Merge tag 'rcu.2022.07.26a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu

Pull RCU updates from Paul McKenney:

 - Documentation updates

 - Miscellaneous fixes

 - Callback-offload updates, perhaps most notably a new
   RCU_NOCB_CPU_DEFAULT_ALL Kconfig option that causes all CPUs to be
   offloaded at boot time, regardless of kernel boot parameters.

   This is useful to battery-powered systems such as ChromeOS and
   Android. In addition, a new RCU_NOCB_CPU_CB_BOOST kernel boot
   parameter prevents offloaded callbacks from interfering with
   real-time workloads and with energy-efficiency mechanisms

 - Polled grace-period updates, perhaps most notably making these APIs
   account for both normal and expedited grace periods

 - Tasks RCU updates, perhaps most notably reducing the CPU overhead of
   RCU tasks trace grace periods by more than a factor of two on a
   system with 15,000 tasks.

   The reduction is expected to increase with the number of tasks, so it
   seems reasonable to hypothesize that a system with 150,000 tasks
   might see a 20-fold reduction in CPU overhead

 - Torture-test updates

 - Updates that merge RCU's dyntick-idle tracking into context tracking,
   thus reducing the overhead of transitioning to kernel mode from
   either idle or nohz_full userspace execution for kernels that track
   context independently of RCU.

   This is expected to be helpful primarily for kernels built with
   CONFIG_NO_HZ_FULL=y

* tag 'rcu.2022.07.26a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (98 commits)
  rcu: Add irqs-disabled indicator to expedited RCU CPU stall warnings
  rcu: Diagnose extended sync_rcu_do_polled_gp() loops
  rcu: Put panic_on_rcu_stall() after expedited RCU CPU stall warnings
  rcutorture: Test polled expedited grace-period primitives
  rcu: Add polled expedited grace-period primitives
  rcutorture: Verify that polled GP API sees synchronous grace periods
  rcu: Make Tiny RCU grace periods visible to polled APIs
  rcu: Make polled grace-period API account for expedited grace periods
  rcu: Switch polled grace-period APIs to ->gp_seq_polled
  rcu/nocb: Avoid polling when my_rdp->nocb_head_rdp list is empty
  rcu/nocb: Add option to opt rcuo kthreads out of RT priority
  rcu: Add nocb_cb_kthread check to rcu_is_callbacks_kthread()
  rcu/nocb: Add an option to offload all CPUs on boot
  rcu/nocb: Fix NOCB kthreads spawn failure with rcu_nocb_rdp_deoffload() direct call
  rcu/nocb: Invert rcu_state.barrier_mutex VS hotplug lock locking order
  rcu/nocb: Add/del rdp to iterate from rcuog itself
  rcu/tree: Add comment to describe GP-done condition in fqs loop
  rcu: Initialize first_gp_fqs at declaration in rcu_gp_fqs()
  rcu/kvfree: Remove useless monitor_todo flag
  rcu: Cleanup RCU urgency state for offline CPU
  ...

Author: torvalds

Documentation/RCU/Design/Requirements/Requirements.rst

@@ -1844,10 +1844,10 @@ that meets this requirement.
 
 Furthermore, NMI handlers can be interrupted by what appear to RCU to be
 normal interrupts. One way that this can happen is for code that
-directly invokes rcu_irq_enter() and rcu_irq_exit() to be called
+directly invokes ct_irq_enter() and ct_irq_exit() to be called
 from an NMI handler. This astonishing fact of life prompted the current
-code structure, which has rcu_irq_enter() invoking
-rcu_nmi_enter() and rcu_irq_exit() invoking rcu_nmi_exit().
+code structure, which has ct_irq_enter() invoking
+ct_nmi_enter() and ct_irq_exit() invoking ct_nmi_exit().
 And yes, I also learned of this requirement the hard way.
 
 Loadable Modules
@@ -2195,7 +2195,7 @@ scheduling-clock interrupt be enabled when RCU needs it to be:
    sections, and RCU believes this CPU to be idle, no problem. This
    sort of thing is used by some architectures for light-weight
    exception handlers, which can then avoid the overhead of
-   rcu_irq_enter() and rcu_irq_exit() at exception entry and
+   ct_irq_enter() and ct_irq_exit() at exception entry and
    exit, respectively. Some go further and avoid the entireties of
    irq_enter() and irq_exit().
    Just make very sure you are running some of your tests with
@@ -2226,7 +2226,7 @@ scheduling-clock interrupt be enabled when RCU needs it to be:
 +-----------------------------------------------------------------------+
 | **Answer**:                                                           |
 +-----------------------------------------------------------------------+
-| One approach is to do ``rcu_irq_exit();rcu_irq_enter();`` every so    |
+| One approach is to do ``ct_irq_exit();ct_irq_enter();`` every so      |
 | often. But given that long-running interrupt handlers can cause other |
 | problems, not least for response time, shouldn't you work to keep     |
 | your interrupt handler's runtime within reasonable bounds?            |

Documentation/RCU/stallwarn.rst

@@ -97,12 +97,12 @@ warnings:
 	which will include additional debugging information.
 
 -	A low-level kernel issue that either fails to invoke one of the
-	variants of rcu_user_enter(), rcu_user_exit(), rcu_idle_enter(),
-	rcu_idle_exit(), rcu_irq_enter(), or rcu_irq_exit() on the one
+	variants of rcu_eqs_enter(true), rcu_eqs_exit(true), ct_idle_enter(),
+	ct_idle_exit(), ct_irq_enter(), or ct_irq_exit() on the one
 	hand, or that invokes one of them too many times on the other.
 	Historically, the most frequent issue has been an omission
 	of either irq_enter() or irq_exit(), which in turn invoke
-	rcu_irq_enter() or rcu_irq_exit(), respectively.  Building your
+	ct_irq_enter() or ct_irq_exit(), respectively.  Building your
 	kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types
 	of issues, which sometimes arise in architecture-specific code.
 

Documentation/admin-guide/kernel-parameters.txt

@@ -3667,6 +3667,9 @@
 			just as if they had also been called out in the
 			rcu_nocbs= boot parameter.
 
+			Note that this argument takes precedence over
+			the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
+
 	noiotrap	[SH] Disables trapped I/O port accesses.
 
 	noirqdebug	[X86-32] Disables the code which attempts to detect and
@@ -4560,6 +4563,9 @@
 			no-callback mode from boot but the mode may be
 			toggled at runtime via cpusets.
 
+			Note that this argument takes precedence over
+			the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
+
 	rcu_nocb_poll	[KNL]
 			Rather than requiring that offloaded CPUs
 			(specified by rcu_nocbs= above) explicitly
@@ -4669,6 +4675,34 @@
 			When RCU_NOCB_CPU is set, also adjust the
 			priority of NOCB callback kthreads.
 
+	rcutree.rcu_divisor= [KNL]
+			Set the shift-right count to use to compute
+			the callback-invocation batch limit bl from
+			the number of callbacks queued on this CPU.
+			The result will be bounded below by the value of
+			the rcutree.blimit kernel parameter.  Every bl
+			callbacks, the softirq handler will exit in
+			order to allow the CPU to do other work.
+
+			Please note that this callback-invocation batch
+			limit applies only to non-offloaded callback
+			invocation.  Offloaded callbacks are instead
+			invoked in the context of an rcuoc kthread, which
+			scheduler will preempt as it does any other task.
+
+	rcutree.nocb_nobypass_lim_per_jiffy= [KNL]
+			On callback-offloaded (rcu_nocbs) CPUs,
+			RCU reduces the lock contention that would
+			otherwise be caused by callback floods through
+			use of the ->nocb_bypass list.	However, in the
+			common non-flooded case, RCU queues directly to
+			the main ->cblist in order to avoid the extra
+			overhead of the ->nocb_bypass list and its lock.
+			But if there are too many callbacks queued during
+			a single jiffy, RCU pre-queues the callbacks into
+			the ->nocb_bypass queue.  The definition of "too
+			many" is supplied by this kernel boot parameter.
+
 	rcutree.rcu_nocb_gp_stride= [KNL]
 			Set the number of NOCB callback kthreads in
 			each group, which defaults to the square root

Documentation/features/time/context-tracking/arch-support.txt

@@ -1,7 +1,7 @@
 #
-# Feature name:          context-tracking
-#         Kconfig:       HAVE_CONTEXT_TRACKING
-#         description:   arch supports context tracking for NO_HZ_FULL
+# Feature name:          user-context-tracking
+#         Kconfig:       HAVE_CONTEXT_TRACKING_USER
+#         description:   arch supports user context tracking for NO_HZ_FULL
 #
     -----------------------
     |         arch |status|

MAINTAINERS

@@ -5165,6 +5165,7 @@ F:	include/linux/console*
 
 CONTEXT TRACKING
 M:	Frederic Weisbecker <frederic@kernel.org>
+M:	"Paul E. McKenney" <paulmck@kernel.org>
 S:	Maintained
 F:	kernel/context_tracking.c
 F:	include/linux/context_tracking*

arch/Kconfig

@@ -784,18 +784,18 @@ config HAVE_ARCH_WITHIN_STACK_FRAMES
 	  and similar) by implementing an inline arch_within_stack_frames(),
 	  which is used by CONFIG_HARDENED_USERCOPY.
 
-config HAVE_CONTEXT_TRACKING
+config HAVE_CONTEXT_TRACKING_USER
 	bool
 	help
 	  Provide kernel/user boundaries probes necessary for subsystems
 	  that need it, such as userspace RCU extended quiescent state.
 	  Syscalls need to be wrapped inside user_exit()-user_enter(), either
 	  optimized behind static key or through the slow path using TIF_NOHZ
 	  flag. Exceptions handlers must be wrapped as well. Irqs are already
-	  protected inside rcu_irq_enter/rcu_irq_exit() but preemption or signal
+	  protected inside ct_irq_enter/ct_irq_exit() but preemption or signal
 	  handling on irq exit still need to be protected.
 
-config HAVE_CONTEXT_TRACKING_OFFSTACK
+config HAVE_CONTEXT_TRACKING_USER_OFFSTACK
 	bool
 	help
 	  Architecture neither relies on exception_enter()/exception_exit()
@@ -807,7 +807,7 @@ config HAVE_CONTEXT_TRACKING_OFFSTACK
 
 	  - Critical entry code isn't preemptible (or better yet:
 	    not interruptible).
-	  - No use of RCU read side critical sections, unless rcu_nmi_enter()
+	  - No use of RCU read side critical sections, unless ct_nmi_enter()
 	    got called.
 	  - No use of instrumentation, unless instrumentation_begin() got
 	    called.

arch/arm/Kconfig

@@ -84,7 +84,7 @@ config ARM
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARM_LPAE
 	select HAVE_ARM_SMCCC if CPU_V7
 	select HAVE_EBPF_JIT if !CPU_ENDIAN_BE32
-	select HAVE_CONTEXT_TRACKING
+	select HAVE_CONTEXT_TRACKING_USER
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_BUILDTIME_MCOUNT_SORT
 	select HAVE_DEBUG_KMEMLEAK if !XIP_KERNEL

arch/arm/kernel/entry-common.S

@@ -28,7 +28,7 @@
 #include "entry-header.S"
 
 saved_psr	.req	r8
-#if defined(CONFIG_TRACE_IRQFLAGS) || defined(CONFIG_CONTEXT_TRACKING)
+#if defined(CONFIG_TRACE_IRQFLAGS) || defined(CONFIG_CONTEXT_TRACKING_USER)
 saved_pc	.req	r9
 #define TRACE(x...) x
 #else
@@ -38,7 +38,7 @@ saved_pc	.req	lr
 
 	.section .entry.text,"ax",%progbits
 	.align	5
-#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING) || \
+#if !(IS_ENABLED(CONFIG_TRACE_IRQFLAGS) || IS_ENABLED(CONFIG_CONTEXT_TRACKING_USER) || \
 	IS_ENABLED(CONFIG_DEBUG_RSEQ))
 /*
  * This is the fast syscall return path.  We do as little as possible here,

arch/arm/kernel/entry-header.S

@@ -366,25 +366,25 @@ ALT_UP_B(.L1_\@)
  * between user and kernel mode.
  */
 	.macro ct_user_exit, save = 1
-#ifdef CONFIG_CONTEXT_TRACKING
+#ifdef CONFIG_CONTEXT_TRACKING_USER
 	.if	\save
 	stmdb   sp!, {r0-r3, ip, lr}
-	bl	context_tracking_user_exit
+	bl	user_exit_callable
 	ldmia	sp!, {r0-r3, ip, lr}
 	.else
-	bl	context_tracking_user_exit
+	bl	user_exit_callable
 	.endif
 #endif
 	.endm
 
 	.macro ct_user_enter, save = 1
-#ifdef CONFIG_CONTEXT_TRACKING
+#ifdef CONFIG_CONTEXT_TRACKING_USER
 	.if	\save
 	stmdb   sp!, {r0-r3, ip, lr}
-	bl	context_tracking_user_enter
+	bl	user_enter_callable
 	ldmia	sp!, {r0-r3, ip, lr}
 	.else
-	bl	context_tracking_user_enter
+	bl	user_enter_callable
 	.endif
 #endif
 	.endm

arch/arm/mach-imx/cpuidle-imx6q.c

@@ -3,6 +3,7 @@
  * Copyright (C) 2012 Freescale Semiconductor, Inc.
  */
 
+#include <linux/context_tracking.h>
 #include <linux/cpuidle.h>
 #include <linux/module.h>
 #include <asm/cpuidle.h>
@@ -24,9 +25,9 @@ static int imx6q_enter_wait(struct cpuidle_device *dev,
 		imx6_set_lpm(WAIT_UNCLOCKED);
 	raw_spin_unlock(&cpuidle_lock);
 
-	rcu_idle_enter();
+	ct_idle_enter();
 	cpu_do_idle();
-	rcu_idle_exit();
+	ct_idle_exit();
 
 	raw_spin_lock(&cpuidle_lock);
 	if (num_idle_cpus-- == num_online_cpus())