrent()) {
auto& ioContext = IoContext::current();
outputGateHoldSpan = ioContext.makeTraceSpan("actor_output_gate_hold"_kjc);
}
}
void outputGateReleased() override {
metrics.outputGateReleased();
outputGateHoldSpan = kj::none;
}
void outputGateWaiterAdded() override {
metrics.outputGateWaiterAdded();
if (IoContext::hasCurrent()) {
auto& ioContext = IoContext::current();
outputGateWaitSpan = ioContext.makeTraceSpan("actor_output_gate_wait"_kjc);
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@a-robinson a-robinson Nov 20, 2025

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This is naively overwriting any existing span, but it's possible for an OutputGate to have multiple waiters. I'll look more closely in a minute, but I'd expect us to want to attach a span to each (traced) waiter

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@a-robinson a-robinson Nov 20, 2025 •
edited
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Yeah, I'd think we could hold these WaitSpan variables in the InputGate::Waiter for InputGates, and for OutputGates we could attach a span on the branch promise returned from OutputGate::wait (similar to the defer for hooks.outputGateWaiterRemoved())

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@shrima-cf shrima-cf Nov 20, 2025

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I tested with one request and this works, but or input gates the IoContext is not set at the point where the gates are held/waited, so have to use the parent's IoContext or pass in a parent span. I am working on that change now, will mostly switch to same model for outpute gate too

}
}
void outputGateWaiterRemoved() override {
metrics.outputGateWaiterRemoved();
outputGateWaitSpan = kj::none;
}

// Implements ActorCache::Hooks
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