linux-kernel - [PATCH 16/34] rcu: Remove now-unused rcu_task_ipi_delay and TASKS_TRACE_RCU_READ

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Message-Id: <20250923142036.112290-16-paulmck@kernel.org>
Date: Tue, 23 Sep 2025 07:20:18 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	bpf@...r.kernel.org
Subject: [PATCH 16/34] rcu: Remove now-unused rcu_task_ipi_delay and TASKS_TRACE_RCU_READ_MB

Now that RCU Tasks Trace being defined in terms of SRCU-fast,
the rcupdate.rcu_task_ipi_delay kernel boot parameter and the
TASKS_TRACE_RCU_READ_MB Kconfig option are no longer used.  This commit
therefore removes them.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: <bpf@...r.kernel.org>
---
 .../admin-guide/kernel-parameters.txt          |  7 -------
 kernel/rcu/Kconfig                             | 18 ------------------
 kernel/rcu/tasks.h                             |  5 -----
 .../selftests/rcutorture/configs/rcu/TRACE01   |  1 -
 .../selftests/rcutorture/configs/rcu/TRACE02   |  1 -
 5 files changed, 32 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 54d31a5e46e1cf..40fc198bed4db9 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6078,13 +6078,6 @@
 			dynamically) adjusted.	This parameter is intended
 			for use in testing.
 
-	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_lazy_lim= [KNL]
 			Number of callbacks on a given CPU that will
 			cancel laziness on that CPU.  Use -1 to disable
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index eeaa1cd47c6c4c..73a6cc364628b5 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -313,24 +313,6 @@ config RCU_NOCB_CPU_CB_BOOST
 	  Say Y here if you want to set RT priority for offloading kthreads.
 	  Say N here if you are building a !PREEMPT_RT kernel and are unsure.
 
-config TASKS_TRACE_RCU_READ_MB
-	bool "Tasks Trace RCU readers use memory barriers in user and idle"
-	depends on RCU_EXPERT && TASKS_TRACE_RCU
-	default PREEMPT_RT || NR_CPUS < 8
-	help
-	  Use this option to further reduce the number of IPIs sent
-	  to CPUs executing in userspace or idle during tasks trace
-	  RCU grace periods.  Given that a reasonable setting of
-	  the rcupdate.rcu_task_ipi_delay kernel boot parameter
-	  eliminates such IPIs for many workloads, proper setting
-	  of this Kconfig option is important mostly for aggressive
-	  real-time installations and for battery-powered devices,
-	  hence the default chosen above.
-
-	  Say Y here if you hate IPIs.
-	  Say N here if you hate read-side memory barriers.
-	  Take the default if you are unsure.
-
 config RCU_LAZY
 	bool "RCU callback lazy invocation functionality"
 	depends on RCU_NOCB_CPU
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index a3c16e65e16812..833e180db744f2 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -161,11 +161,6 @@ static void tasks_rcu_exit_srcu_stall(struct timer_list *unused);
 static DEFINE_TIMER(tasks_rcu_exit_srcu_stall_timer, tasks_rcu_exit_srcu_stall);
 #endif
 
-/* Avoid IPIing CPUs early in the grace period. */
-#define RCU_TASK_IPI_DELAY (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) ? HZ / 2 : 0)
-static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
-module_param(rcu_task_ipi_delay, int, 0644);
-
 /* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
 #define RCU_TASK_BOOT_STALL_TIMEOUT (HZ * 30)
 #define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TRACE01 b/tools/testing/selftests/rcutorture/configs/rcu/TRACE01
index 85b407467454a2..18efab346381a4 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TRACE01
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TRACE01
@@ -10,5 +10,4 @@ CONFIG_PROVE_LOCKING=n
 #CHECK#CONFIG_PROVE_RCU=n
 CONFIG_FORCE_TASKS_TRACE_RCU=y
 #CHECK#CONFIG_TASKS_TRACE_RCU=y
-CONFIG_TASKS_TRACE_RCU_READ_MB=y
 CONFIG_RCU_EXPERT=y
diff --git a/tools/testing/selftests/rcutorture/configs/rcu/TRACE02 b/tools/testing/selftests/rcutorture/configs/rcu/TRACE02
index 9003c56cd76484..8da390e8282977 100644
--- a/tools/testing/selftests/rcutorture/configs/rcu/TRACE02
+++ b/tools/testing/selftests/rcutorture/configs/rcu/TRACE02
@@ -9,6 +9,5 @@ CONFIG_PROVE_LOCKING=y
 #CHECK#CONFIG_PROVE_RCU=y
 CONFIG_FORCE_TASKS_TRACE_RCU=y
 #CHECK#CONFIG_TASKS_TRACE_RCU=y
-CONFIG_TASKS_TRACE_RCU_READ_MB=n
 CONFIG_RCU_EXPERT=y
 CONFIG_DEBUG_OBJECTS=y
-- 
2.40.1