[base] Check CPU affinity when counting CPUs

On Linux, use sched_getaffinity to count usable CPUs, so that runners of
Chromium can restrict cores with e.g. taskset.

This is observable locally by running

    taskset ./chrome

recording a trace with chrome://tracing, and checking the number of
foreground worker pool threads created.

Change-Id: I0402c1323a6e26a8fae6ce845e89a7c69d0acc25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270183
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785807}

base/system/sys_info_posix.cc

@@ -5,6 +5,7 @@
 #include "base/system/sys_info.h"
 
 #include <errno.h>
+#include <sched.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>
@@ -56,7 +57,21 @@ int NumberOfProcessors() {
     return 1;
   }
 
-  return static_cast<int>(res);
+  int num_cpus = static_cast<int>(res);
+
+#ifdef OS_LINUX
+  // Restrict the CPU count based on the process's CPU affinity mask, if
+  // available.
+  cpu_set_t* cpu_set = CPU_ALLOC(num_cpus);
+  size_t cpu_set_size = CPU_ALLOC_SIZE(num_cpus);
+  int ret = sched_getaffinity(0, cpu_set_size, cpu_set);
+  if (ret == 0) {
+    num_cpus = CPU_COUNT_S(cpu_set_size, cpu_set);
+  }
+  CPU_FREE(cpu_set);
+#endif  // OS_LINUX
+
+  return num_cpus;
 }
 
 base::LazyInstance<base::internal::LazySysInfoValue<int, NumberOfProcessors>>::