Subject: Re: INFO: rcu detected stall in ext4_write_checks

From: "Theodore Ts'o" <[email-masked]>
Date: Wed, 26 Jun 2019 18:47:09 -0400

More details about what is going on.  First, it requires root, because
one of that is required is using sched_setattr (which is enough to
shoot yourself in the foot):

sched_setattr(0, {size=0, sched_policy=0x6 /* SCHED_??? */, sched_flags=0, sched_nice=0, sched_priority=0, sched_runtime=2251799813724439, sched_deadline=4611686018427453437, sched_period=0}, 0) = 0

This is setting the scheduler policy to be SCHED_DEADLINE, with a
runtime parameter of 2251799.813724439 seconds (or 26 days) and a
deadline of 4611686018.427453437 seconds (or 146 *years*).  This means
a particular kernel thread can run for up to 26 **days** before it is
scheduled away, and if a kernel reads gets woken up or sent a signal,
no worries, it will wake up roughly seven times the interval that Rip
Van Winkle spent snoozing in a cave in the Catskill Mountains (in
Washington Irving's short story).

We then kick off a half-dozen threads all running:

   sendfile(fd, fd, &pos, 0x8080fffffffe);

(and since count is a ridiculously large number, this gets cut down to):

   sendfile(fd, fd, &pos, 2147479552);

Is it any wonder that we are seeing RCU stalls?   :-)

      	  	      	     	    - Ted