base/process: Use /proc/self/stat to read startup time.

base/process/internal_linux.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file contains internal routines that are called by other files in
 // base/process/.
 
 #ifndef BASE_PROCESS_INTERNAL_LINUX_H_
 #define BASE_PROCESS_INTERNAL_LINUX_H_
 
@@ skipping 54 matching lines @@
 // Reads the |field_num|th field from |proc_stats|. Returns 0 on failure.
 // This version does not handle the first 3 values, since the first value is
 // simply |pid|, and the next two values are strings.
 int64_t GetProcStatsFieldAsInt64(const std::vector<std::string>& proc_stats,
                                  ProcStatsFields field_num);
 
 // Same as GetProcStatsFieldAsInt64(), but for size_t values.
 size_t GetProcStatsFieldAsSizeT(const std::vector<std::string>& proc_stats,
                                 ProcStatsFields field_num);
 
-// Convenience wrapper around GetProcStatsFieldAsInt64(), ParseProcStats() and
+// Convenience wrappers around GetProcStatsFieldAsInt64(), ParseProcStats() and
 // ReadProcStats(). See GetProcStatsFieldAsInt64() for details.
+int64_t ReadProcStatsAndGetFieldAsInt64(const std::string& pid,

[jln (very slow on Chromium), 2017/02/22 22:37:15]

Nit: I think we still don't allow overloading like this. Also it seems weird to
take pid as a string.

Let's just have a ReadProcSelfStatsAndGet...() instead ?

[Daniele Castagna, 2017/02/23 18:04:10]

Overloading should be fine as long as it's not confusing from the caller POV.
I like this suggestion though. Done.

+                                        ProcStatsFields field_num);
 int64_t ReadProcStatsAndGetFieldAsInt64(pid_t pid, ProcStatsFields field_num);
 
 // Same as ReadProcStatsAndGetFieldAsInt64() but for size_t values.
 size_t ReadProcStatsAndGetFieldAsSizeT(pid_t pid,
                                        ProcStatsFields field_num);
 
 // Returns the time that the OS started. Clock ticks are relative to this.
 Time GetBootTime();
 
 // Returns the amount of time spent in user space since boot across all CPUs.
 TimeDelta GetUserCpuTimeSinceBoot();
 
 // Converts Linux clock ticks to a wall time delta.
 TimeDelta ClockTicksToTimeDelta(int clock_ticks);
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_PROCESS_INTERNAL_LINUX_H_