Re: [PATCH] Make rcutorture more vicious: reinstate boot-time testing

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> Hello again!
> 
> This patch re-institutes the ability to build rcutorture directly into 
> the Linux kernel.  The reason that this capability was removed was 
> that this could result in your kernel being pretty much useless, as 
> rcutorture would be running starting from early boot.  This problem 
> has been avoided by (1) making rcutorture run only three seconds of 
> every six by default, (2) adding a CONFIG_RCU_TORTURE_TEST_RUNNABLE 
> that permits rcutorture to be quiesced at boot time, and (3) adding a 
> sysctl in /proc named /proc/sys/kernel/rcutorture_runnable that 
> permits rcutorture to be quiesced and unquiesced when built into the 
> kernel.
> 
> Please note that this /proc file is -not- available when rcutorture is 
> built as a module.  Please also note that to get the earlier 
> take-no-prisoners behavior, you must use the boot command line to set 
> rcutorture's "stutter" parameter to zero.
> 
> The rcutorture quiescing mechanism is currently quite crude: loops in 
> each rcutorture process that poll a global variable once per tick. 
> Suggestions for improvement are welcome.  The default action will be 
> to reduce the polling rate to a few times per second.

applied to tip/core/rcu - thanks Paul!

FYI, yesterday's rcutorture-stutter feature survived a few hundred 
random bootup tests in -tip testing already.

a possible area for enhancement would be the following code:

static void
rcu_stutter_wait(void)
{
        while (stutter_pause_test || !rcutorture_runnable)
                schedule_timeout_interruptible(1);
}

will cause HZ number of IRQs on nohz systems, even if rcutorture is 
disabled. While it's fine to poll if rcutorture_runnable==1, the sleep 
should be at least 1 second when !rcutorture_runnable.

[ Or it might even make sense to completely stop the threads from a 
  sysctl handler - to make it event-driven and to cause no extra IRQs at 
  all when rcutorture is disabled via /proc. ]

	Ingo