Auto merge of #2207 - RalfJung:ui_test_parallelism, r=oli-obk

ui_test: ensure all worker threads stay around

Also organize files such that the by far slowest test (weak_memory/consistency) always starts first. It still finishes last on my system... even after I halved the iteration count.

Fixes https://github.com/rust-lang/miri/issues/2204

tests/pass/0weak_memory_consistency.rs

@@ -214,7 +214,7 @@ fn test_single_thread() {
 }
 
 pub fn main() {
-    for _ in 0..100 {
+    for _ in 0..50 {
         test_single_thread();
         test_mixed_access();
         test_load_buffering_acq_rel();

ui_test/README.md

@@ -1,5 +1,10 @@
 A smaller version of compiletest-rs
 
+## Magic behavior
+
+* Tests are run in order of their filenames (files first, then recursing into folders).
+  So if you have any slow tests, prepend them with a small integral number to make them get run first, taking advantage of parallelism as much as possible (instead of waiting for the slow tests at the end).
+
 ## Supported magic comment annotations
 
 Note that the space after `//`, when it is present, is *not* optional -- it must be exactly one.

ui_test/src/lib.rs

@@ -1,3 +1,4 @@
+use std::collections::VecDeque;
 use std::fmt::Write;
 use std::path::{Path, PathBuf};
 use std::process::{Command, ExitStatus};
@@ -6,7 +7,6 @@
 
 use colored::*;
 use comments::ErrorMatch;
-use crossbeam::queue::SegQueue;
 use regex::Regex;
 use rustc_stderr::{Level, Message};
 
@@ -55,9 +55,8 @@ pub fn run_tests(config: Config) {
     // Get the triple with which to run the tests
     let target = config.target.clone().unwrap_or_else(|| config.get_host());
 
-    // A queue for files or folders to process
-    let todo = SegQueue::new();
-    todo.push(config.root_dir.clone());
+    // A channel for files to process
+    let (submit, receive) = crossbeam::channel::unbounded();
 
     // Some statistics and failure reports.
     let failures = Mutex::new(vec![]);
@@ -66,20 +65,35 @@ pub fn run_tests(config: Config) {
     let filtered = AtomicUsize::default();
 
     crossbeam::scope(|s| {
+        // Create a thread that is in charge of walking the directory and submitting jobs.
+        // It closes the channel when it is done.
+        s.spawn(|_| {
+            let mut todo = VecDeque::new();
+            todo.push_back(config.root_dir.clone());
+            while let Some(path) = todo.pop_front() {
+                if path.is_dir() {
+                    // Enqueue everything inside this directory.
+                    // We want it sorted, to have some control over scheduling of slow tests.
+                    let mut entries =
+                        std::fs::read_dir(path).unwrap().collect::<Result<Vec<_>, _>>().unwrap();
+                    entries.sort_by_key(|e| e.file_name());
+                    for entry in entries {
+                        todo.push_back(entry.path());
+                    }
+                } else if path.extension().map(|ext| ext == "rs").unwrap_or(false) {
+                    // Forward .rs files to the test workers.
+                    submit.send(path).unwrap();
+                }
+            }
+            // There will be no more jobs. This signals the workers to quit.
+            // (This also ensures `submit` is moved into this closure.)
+            drop(submit);
+        });
+
+        // Create N worker threads that receive files to test.
         for _ in 0..std::thread::available_parallelism().unwrap().get() {
             s.spawn(|_| {
-                while let Some(path) = todo.pop() {
-                    // Collect everything inside directories
-                    if path.is_dir() {
-                        for entry in std::fs::read_dir(path).unwrap() {
-                            todo.push(entry.unwrap().path());
-                        }
-                        continue;
-                    }
-                    // Only look at .rs files
-                    if !path.extension().map(|ext| ext == "rs").unwrap_or(false) {
-                        continue;
-                    }
+                for path in &receive {
                     if !config.path_filter.is_empty() {
                         let path_display = path.display().to_string();
                         if !config.path_filter.iter().any(|filter| path_display.contains(filter)) {