sched/core: Enable increased load resolution on 64-bit kernels

Peter Zijlstra · Ingo Molnar · commit 2159197d6677 · 2016-05-05T09:24:00.000+02:00
Mike ran into the low load resolution limitation on his big machine.

So reenable these bits; nobody could ever reproduce/analyze the
reported power usage claim and Google has been running with this for
years as well.

Reported-by: Mike Galbraith &lt;umgwanakikbuti@gmail.com&gt;
Tested-by: Mike Galbraith &lt;umgwanakikbuti@gmail.com&gt;
Signed-off-by: Peter Zijlstra (Intel) &lt;peterz@infradead.org&gt;
Cc: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
Cc: Mike Galbraith &lt;efault@gmx.de&gt;
Cc: Peter Zijlstra &lt;peterz@infradead.org&gt;
Cc: Thomas Gleixner &lt;tglx@linutronix.de&gt;
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar &lt;mingo@kernel.org&gt;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
@@ -49,11 +49,13 @@ static inline void cpu_load_update_active(struct rq *this_rq) { }
  * and does not change the user-interface for setting shares/weights.
  *
  * We increase resolution only if we have enough bits to allow this increased
- * resolution (i.e. BITS_PER_LONG > 32). The costs for increasing resolution
- * when BITS_PER_LONG <= 32 are pretty high and the returns do not justify the
- * increased costs.
+ * resolution (i.e. 64bit). The costs for increasing resolution when 32bit are
+ * pretty high and the returns do not justify the increased costs.
+ *
+ * Really only required when CONFIG_FAIR_GROUP_SCHED is also set, but to
+ * increase coverage and consistency always enable it on 64bit platforms.
  */
-#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power usage under light load  */
+#ifdef CONFIG_64BIT
 # define SCHED_LOAD_RESOLUTION	10
 # define scale_load(w)		((w) << SCHED_LOAD_RESOLUTION)
 # define scale_load_down(w)	((w) >> SCHED_LOAD_RESOLUTION)
