Merge tag 'core-rcu-2020-06-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull RCU updates from Ingo Molnar:
 "The RCU updates for this cycle were:

   - RCU-tasks update, including addition of RCU Tasks Trace for BPF use
     and TASKS_RUDE_RCU

   - kfree_rcu() updates.

   - Remove scheduler locking restriction

   - RCU CPU stall warning updates.

   - Torture-test updates.

   - Miscellaneous fixes and other updates"

* tag 'core-rcu-2020-06-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (103 commits)
  rcu: Allow for smp_call_function() running callbacks from idle
  rcu: Provide rcu_irq_exit_check_preempt()
  rcu: Abstract out rcu_irq_enter_check_tick() from rcu_nmi_enter()
  rcu: Provide __rcu_is_watching()
  rcu: Provide rcu_irq_exit_preempt()
  rcu: Make RCU IRQ enter/exit functions rely on in_nmi()
  rcu/tree: Mark the idle relevant functions noinstr
  x86: Replace ist_enter() with nmi_enter()
  x86/mce: Send #MC singal from task work
  x86/entry: Get rid of ist_begin/end_non_atomic()
  sched,rcu,tracing: Avoid tracing before in_nmi() is correct
  sh/ftrace: Move arch_ftrace_nmi_{enter,exit} into nmi exception
  lockdep: Always inline lockdep_{off,on}()
  hardirq/nmi: Allow nested nmi_enter()
  arm64: Prepare arch_nmi_enter() for recursion
  printk: Disallow instrumenting print_nmi_enter()
  printk: Prepare for nested printk_nmi_enter()
  rcutorture: Convert ULONG_CMP_LT() to time_before()
  torture: Add a --kasan argument
  torture: Save a few lines by using config_override_param initially
  ...

3 files changed, 35 insertions, 53 deletions

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index fd5e2cbc4935..75b8ca007a11 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -1943,56 +1943,27 @@ invoked from a CPU-hotplug notifier.
 Scheduler and RCU
 ~~~~~~~~~~~~~~~~~
 
-RCU depends on the scheduler, and the scheduler uses RCU to protect some
-of its data structures. The preemptible-RCU ``rcu_read_unlock()``
-implementation must therefore be written carefully to avoid deadlocks
-involving the scheduler's runqueue and priority-inheritance locks. In
-particular, ``rcu_read_unlock()`` must tolerate an interrupt where the
-interrupt handler invokes both ``rcu_read_lock()`` and
-``rcu_read_unlock()``. This possibility requires ``rcu_read_unlock()``
-to use negative nesting levels to avoid destructive recursion via
-interrupt handler's use of RCU.
-
-This scheduler-RCU requirement came as a `complete
-surprise <https://lwn.net/Articles/453002/>`__.
-
-As noted above, RCU makes use of kthreads, and it is necessary to avoid
-excessive CPU-time accumulation by these kthreads. This requirement was
-no surprise, but RCU's violation of it when running context-switch-heavy
-workloads when built with ``CONFIG_NO_HZ_FULL=y`` `did come as a
-surprise
+RCU makes use of kthreads, and it is necessary to avoid excessive CPU-time
+accumulation by these kthreads. This requirement was no surprise, but
+RCU's violation of it when running context-switch-heavy workloads when
+built with ``CONFIG_NO_HZ_FULL=y`` `did come as a surprise
 [PDF] <http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf>`__.
 RCU has made good progress towards meeting this requirement, even for
 context-switch-heavy ``CONFIG_NO_HZ_FULL=y`` workloads, but there is
 room for further improvement.
 
-It is forbidden to hold any of scheduler's runqueue or
-priority-inheritance spinlocks across an ``rcu_read_unlock()`` unless
-interrupts have been disabled across the entire RCU read-side critical
-section, that is, up to and including the matching ``rcu_read_lock()``.
-Violating this restriction can result in deadlocks involving these
-scheduler spinlocks. There was hope that this restriction might be
-lifted when interrupt-disabled calls to ``rcu_read_unlock()`` started
-deferring the reporting of the resulting RCU-preempt quiescent state
-until the end of the corresponding interrupts-disabled region.
-Unfortunately, timely reporting of the corresponding quiescent state to
-expedited grace periods requires a call to ``raise_softirq()``, which
-can acquire these scheduler spinlocks. In addition, real-time systems
-using RCU priority boosting need this restriction to remain in effect
-because deferred quiescent-state reporting would also defer deboosting,
-which in turn would degrade real-time latencies.
-
-In theory, if a given RCU read-side critical section could be guaranteed
-to be less than one second in duration, holding a scheduler spinlock
-across that critical section's ``rcu_read_unlock()`` would require only
-that preemption be disabled across the entire RCU read-side critical
-section, not interrupts. Unfortunately, given the possibility of vCPU
-preemption, long-running interrupts, and so on, it is not possible in
-practice to guarantee that a given RCU read-side critical section will
-complete in less than one second. Therefore, as noted above, if
-scheduler spinlocks are held across a given call to
-``rcu_read_unlock()``, interrupts must be disabled across the entire RCU
-read-side critical section.
+There is no longer any prohibition against holding any of
+scheduler's runqueue or priority-inheritance spinlocks across an
+``rcu_read_unlock()``, even if interrupts and preemption were enabled
+somewhere within the corresponding RCU read-side critical section.
+Therefore, it is now perfectly legal to execute ``rcu_read_lock()``
+with preemption enabled, acquire one of the scheduler locks, and hold
+that lock across the matching ``rcu_read_unlock()``.
+
+Similarly, the RCU flavor consolidation has removed the need for negative
+nesting.  The fact that interrupt-disabled regions of code act as RCU
+read-side critical sections implicitly avoids earlier issues that used
+to result in destructive recursion via interrupt handler's use of RCU.
 
 Tracing and RCU
 ~~~~~~~~~~~~~~~
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 7bc83f3d9bdf..46df119c931a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4210,12 +4210,24 @@
 			Duration of CPU stall (s) to test RCU CPU stall
 			warnings, zero to disable.
 
+	rcutorture.stall_cpu_block= [KNL]
+			Sleep while stalling if set.  This will result
+			in warnings from preemptible RCU in addition
+			to any other stall-related activity.
+
 	rcutorture.stall_cpu_holdoff= [KNL]
 			Time to wait (s) after boot before inducing stall.
 
 	rcutorture.stall_cpu_irqsoff= [KNL]
 			Disable interrupts while stalling if set.
 
+	rcutorture.stall_gp_kthread= [KNL]
+			Duration (s) of forced sleep within RCU
+			grace-period kthread to test RCU CPU stall
+			warnings, zero to disable.  If both stall_cpu
+			and stall_gp_kthread are specified, the
+			kthread is starved first, then the CPU.
+
 	rcutorture.stat_interval= [KNL]
 			Time (s) between statistics printk()s.
 
@@ -4286,6 +4298,13 @@
 			only normal grace-period primitives.  No effect
 			on CONFIG_TINY_RCU kernels.
 
+	rcupdate.rcu_task_ipi_delay= [KNL]
+			Set time in jiffies during which RCU tasks will
+			avoid sending IPIs, starting with the beginning
+			of a given grace period.  Setting a large
+			number avoids disturbing real-time workloads,
+			but lengthens grace periods.
+
 	rcupdate.rcu_task_stall_timeout= [KNL]
 			Set timeout in jiffies for RCU task stall warning
 			messages.  Disable with a value less than or equal
diff --git a/Documentation/trace/ftrace-design.rst b/Documentation/trace/ftrace-design.rst
index a8e22e0db63c..6893399157f0 100644
--- a/Documentation/trace/ftrace-design.rst
+++ b/Documentation/trace/ftrace-design.rst
@@ -229,14 +229,6 @@ Adding support for it is easy: just define the macro in asm/ftrace.h and
 pass the return address pointer as the 'retp' argument to
 ftrace_push_return_trace().
 
-HAVE_FTRACE_NMI_ENTER
----------------------
-
-If you can't trace NMI functions, then skip this option.
-
-<details to be filled>
-
-
 HAVE_SYSCALL_TRACEPOINTS
 ------------------------