psi: clarify the Kconfig text for the default-disable option

The current help text caused some confusion in online forums about
whether or not to default-enable or default-disable psi in vendor
kernels.  This is because it doesn't communicate the reason for why we
made this setting configurable in the first place: that the overhead is
non-zero in an artificial scheduler stress test.

Since this isn't representative of real workloads, and the effect was
not measurable in scheduler-heavy real world applications such as the
webservers and memcache installations at Facebook, it's fair to point
out that this is a pretty cautious option to select.

Link: http://lkml.kernel.org/r/20190129233617.16767-1-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -512,6 +512,17 @@ config PSI_DEFAULT_DISABLED
 	  per default but can be enabled through passing psi=1 on the
 	  kernel commandline during boot.
 
+	  This feature adds some code to the task wakeup and sleep
+	  paths of the scheduler. The overhead is too low to affect
+	  common scheduling-intense workloads in practice (such as
+	  webservers, memcache), but it does show up in artificial
+	  scheduler stress tests, such as hackbench.
+
+	  If you are paranoid and not sure what the kernel will be
+	  used for, say Y.
+
+	  Say N if unsure.
+
 endmenu # "CPU/Task time and stats accounting"
 
 config CPU_ISOLATION