* [PATCH rcu 3/4] rcu: Remove the RCU_FAST_NO_HZ Kconfig option
2021-12-02 0:18 [PATCH rcu 0/4] CONFIG_RCU_FAST_NO_HZ removal for v5.17 Paul E. McKenney
2021-12-02 0:19 ` [PATCH rcu 1/4] torture: Remove RCU_FAST_NO_HZ from rcuscale and refscale scenarios Paul E. McKenney
2021-12-02 0:19 ` [PATCH rcu 2/4] torture: Remove RCU_FAST_NO_HZ from rcu scenarios Paul E. McKenney
@ 2021-12-02 0:19 ` Paul E. McKenney
2021-12-02 0:19 ` [PATCH rcu 4/4] rcu: Move rcu_needs_cpu() to tree.c Paul E. McKenney
3 siblings, 0 replies; 5+ messages in thread
From: Paul E. McKenney @ 2021-12-02 0:19 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, kernel-team, mingo, jiangshanlai, akpm,
mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, Paul E. McKenney
All of the uses of CONFIG_RCU_FAST_NO_HZ=y that I have seen involve
systems with RCU callbacks offloaded. In this situation, all that this
Kconfig option does is slow down idle entry/exit with an additional
allways-taken early exit. If this is the only use case, then this
Kconfig option nothing but an attractive nuisance that needs to go away.
This commit therefore removes the RCU_FAST_NO_HZ Kconfig option.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
Documentation/RCU/stallwarn.rst | 11 --
.../admin-guide/kernel-parameters.txt | 4 -
Documentation/timers/no_hz.rst | 10 +-
kernel/rcu/Kconfig | 18 --
kernel/rcu/tree.c | 11 --
kernel/rcu/tree.h | 7 -
kernel/rcu/tree_plugin.h | 185 +-----------------
kernel/rcu/tree_stall.h | 27 +--
.../selftests/rcutorture/configs/rcu/TREE01 | 1 -
.../selftests/rcutorture/configs/rcu/TREE04 | 1 -
.../rcutorture/doc/TREE_RCU-kconfig.txt | 1 -
11 files changed, 7 insertions(+), 269 deletions(-)
diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
index 28f8ad16db250..78404625bad26 100644
--- a/Documentation/RCU/stallwarn.rst
+++ b/Documentation/RCU/stallwarn.rst
@@ -254,17 +254,6 @@ period (in this case 2603), the grace-period sequence number (7075), and
an estimate of the total number of RCU callbacks queued across all CPUs
(625 in this case).
-In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
-for each CPU::
-
- 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1
-
-The "last_accelerate:" prints the low-order 16 bits (in hex) of the
-jiffies counter when this CPU last invoked rcu_try_advance_all_cbs()
-from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from
-rcu_prepare_for_idle(). "dyntick_enabled: 1" indicates that dyntick-idle
-processing is enabled.
-
If the grace period ends just as the stall warning starts printing,
there will be a spurious stall-warning message, which will include
the following::
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9725c546a0d46..36b83a47e54a6 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4489,10 +4489,6 @@
on rcutree.qhimark at boot time and to zero to
disable more aggressive help enlistment.
- rcutree.rcu_idle_gp_delay= [KNL]
- Set wakeup interval for idle CPUs that have
- RCU callbacks (RCU_FAST_NO_HZ=y).
-
rcutree.rcu_kick_kthreads= [KNL]
Cause the grace-period kthread to get an extra
wake_up() if it sleeps three times longer than
diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
index 20ad23a6c618e..f8786be15183c 100644
--- a/Documentation/timers/no_hz.rst
+++ b/Documentation/timers/no_hz.rst
@@ -184,16 +184,12 @@ There are situations in which idle CPUs cannot be permitted to
enter either dyntick-idle mode or adaptive-tick mode, the most
common being when that CPU has RCU callbacks pending.
-The CONFIG_RCU_FAST_NO_HZ=y Kconfig option may be used to cause such CPUs
-to enter dyntick-idle mode or adaptive-tick mode anyway. In this case,
-a timer will awaken these CPUs every four jiffies in order to ensure
-that the RCU callbacks are processed in a timely fashion.
-
-Another approach is to offload RCU callback processing to "rcuo" kthreads
+Avoid this by offloading RCU callback processing to "rcuo" kthreads
using the CONFIG_RCU_NOCB_CPU=y Kconfig option. The specific CPUs to
offload may be selected using The "rcu_nocbs=" kernel boot parameter,
which takes a comma-separated list of CPUs and CPU ranges, for example,
-"1,3-5" selects CPUs 1, 3, 4, and 5.
+"1,3-5" selects CPUs 1, 3, 4, and 5. Note that CPUs specified by
+the "nohz_full" kernel boot parameter are also offloaded.
The offloaded CPUs will never queue RCU callbacks, and therefore RCU
never prevents offloaded CPUs from entering either dyntick-idle mode
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 3128b7cf8e1fd..4005f2728f1a6 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -169,24 +169,6 @@ config RCU_FANOUT_LEAF
Take the default if unsure.
-config RCU_FAST_NO_HZ
- bool "Accelerate last non-dyntick-idle CPU's grace periods"
- depends on NO_HZ_COMMON && SMP && RCU_EXPERT
- default n
- help
- This option permits CPUs to enter dynticks-idle state even if
- they have RCU callbacks queued, and prevents RCU from waking
- these CPUs up more than roughly once every four jiffies (by
- default, you can adjust this using the rcutree.rcu_idle_gp_delay
- parameter), thus improving energy efficiency. On the other
- hand, this option increases the duration of RCU grace periods,
- for example, slowing down synchronize_rcu().
-
- Say Y if energy efficiency is critically important, and you
- don't care about increased grace-period durations.
-
- Say N if you are unsure.
-
config RCU_BOOST
bool "Enable RCU priority boosting"
depends on (RT_MUTEXES && PREEMPT_RCU && RCU_EXPERT) || PREEMPT_RT
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ef8d36f580fc3..5e98257c2910e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -624,7 +624,6 @@ static noinstr void rcu_eqs_enter(bool user)
instrumentation_begin();
trace_rcu_dyntick(TPS("Start"), rdp->dynticks_nesting, 0, atomic_read(&rdp->dynticks));
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
- rcu_prepare_for_idle();
rcu_preempt_deferred_qs(current);
// instrumentation for the noinstr rcu_dynticks_eqs_enter()
@@ -768,9 +767,6 @@ noinstr void rcu_nmi_exit(void)
trace_rcu_dyntick(TPS("Startirq"), rdp->dynticks_nmi_nesting, 0, atomic_read(&rdp->dynticks));
WRITE_ONCE(rdp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
- if (!in_nmi())
- rcu_prepare_for_idle();
-
// instrumentation for the noinstr rcu_dynticks_eqs_enter()
instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
instrumentation_end();
@@ -872,7 +868,6 @@ static void noinstr rcu_eqs_exit(bool user)
// instrumentation for the noinstr rcu_dynticks_eqs_exit()
instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks));
- rcu_cleanup_after_idle();
trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
WRITE_ONCE(rdp->dynticks_nesting, 1);
@@ -1014,12 +1009,6 @@ noinstr void rcu_nmi_enter(void)
rcu_dynticks_eqs_exit();
// ... but is watching here.
- if (!in_nmi()) {
- instrumentation_begin();
- rcu_cleanup_after_idle();
- instrumentation_end();
- }
-
instrumentation_begin();
// instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
instrument_atomic_read(&rdp->dynticks, sizeof(rdp->dynticks));
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 305cf6aeb4086..7ca1aa46083cc 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -189,11 +189,6 @@ struct rcu_data {
bool rcu_urgent_qs; /* GP old need light quiescent state. */
bool rcu_forced_tick; /* Forced tick to provide QS. */
bool rcu_forced_tick_exp; /* ... provide QS to expedited GP. */
-#ifdef CONFIG_RCU_FAST_NO_HZ
- unsigned long last_accelerate; /* Last jiffy CBs were accelerated. */
- unsigned long last_advance_all; /* Last jiffy CBs were all advanced. */
- int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
-#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
/* 4) rcu_barrier(), OOM callbacks, and expediting. */
struct rcu_head barrier_head;
@@ -419,8 +414,6 @@ static bool rcu_is_callbacks_kthread(void);
static void rcu_cpu_kthread_setup(unsigned int cpu);
static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp);
static void __init rcu_spawn_boost_kthreads(void);
-static void rcu_cleanup_after_idle(void);
-static void rcu_prepare_for_idle(void);
static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
static bool rcu_preempt_need_deferred_qs(struct task_struct *t);
static void rcu_preempt_deferred_qs(struct task_struct *t);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 5199559fbbf0c..19f7d578cedb3 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -51,8 +51,6 @@ static void __init rcu_bootup_announce_oddness(void)
RCU_FANOUT);
if (rcu_fanout_exact)
pr_info("\tHierarchical RCU autobalancing is disabled.\n");
- if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))
- pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
if (IS_ENABLED(CONFIG_PROVE_RCU))
pr_info("\tRCU lockdep checking is enabled.\n");
if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
@@ -1253,16 +1251,14 @@ static void __init rcu_spawn_boost_kthreads(void)
#endif /* #else #ifdef CONFIG_RCU_BOOST */
-#if !defined(CONFIG_RCU_FAST_NO_HZ)
-
/*
* Check to see if any future non-offloaded RCU-related work will need
* to be done by the current CPU, even if none need be done immediately,
* returning 1 if so. This function is part of the RCU implementation;
* it is -not- an exported member of the RCU API.
*
- * Because we not have RCU_FAST_NO_HZ, just check whether or not this
- * CPU has RCU callbacks queued.
+ * Just check whether or not this CPU has non-offloaded RCU callbacks
+ * queued.
*/
int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
@@ -1271,183 +1267,6 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
!rcu_rdp_is_offloaded(this_cpu_ptr(&rcu_data));
}
-/*
- * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
- * after it.
- */
-static void rcu_cleanup_after_idle(void)
-{
-}
-
-/*
- * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
- * is nothing.
- */
-static void rcu_prepare_for_idle(void)
-{
-}
-
-#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
-
-/*
- * This code is invoked when a CPU goes idle, at which point we want
- * to have the CPU do everything required for RCU so that it can enter
- * the energy-efficient dyntick-idle mode.
- *
- * The following preprocessor symbol controls this:
- *
- * RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
- * to sleep in dyntick-idle mode with RCU callbacks pending. This
- * is sized to be roughly one RCU grace period. Those energy-efficiency
- * benchmarkers who might otherwise be tempted to set this to a large
- * number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
- * system. And if you are -that- concerned about energy efficiency,
- * just power the system down and be done with it!
- *
- * The value below works well in practice. If future workloads require
- * adjustment, they can be converted into kernel config parameters, though
- * making the state machine smarter might be a better option.
- */
-#define RCU_IDLE_GP_DELAY 4 /* Roughly one grace period. */
-
-static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
-module_param(rcu_idle_gp_delay, int, 0644);
-
-/*
- * Try to advance callbacks on the current CPU, but only if it has been
- * awhile since the last time we did so. Afterwards, if there are any
- * callbacks ready for immediate invocation, return true.
- */
-static bool __maybe_unused rcu_try_advance_all_cbs(void)
-{
- bool cbs_ready = false;
- struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
- struct rcu_node *rnp;
-
- /* Exit early if we advanced recently. */
- if (jiffies == rdp->last_advance_all)
- return false;
- rdp->last_advance_all = jiffies;
-
- rnp = rdp->mynode;
-
- /*
- * Don't bother checking unless a grace period has
- * completed since we last checked and there are
- * callbacks not yet ready to invoke.
- */
- if ((rcu_seq_completed_gp(rdp->gp_seq,
- rcu_seq_current(&rnp->gp_seq)) ||
- unlikely(READ_ONCE(rdp->gpwrap))) &&
- rcu_segcblist_pend_cbs(&rdp->cblist))
- note_gp_changes(rdp);
-
- if (rcu_segcblist_ready_cbs(&rdp->cblist))
- cbs_ready = true;
- return cbs_ready;
-}
-
-/*
- * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
- * to invoke. If the CPU has callbacks, try to advance them. Tell the
- * caller about what to set the timeout.
- *
- * The caller must have disabled interrupts.
- */
-int rcu_needs_cpu(u64 basemono, u64 *nextevt)
-{
- struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
- unsigned long dj;
-
- lockdep_assert_irqs_disabled();
-
- /* If no non-offloaded callbacks, RCU doesn't need the CPU. */
- if (rcu_segcblist_empty(&rdp->cblist) ||
- rcu_rdp_is_offloaded(rdp)) {
- *nextevt = KTIME_MAX;
- return 0;
- }
-
- /* Attempt to advance callbacks. */
- if (rcu_try_advance_all_cbs()) {
- /* Some ready to invoke, so initiate later invocation. */
- invoke_rcu_core();
- return 1;
- }
- rdp->last_accelerate = jiffies;
-
- /* Request timer and round. */
- dj = round_up(rcu_idle_gp_delay + jiffies, rcu_idle_gp_delay) - jiffies;
-
- *nextevt = basemono + dj * TICK_NSEC;
- return 0;
-}
-
-/*
- * Prepare a CPU for idle from an RCU perspective. The first major task is to
- * sense whether nohz mode has been enabled or disabled via sysfs. The second
- * major task is to accelerate (that is, assign grace-period numbers to) any
- * recently arrived callbacks.
- *
- * The caller must have disabled interrupts.
- */
-static void rcu_prepare_for_idle(void)
-{
- bool needwake;
- struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
- struct rcu_node *rnp;
- int tne;
-
- lockdep_assert_irqs_disabled();
- if (rcu_rdp_is_offloaded(rdp))
- return;
-
- /* Handle nohz enablement switches conservatively. */
- tne = READ_ONCE(tick_nohz_active);
- if (tne != rdp->tick_nohz_enabled_snap) {
- if (!rcu_segcblist_empty(&rdp->cblist))
- invoke_rcu_core(); /* force nohz to see update. */
- rdp->tick_nohz_enabled_snap = tne;
- return;
- }
- if (!tne)
- return;
-
- /*
- * If we have not yet accelerated this jiffy, accelerate all
- * callbacks on this CPU.
- */
- if (rdp->last_accelerate == jiffies)
- return;
- rdp->last_accelerate = jiffies;
- if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
- rnp = rdp->mynode;
- raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
- needwake = rcu_accelerate_cbs(rnp, rdp);
- raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
- if (needwake)
- rcu_gp_kthread_wake();
- }
-}
-
-/*
- * Clean up for exit from idle. Attempt to advance callbacks based on
- * any grace periods that elapsed while the CPU was idle, and if any
- * callbacks are now ready to invoke, initiate invocation.
- */
-static void rcu_cleanup_after_idle(void)
-{
- struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
-
- lockdep_assert_irqs_disabled();
- if (rcu_rdp_is_offloaded(rdp))
- return;
- if (rcu_try_advance_all_cbs())
- invoke_rcu_core();
-}
-
-#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
-
/*
* Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
* grace-period kthread will do force_quiescent_state() processing?
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 5e2fa6fd97f12..21bebf7c9030b 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -347,26 +347,6 @@ static void rcu_dump_cpu_stacks(void)
}
}
-#ifdef CONFIG_RCU_FAST_NO_HZ
-
-static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
-{
- struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
-
- sprintf(cp, "last_accelerate: %04lx/%04lx dyntick_enabled: %d",
- rdp->last_accelerate & 0xffff, jiffies & 0xffff,
- !!rdp->tick_nohz_enabled_snap);
-}
-
-#else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
-
-static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
-{
- *cp = '\0';
-}
-
-#endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
-
static const char * const gp_state_names[] = {
[RCU_GP_IDLE] = "RCU_GP_IDLE",
[RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
@@ -408,13 +388,12 @@ static bool rcu_is_gp_kthread_starving(unsigned long *jp)
* of RCU grace periods that this CPU is ignorant of, for example, "1"
* if the CPU was aware of the previous grace period.
*
- * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
+ * Also print out idle info.
*/
static void print_cpu_stall_info(int cpu)
{
unsigned long delta;
bool falsepositive;
- char fast_no_hz[72];
struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
char *ticks_title;
unsigned long ticks_value;
@@ -432,11 +411,10 @@ static void print_cpu_stall_info(int cpu)
ticks_title = "ticks this GP";
ticks_value = rdp->ticks_this_gp;
}
- print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
falsepositive = rcu_is_gp_kthread_starving(NULL) &&
rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp));
- pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s%s\n",
+ pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s\n",
cpu,
"O."[!!cpu_online(cpu)],
"o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
@@ -449,7 +427,6 @@ static void print_cpu_stall_info(int cpu)
rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
- fast_no_hz,
falsepositive ? " (false positive?)" : "");
}
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE01 b/tools/testing/selftests/rcutorture/configs/rcu/TREE01
index b5b53973c01e4..8ae41d5f81a3e 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE01
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE01
@@ -6,7 +6,6 @@ CONFIG_PREEMPT=y
CONFIG_HZ_PERIODIC=n
CONFIG_NO_HZ_IDLE=y
CONFIG_NO_HZ_FULL=n
-CONFIG_RCU_FAST_NO_HZ=y
CONFIG_RCU_TRACE=y
CONFIG_HOTPLUG_CPU=y
CONFIG_MAXSMP=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TREE04 b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
index f6d6a40c05768..22ad0261728d0 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TREE04
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TREE04
@@ -7,7 +7,6 @@ CONFIG_PREEMPT=n
CONFIG_HZ_PERIODIC=n
CONFIG_NO_HZ_IDLE=n
CONFIG_NO_HZ_FULL=y
-CONFIG_RCU_FAST_NO_HZ=y
CONFIG_RCU_TRACE=y
CONFIG_RCU_FANOUT=4
CONFIG_RCU_FANOUT_LEAF=3
diff --git a/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt b/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
index 1b96d68473b8f..42acb1a64ce10 100644
--- a/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
+++ b/tools/testing/selftests/rcutorture/doc/TREE_RCU-kconfig.txt
@@ -15,7 +15,6 @@ CONFIG_PROVE_RCU -- Hardwired to CONFIG_PROVE_LOCKING.
CONFIG_RCU_BOOST -- one of PREEMPT_RCU.
CONFIG_RCU_FANOUT -- Cover hierarchy, but overlap with others.
CONFIG_RCU_FANOUT_LEAF -- Do one non-default.
-CONFIG_RCU_FAST_NO_HZ -- Do one, but not with all nohz_full CPUs.
CONFIG_RCU_NOCB_CPU -- Do three, one with no rcu_nocbs CPUs, one with
rcu_nocbs=0, and one with all rcu_nocbs CPUs.
CONFIG_RCU_TRACE -- Do half.
--
2.31.1.189.g2e36527f23
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