Commit e2c73a68 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Remove the RCU_FAST_NO_HZ Kconfig option 



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: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 24eab6e1

Documentation/RCU/stallwarn.rst

+0 −11
Original line number Diff line number Diff line
@@ -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::

Documentation/admin-guide/kernel-parameters.txt

+0 −4
Original line number Diff line number Diff line
@@ -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

Documentation/timers/no_hz.rst

+3 −7
Original line number Diff line number Diff line
@@ -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

kernel/rcu/Kconfig

+0 −18
Original line number Diff line number Diff line
@@ -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

kernel/rcu/tree.c

+0 −11
Original line number Diff line number Diff line
@@ -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));