[ResourceScheduler] Yield after starting several requests

content/browser/loader/resource_scheduler.cc

Index: content/browser/loader/resource_scheduler.cc
diff --git a/content/browser/loader/resource_scheduler.cc b/content/browser/loader/resource_scheduler.cc
index 6d68cf1c438dd7b22621500e2dcc70af6fa917b0..ca26127088ad64affc5989e7cefc62c51c5434f3 100644
--- a/content/browser/loader/resource_scheduler.cc
+++ b/content/browser/loader/resource_scheduler.cc
@@ -13,6 +13,7 @@
 #include "base/feature_list.h"
 #include "base/macros.h"
 #include "base/metrics/field_trial.h"
+#include "base/metrics/field_trial_params.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "base/supports_user_data.h"
@@ -39,6 +40,16 @@ namespace {
 const base::Feature kPrioritySupportedRequestsDelayable{
     "PrioritySupportedRequestsDelayable", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// In the event that many resource requests are started quickly, this feature
+// will periodically yield (e.g., delaying starting of requests) by posting a
+// task and waiting for the task to run to resume. This allows other
+// operations that rely on the IO thread (e.g., already running network
+// requests) to make progress.
+const base::Feature kNetworkSchedulerYielding{
+    "NetworkSchedulerYielding", base::FEATURE_DISABLED_BY_DEFAULT};
+const char kMaxRequestsBeforeYieldingParam[] = "MaxRequestsBeforeYieldingParam";
+const int kMaxRequestsBeforeYieldingDefault = 5;
+
 enum StartMode {
   START_SYNC,
   START_ASYNC
@@ -61,6 +72,7 @@ enum class RequestStartTrigger {
   CLIENT_KILL,
   SPDY_PROXY_DETECTED,
   REQUEST_REPRIORITIZED,
+  START_WAS_YIELDED,
 };
 
 const char* RequestStartTriggerString(RequestStartTrigger trigger) {
@@ -79,6 +91,8 @@ const char* RequestStartTriggerString(RequestStartTrigger trigger) {
       return "SPDY_PROXY_DETECTED";
     case RequestStartTrigger::REQUEST_REPRIORITIZED:
       return "REQUEST_REPRIORITIZED";
+    case RequestStartTrigger::START_WAS_YIELDED:
+      return "START_WAS_YIELDED";
   }
   NOTREACHED();
   return "Unknown";
@@ -348,7 +362,9 @@ void ResourceScheduler::RequestQueue::Insert(
 // Each client represents a tab.
 class ResourceScheduler::Client {
  public:
-  explicit Client(bool priority_requests_delayable)
+  Client(bool priority_requests_delayable,
+         bool yielding_scheduler_enabled,
+         int max_requests_before_yielding)
       : is_loaded_(false),
         has_html_body_(false),
         using_spdy_proxy_(false),
@@ -356,6 +372,9 @@ class ResourceScheduler::Client {
         total_layout_blocking_count_(0),
         priority_requests_delayable_(priority_requests_delayable),
         has_pending_start_task_(false),
+        requests_since_yielding_(0),
+        yielding_scheduler_enabled_(yielding_scheduler_enabled),
+        max_requests_before_yielding_(max_requests_before_yielding),
         weak_ptr_factory_(this) {}
 
   ~Client() {}
@@ -363,7 +382,8 @@ class ResourceScheduler::Client {
   void ScheduleRequest(net::URLRequest* url_request,
                        ScheduledResourceRequest* request) {
     SetRequestAttributes(request, DetermineRequestAttributes(request));
-    if (ShouldStartRequest(request) == START_REQUEST) {
+    if (ShouldStartRequest(request) == START_REQUEST &&
+        (!request->is_async() || !ShouldYieldScheduler())) {

[mtomasz, 2017/02/22 03:52:03]

This doesn't seem right. It was guaranteed that non HTTP/HTTPS requests will
never be delayed
(https://cs.chromium.org/chromium/src/content/browser/loader/resource_schedule...).


This CL releases this guarantee. As a result, blob requests became delayable,
which caused data corruption for filed provided via File System Provider API.


I think the logic here could be simpler. It used to be ShouldStartRequest which
would contain the entire logic for deciding whether to start a request
immediately or later. However, now part of the logic lays is moved to
ShouldYieldScheduler. Can we move ShouldYieldScheduler to ShouldStartRequest
while fixing the blob requests?

       // New requests can be started synchronously without issue.
       StartRequest(request, START_SYNC, RequestStartTrigger::NONE);
     } else {
@@ -738,6 +758,38 @@ class ResourceScheduler::Client {
                               weak_ptr_factory_.GetWeakPtr(), trigger));
   }
 
+  void ResumeAfterYielding() {
+    bool yielded_requests =
+        requests_since_yielding_ > max_requests_before_yielding_;
+    requests_since_yielding_ = 0;
+
+    if (yielded_requests)
+      LoadAnyStartablePendingRequests(RequestStartTrigger::START_WAS_YIELDED);
+  }
+
+  // Returns true if the scheduler should not start the next request and
+  // instead return so that other tasks can run on the IO thread (e.g.,
+  // existing network requests). If it returns true, ShouldYieldScheduler will
+  // have already scheduled a task to resume after yielding.
+  bool ShouldYieldScheduler() {
+    if (!yielding_scheduler_enabled_)
+      return false;
+
+    requests_since_yielding_ += 1;
+
+    if (requests_since_yielding_ == 1) {
+      // This is the first request since last yielding. Post a task to reset the
+      // counter and start any yielded tasks if necessary. We post this now
+      // instead of when we first yield so that if there is a pause between
+      // requests the counter is reset.
+      base::ThreadTaskRunnerHandle::Get()->PostTask(
+          FROM_HERE, base::Bind(&Client::ResumeAfterYielding,
+                                weak_ptr_factory_.GetWeakPtr()));
+    }
+
+    return requests_since_yielding_ > max_requests_before_yielding_;
+  }
+
   void LoadAnyStartablePendingRequests(RequestStartTrigger trigger) {
     // We iterate through all the pending requests, starting with the highest
     // priority one. For each entry, one of three things can happen:
@@ -756,6 +808,9 @@ class ResourceScheduler::Client {
       ShouldStartReqResult query_result = ShouldStartRequest(request);
 
       if (query_result == START_REQUEST) {
+        if (ShouldYieldScheduler())
+          break;
+
         pending_requests_.Erase(request);
         StartRequest(request, START_ASYNC, trigger);
 
@@ -794,12 +849,28 @@ class ResourceScheduler::Client {
 
   bool has_pending_start_task_;
 
+  // The number of requests that have been started since the last
+  // ResumeAfterYielding task was posted.
+  int requests_since_yielding_;
+
+  // Whether or not to periodically yield when starting lots of requests.
+  bool yielding_scheduler_enabled_;
+
+  // The number of requests that can start before yielding.
+  int max_requests_before_yielding_;
+
   base::WeakPtrFactory<ResourceScheduler::Client> weak_ptr_factory_;
 };
 
 ResourceScheduler::ResourceScheduler()
     : priority_requests_delayable_(
-          base::FeatureList::IsEnabled(kPrioritySupportedRequestsDelayable)) {}
+          base::FeatureList::IsEnabled(kPrioritySupportedRequestsDelayable)),
+      yielding_scheduler_enabled_(
+          base::FeatureList::IsEnabled(kNetworkSchedulerYielding)),
+      max_requests_before_yielding_(base::GetFieldTrialParamByFeatureAsInt(
+          kNetworkSchedulerYielding,
+          kMaxRequestsBeforeYieldingParam,
+          kMaxRequestsBeforeYieldingDefault)) {}
 
 ResourceScheduler::~ResourceScheduler() {
   DCHECK(unowned_requests_.empty());
@@ -856,7 +927,9 @@ void ResourceScheduler::OnClientCreated(int child_id,
   ClientId client_id = MakeClientId(child_id, route_id);
   DCHECK(!base::ContainsKey(client_map_, client_id));
 
-  Client* client = new Client(priority_requests_delayable_);
+  Client* client =
+      new Client(priority_requests_delayable_, yielding_scheduler_enabled_,
+                 max_requests_before_yielding_);
   client_map_[client_id] = client;
 }