[PATCH rcu 0/4] Refactor TASKS_RCU for lazy preemption

[PATCH rcu 0/4] Refactor TASKS_RCU for lazy preemption

[Paul E. McKenney]

Hello!

This series refactors the NEED_TASKS_RCU option so that the conditional
processing is in one place in kernel/rcu/Kconfig.  This makes it easier
to make sure that Tasks RCU is enabled in PREEMPT_AUTO kernels, given
that such kernels are preemptible, even if only lazily so.  After all,
it takes only one lazy preemption from a trampoline to significantly
degrade your kernel's actuarial statistics.

1.	Create NEED_TASKS_RCU to factor out enablement logic.

2.	Select new NEED_TASKS_RCU Kconfig option.

3.	Select new NEED_TASKS_RCU Kconfig option.

4.	Select new NEED_TASKS_RCU Kconfig option.

						Thanx, Paul

------------------------------------------------------------------------

 arch/Kconfig         |    4 ++--
 kernel/bpf/Kconfig   |    2 +-
 kernel/rcu/Kconfig   |    6 +++++-
 kernel/trace/Kconfig |    4 ++--
 4 files changed, 10 insertions(+), 6 deletions(-)

[PATCH rcu 1/4] rcu: Create NEED_TASKS_RCU to factor out enablement logic

[Paul E. McKenney]

Currently, if a Kconfig option depends on TASKS_RCU, it conditionally does
"select TASKS_RCU if PREEMPTION".  This works, but requires any change in
this enablement logic to be replicated across all such "select" clauses.
This commit therefore creates a new NEED_TASKS_RCU Kconfig option so
that the default value of TASKS_RCU can depend on a combination of this
new option and any needed enablement logic, so that this logic is in
one place.

While in the area, also anticipate a likely future change by adding
PREEMPT_AUTO to that logic.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/rcu/Kconfig | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 7dca0138260c3..3e079de0f5b43 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -85,9 +85,13 @@ config FORCE_TASKS_RCU
 	  idle, and user-mode execution as quiescent states.  Not for
 	  manual selection in most cases.
 
-config TASKS_RCU
+config NEED_TASKS_RCU
 	bool
 	default n
+
+config TASKS_RCU
+	bool
+	default NEED_TASKS_RCU && (PREEMPTION || PREEMPT_AUTO)
 	select IRQ_WORK
 
 config FORCE_TASKS_RUDE_RCU
-- 
2.40.1

[PATCH rcu 2/4] bpf: Select new NEED_TASKS_RCU Kconfig option

[Paul E. McKenney]

Currently, if a Kconfig option depends on TASKS_RCU, it conditionally does
"select TASKS_RCU if PREEMPTION".  This works, but requires any change in
this enablement logic to be replicated across all such "select" clauses.
A new NEED_TASKS_RCU Kconfig option has been created to allow this
enablement logic to be in one place in kernel/rcu/Kconfig.

Therefore, make BPF select the new NEED_TASKS_RCU Kconfig option.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Song Liu <song@kernel.org>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: <bpf@vger.kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/bpf/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
index 6a906ff930065..ce9fbc3b27ecf 100644
--- a/kernel/bpf/Kconfig
+++ b/kernel/bpf/Kconfig
@@ -27,7 +27,7 @@ config BPF_SYSCALL
 	bool "Enable bpf() system call"
 	select BPF
 	select IRQ_WORK
-	select TASKS_RCU if PREEMPTION
+	select NEED_TASKS_RCU
 	select TASKS_TRACE_RCU
 	select BINARY_PRINTF
 	select NET_SOCK_MSG if NET
-- 
2.40.1

[PATCH rcu 3/4] arch: Select new NEED_TASKS_RCU Kconfig option

[Paul E. McKenney]

Currently, if a Kconfig option depends on TASKS_RCU, it conditionally does
"select TASKS_RCU if PREEMPTION".  This works, but requires any change in
this enablement logic to be replicated across all such "select" clauses.
A new NEED_TASKS_RCU Kconfig option has been created to allow this
enablement logic to be in one place in kernel/rcu/Kconfig.

Therefore, select the new NEED_TASKS_RCU Kconfig option instead of the
old TASKS_RCU option.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 arch/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index c91917b508736..154f994547632 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -55,7 +55,7 @@ config KPROBES
 	depends on MODULES
 	depends on HAVE_KPROBES
 	select KALLSYMS
-	select TASKS_RCU if PREEMPTION
+	select NEED_TASKS_RCU
 	help
 	  Kprobes allows you to trap at almost any kernel address and
 	  execute a callback function.  register_kprobe() establishes
@@ -104,7 +104,7 @@ config STATIC_CALL_SELFTEST
 config OPTPROBES
 	def_bool y
 	depends on KPROBES && HAVE_OPTPROBES
-	select TASKS_RCU if PREEMPTION
+	select NEED_TASKS_RCU
 
 config KPROBES_ON_FTRACE
 	def_bool y
-- 
2.40.1

Re: [PATCH rcu 3/4] arch: Select new NEED_TASKS_RCU Kconfig option

[Steven Rostedt]

On Thu, 22 Feb 2024 16:26:26 -0800
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> Currently, if a Kconfig option depends on TASKS_RCU, it conditionally does
> "select TASKS_RCU if PREEMPTION".  This works, but requires any change in
> this enablement logic to be replicated across all such "select" clauses.
> A new NEED_TASKS_RCU Kconfig option has been created to allow this
> enablement logic to be in one place in kernel/rcu/Kconfig.
> 
> Therefore, select the new NEED_TASKS_RCU Kconfig option instead of the
> old TASKS_RCU option.
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Ankur Arora <ankur.a.arora@oracle.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

> ---
>  arch/Kconfig | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index c91917b508736..154f994547632 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -55,7 +55,7 @@ config KPROBES
>  	depends on MODULES
>  	depends on HAVE_KPROBES
>  	select KALLSYMS
> -	select TASKS_RCU if PREEMPTION
> +	select NEED_TASKS_RCU
>  	help
>  	  Kprobes allows you to trap at almost any kernel address and
>  	  execute a callback function.  register_kprobe() establishes
> @@ -104,7 +104,7 @@ config STATIC_CALL_SELFTEST
>  config OPTPROBES
>  	def_bool y
>  	depends on KPROBES && HAVE_OPTPROBES
> -	select TASKS_RCU if PREEMPTION
> +	select NEED_TASKS_RCU
>  
>  config KPROBES_ON_FTRACE
>  	def_bool y

Re: [PATCH rcu 3/4] arch: Select new NEED_TASKS_RCU Kconfig option

[Paul E. McKenney]

On Thu, Feb 22, 2024 at 08:36:51PM -0500, Steven Rostedt wrote:
> On Thu, 22 Feb 2024 16:26:26 -0800
> "Paul E. McKenney" <paulmck@kernel.org> wrote:
> 
> > Currently, if a Kconfig option depends on TASKS_RCU, it conditionally does
> > "select TASKS_RCU if PREEMPTION".  This works, but requires any change in
> > this enablement logic to be replicated across all such "select" clauses.
> > A new NEED_TASKS_RCU Kconfig option has been created to allow this
> > enablement logic to be in one place in kernel/rcu/Kconfig.
> > 
> > Therefore, select the new NEED_TASKS_RCU Kconfig option instead of the
> > old TASKS_RCU option.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Heiko Carstens <hca@linux.ibm.com>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Douglas Anderson <dianders@chromium.org>
> > Cc: Ankur Arora <ankur.a.arora@oracle.com>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Thank you!  I will apply on my next rebase.

							Thanx, Paul

> -- Steve
> 
> > ---
> >  arch/Kconfig | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index c91917b508736..154f994547632 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -55,7 +55,7 @@ config KPROBES
> >  	depends on MODULES
> >  	depends on HAVE_KPROBES
> >  	select KALLSYMS
> > -	select TASKS_RCU if PREEMPTION
> > +	select NEED_TASKS_RCU
> >  	help
> >  	  Kprobes allows you to trap at almost any kernel address and
> >  	  execute a callback function.  register_kprobe() establishes
> > @@ -104,7 +104,7 @@ config STATIC_CALL_SELFTEST
> >  config OPTPROBES
> >  	def_bool y
> >  	depends on KPROBES && HAVE_OPTPROBES
> > -	select TASKS_RCU if PREEMPTION
> > +	select NEED_TASKS_RCU
> >  
> >  config KPROBES_ON_FTRACE
> >  	def_bool y
> 

[PATCH rcu 4/4] tracing: Select new NEED_TASKS_RCU Kconfig option

[Paul E. McKenney]

Currently, if a Kconfig option depends on TASKS_RCU, it conditionally does
"select TASKS_RCU if PREEMPTION".  This works, but requires any change in
this enablement logic to be replicated across all such "select" clauses.
A new NEED_TASKS_RCU Kconfig option has been created to allow this
enablement logic to be in one place in kernel/rcu/Kconfig.

Therefore, select the new NEED_TASKS_RCU Kconfig option instead of the
old TASKS_RCU option.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: <linux-trace-kernel@vger.kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
---
 kernel/trace/Kconfig | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 61c541c36596d..6cdc5ff919b09 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -163,7 +163,7 @@ config TRACING
 	select BINARY_PRINTF
 	select EVENT_TRACING
 	select TRACE_CLOCK
-	select TASKS_RCU if PREEMPTION
+	select NEED_TASKS_RCU
 
 config GENERIC_TRACER
 	bool
@@ -204,7 +204,7 @@ config FUNCTION_TRACER
 	select GENERIC_TRACER
 	select CONTEXT_SWITCH_TRACER
 	select GLOB
-	select TASKS_RCU if PREEMPTION
+	select NEED_TASKS_RCU
 	select TASKS_RUDE_RCU
 	help
 	  Enable the kernel to trace every kernel function. This is done
-- 
2.40.1

Re: [PATCH rcu 0/4] Refactor TASKS_RCU for lazy preemption

[Mark Rutland]

On Thu, Feb 22, 2024 at 04:26:18PM -0800, Paul E. McKenney wrote:
> Hello!

Hi!

> This series refactors the NEED_TASKS_RCU option so that the conditional
> processing is in one place in kernel/rcu/Kconfig.  This makes it easier
> to make sure that Tasks RCU is enabled in PREEMPT_AUTO kernels, given
> that such kernels are preemptible, even if only lazily so.  After all,
> it takes only one lazy preemption from a trampoline to significantly
> degrade your kernel's actuarial statistics.
> 
> 1.	Create NEED_TASKS_RCU to factor out enablement logic.
> 
> 2.	Select new NEED_TASKS_RCU Kconfig option.
> 
> 3.	Select new NEED_TASKS_RCU Kconfig option.
> 
> 4.	Select new NEED_TASKS_RCU Kconfig option.

For the series:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.