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* [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2
@ 2020-10-27 15:08 Frederic Weisbecker
  2020-10-27 15:08 ` [PATCH 1/5] context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
                   ` (4 more replies)
  0 siblings, 5 replies; 10+ messages in thread
From: Frederic Weisbecker @ 2020-10-27 15:08 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Frederic Weisbecker, Paul E . McKenney, Thomas Gleixner,
	Phil Auld, Marcelo Tosatti

In order to be able to turn on/off nohz_full at runtime, we'll need
the archs to give up saving the context tracking state on the task stack
because this forces context tracking to run system wide, even on CPUs
that don't have nohz_full enabled.

Now being able to do so require sane entry code meeting some
requirements that at least x86 just achieved recently (I haven't checked
other archs yet).

Changes since v2:

- Rebase against v5.10-rc1
- Use SCHED_WARN_ON() instead of WARN_ON_ONCE() for the context tracking
  sanity check on schedule().

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
	core/isolation-v2

HEAD: 79f60f3dd0e0aea8b17c825371d8697444ae5faf

Thanks,
	Frederic
---

Frederic Weisbecker (5):
      context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK
      context_tracking:  Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK
      sched: Detect call to schedule from critical entry code
      context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs
      x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK


 arch/Kconfig                     | 17 +++++++++++++++++
 arch/x86/Kconfig                 |  1 +
 include/linux/context_tracking.h |  6 ++++--
 kernel/sched/core.c              |  3 ++-
 4 files changed, 24 insertions(+), 3 deletions(-)

^ permalink raw reply	[flat|nested] 10+ messages in thread

* [PATCH 1/5] context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK
  2020-10-27 15:08 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2 Frederic Weisbecker
@ 2020-10-27 15:08 ` Frederic Weisbecker
  2020-10-27 15:08 ` [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
                   ` (3 subsequent siblings)
  4 siblings, 0 replies; 10+ messages in thread
From: Frederic Weisbecker @ 2020-10-27 15:08 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Frederic Weisbecker, Paul E . McKenney, Thomas Gleixner,
	Phil Auld, Marcelo Tosatti

Historically, context tracking had to deal with fragile entry code path,
ie: before user_exit() is called and after user_enter() is called, in
case some of those spots would call schedule() or use RCU. On such
cases, the site had to be protected between exception_enter() and
exception_exit() that save the context tracking state in the task stack.

Such sleepable fragile code path had many different origins: tracing,
exceptions, early or late calls to context tracking on syscalls...

Aside of that not being pretty, saving the context tracking state on
the task stack forces us to run context tracking on all CPUs, including
housekeepers, and prevents us to completely shutdown nohz_full at
runtime on a CPU in the future as context tracking and its overhead
would still need to run system wide.

Now thanks to the extensive efforts to sanitize x86 entry code, those
conditions have been removed and we can now get rid of these workarounds
in this architecture.

Create a Kconfig feature to express this achievement.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Phil Auld <pauld@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 arch/Kconfig | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 56b6ccc0e32d..090ef3566c56 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -618,6 +618,23 @@ config HAVE_CONTEXT_TRACKING
 	  protected inside rcu_irq_enter/rcu_irq_exit() but preemption or signal
 	  handling on irq exit still need to be protected.
 
+config HAVE_CONTEXT_TRACKING_OFFSTACK
+	bool
+	help
+	  Architecture neither relies on exception_enter()/exception_exit()
+	  nor on schedule_user(). Also preempt_schedule_notrace() and
+	  preempt_schedule_irq() can't be called in a preemptible section
+	  while context tracking is CONTEXT_USER. This feature reflects a sane
+	  entry implementation where the following requirements are met on
+	  critical entry code, ie: before user_exit() or after user_enter():
+
+	  - Critical entry code isn't preemptible (or better yet:
+	    not interruptible).
+	  - No use of RCU read side critical sections, unless rcu_nmi_enter()
+	    got called.
+	  - No use of instrumentation, unless instrumentation_begin() got
+	    called.
+
 config HAVE_TIF_NOHZ
 	bool
 	help
-- 
2.25.1


^ permalink raw reply related	[flat|nested] 10+ messages in thread

* [PATCH 2/5] context_tracking:  Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK
  2020-10-27 15:08 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2 Frederic Weisbecker
  2020-10-27 15:08 ` [PATCH 1/5] context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
@ 2020-10-27 15:08 ` Frederic Weisbecker
  2020-11-11 14:32   ` Peter Zijlstra
  2020-10-27 15:08 ` [PATCH 3/5] sched: Detect call to schedule from critical entry code Frederic Weisbecker
                   ` (2 subsequent siblings)
  4 siblings, 1 reply; 10+ messages in thread
From: Frederic Weisbecker @ 2020-10-27 15:08 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Frederic Weisbecker, Paul E . McKenney, Thomas Gleixner,
	Phil Auld, Marcelo Tosatti

An architecture that provides this Kconfig feature doesn't need to
store the context tracking state on the task stack because its entry
code has been sanitized such that fragile path aren't preemptible
and special use of tracing and RCU read side critical sections in these
areas have been explicitly annotated.

Hence the exception_enter()/exception_exit() couple doesn't need to be
implemented in this case.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Phil Auld <pauld@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/context_tracking.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index d53cd331c4dd..bceb06498521 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -51,7 +51,8 @@ static inline enum ctx_state exception_enter(void)
 {
 	enum ctx_state prev_ctx;
 
-	if (!context_tracking_enabled())
+	if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) ||
+	    !context_tracking_enabled())
 		return 0;
 
 	prev_ctx = this_cpu_read(context_tracking.state);
@@ -63,7 +64,8 @@ static inline enum ctx_state exception_enter(void)
 
 static inline void exception_exit(enum ctx_state prev_ctx)
 {
-	if (context_tracking_enabled()) {
+	if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) &&
+	    context_tracking_enabled()) {
 		if (prev_ctx != CONTEXT_KERNEL)
 			context_tracking_enter(prev_ctx);
 	}
-- 
2.25.1


^ permalink raw reply related	[flat|nested] 10+ messages in thread

* [PATCH 3/5] sched: Detect call to schedule from critical entry code
  2020-10-27 15:08 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2 Frederic Weisbecker
  2020-10-27 15:08 ` [PATCH 1/5] context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
  2020-10-27 15:08 ` [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
@ 2020-10-27 15:08 ` Frederic Weisbecker
  2020-10-27 15:08 ` [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs Frederic Weisbecker
  2020-10-27 15:08 ` [PATCH 5/5] x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
  4 siblings, 0 replies; 10+ messages in thread
From: Frederic Weisbecker @ 2020-10-27 15:08 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Frederic Weisbecker, Paul E . McKenney, Thomas Gleixner,
	Phil Auld, Marcelo Tosatti

Detect calls to schedule() between user_enter() and user_exit(). Those
are symptoms of early entry code that either forgot to protect a call
to schedule() inside exception_enter()/exception_exit() or, in the case
of HAVE_CONTEXT_TRACKING_OFFSTACK, enabled interrupts or preemption in
a wrong spot.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Phil Auld <pauld@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/sched/core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d2003a7d5ab5..c23d7cb5aee3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4291,6 +4291,7 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
 		preempt_count_set(PREEMPT_DISABLED);
 	}
 	rcu_sleep_check();
+	SCHED_WARN_ON(ct_state() == CONTEXT_USER);
 
 	profile_hit(SCHED_PROFILING, __builtin_return_address(0));
 
-- 
2.25.1


^ permalink raw reply related	[flat|nested] 10+ messages in thread

* [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs
  2020-10-27 15:08 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2 Frederic Weisbecker
                   ` (2 preceding siblings ...)
  2020-10-27 15:08 ` [PATCH 3/5] sched: Detect call to schedule from critical entry code Frederic Weisbecker
@ 2020-10-27 15:08 ` Frederic Weisbecker
  2020-11-11 14:34   ` Peter Zijlstra
  2020-10-27 15:08 ` [PATCH 5/5] x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
  4 siblings, 1 reply; 10+ messages in thread
From: Frederic Weisbecker @ 2020-10-27 15:08 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Frederic Weisbecker, Paul E . McKenney, Thomas Gleixner,
	Phil Auld, Marcelo Tosatti

schedule_user() was traditionally used by the entry code's tail to
preempt userspace after the call to user_enter(). Indeed the call to
user_enter() used to be performed upon syscall exit slow path which was
right before the last opportunity to schedule() while resuming to
userspace. The context tracking state had to be saved on the task stack
and set back to CONTEXT_KERNEL temporarily in order to safely switch to
another task.

Only a few archs use it now and those implementing
HAVE_CONTEXT_TRACKING_OFFSTACK definetly can't rely on it.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Phil Auld <pauld@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c23d7cb5aee3..44426e5acde2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4631,7 +4631,7 @@ void __sched schedule_idle(void)
 	} while (need_resched());
 }
 
-#ifdef CONFIG_CONTEXT_TRACKING
+#if defined(CONFIG_CONTEXT_TRACKING) && !defined(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK)
 asmlinkage __visible void __sched schedule_user(void)
 {
 	/*
-- 
2.25.1


^ permalink raw reply related	[flat|nested] 10+ messages in thread

* [PATCH 5/5] x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK
  2020-10-27 15:08 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2 Frederic Weisbecker
                   ` (3 preceding siblings ...)
  2020-10-27 15:08 ` [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs Frederic Weisbecker
@ 2020-10-27 15:08 ` Frederic Weisbecker
  4 siblings, 0 replies; 10+ messages in thread
From: Frederic Weisbecker @ 2020-10-27 15:08 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Frederic Weisbecker, Paul E . McKenney, Thomas Gleixner,
	Phil Auld, Marcelo Tosatti

A lot of ground work has been performed on x86 entry code. Fragile path
between user_enter() and user_exit() have IRQs disabled. Uses of RCU and
intrumentation in these fragile areas have been explicitly annotated
and protected.

This architecture doesn't need exception_enter()/exception_exit()
anymore and has therefore earned CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Phil Auld <pauld@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f6946b81f74a..d793361839b5 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -162,6 +162,7 @@ config X86
 	select HAVE_CMPXCHG_DOUBLE
 	select HAVE_CMPXCHG_LOCAL
 	select HAVE_CONTEXT_TRACKING		if X86_64
+	select HAVE_CONTEXT_TRACKING_OFFSTACK	if HAVE_CONTEXT_TRACKING
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS
-- 
2.25.1


^ permalink raw reply related	[flat|nested] 10+ messages in thread

* Re: [PATCH 2/5] context_tracking:  Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK
  2020-10-27 15:08 ` [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
@ 2020-11-11 14:32   ` Peter Zijlstra
  2020-11-11 22:20     ` Frederic Weisbecker
  0 siblings, 1 reply; 10+ messages in thread
From: Peter Zijlstra @ 2020-11-11 14:32 UTC (permalink / raw)
  To: Frederic Weisbecker
  Cc: LKML, Paul E . McKenney, Thomas Gleixner, Phil Auld, Marcelo Tosatti

On Tue, Oct 27, 2020 at 04:08:24PM +0100, Frederic Weisbecker wrote:
> An architecture that provides this Kconfig feature doesn't need to
> store the context tracking state on the task stack because its entry
> code has been sanitized such that fragile path aren't preemptible
> and special use of tracing and RCU read side critical sections in these
> areas have been explicitly annotated.
> 
> Hence the exception_enter()/exception_exit() couple doesn't need to be
> implemented in this case.

Could you please explain what exception_{enter,exit}() actually do, then
explain what is required to make it superfluous? Because as is, I don't
have enough information to verify the claims made.

> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Phil Auld <pauld@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> ---
>  include/linux/context_tracking.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index d53cd331c4dd..bceb06498521 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -51,7 +51,8 @@ static inline enum ctx_state exception_enter(void)
>  {
>  	enum ctx_state prev_ctx;
>  
> -	if (!context_tracking_enabled())
> +	if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) ||
> +	    !context_tracking_enabled())
>  		return 0;
>  
>  	prev_ctx = this_cpu_read(context_tracking.state);
> @@ -63,7 +64,8 @@ static inline enum ctx_state exception_enter(void)
>  
>  static inline void exception_exit(enum ctx_state prev_ctx)
>  {
> -	if (context_tracking_enabled()) {
> +	if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) &&
> +	    context_tracking_enabled()) {
>  		if (prev_ctx != CONTEXT_KERNEL)
>  			context_tracking_enter(prev_ctx);
>  	}
> -- 
> 2.25.1
> 

^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs
  2020-10-27 15:08 ` [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs Frederic Weisbecker
@ 2020-11-11 14:34   ` Peter Zijlstra
  2020-11-11 22:22     ` Frederic Weisbecker
  0 siblings, 1 reply; 10+ messages in thread
From: Peter Zijlstra @ 2020-11-11 14:34 UTC (permalink / raw)
  To: Frederic Weisbecker
  Cc: LKML, Paul E . McKenney, Thomas Gleixner, Phil Auld, Marcelo Tosatti

On Tue, Oct 27, 2020 at 04:08:26PM +0100, Frederic Weisbecker wrote:
> schedule_user() was traditionally used by the entry code's tail to
> preempt userspace after the call to user_enter(). Indeed the call to
> user_enter() used to be performed upon syscall exit slow path which was
> right before the last opportunity to schedule() while resuming to
> userspace. The context tracking state had to be saved on the task stack
> and set back to CONTEXT_KERNEL temporarily in order to safely switch to
> another task.
> 
> Only a few archs use it now and those implementing
> HAVE_CONTEXT_TRACKING_OFFSTACK definetly can't rely on it.

powerpc64 and sparc64 only, right?

> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Phil Auld <pauld@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> ---
>  kernel/sched/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c23d7cb5aee3..44426e5acde2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4631,7 +4631,7 @@ void __sched schedule_idle(void)
>  	} while (need_resched());
>  }
>  
> -#ifdef CONFIG_CONTEXT_TRACKING
> +#if defined(CONFIG_CONTEXT_TRACKING) && !defined(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK)
>  asmlinkage __visible void __sched schedule_user(void)
>  {
>  	/*
> -- 
> 2.25.1
> 

^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: [PATCH 2/5] context_tracking:  Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK
  2020-11-11 14:32   ` Peter Zijlstra
@ 2020-11-11 22:20     ` Frederic Weisbecker
  0 siblings, 0 replies; 10+ messages in thread
From: Frederic Weisbecker @ 2020-11-11 22:20 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Paul E . McKenney, Thomas Gleixner, Phil Auld, Marcelo Tosatti

On Wed, Nov 11, 2020 at 03:32:18PM +0100, Peter Zijlstra wrote:
> On Tue, Oct 27, 2020 at 04:08:24PM +0100, Frederic Weisbecker wrote:
> > An architecture that provides this Kconfig feature doesn't need to
> > store the context tracking state on the task stack because its entry
> > code has been sanitized such that fragile path aren't preemptible
> > and special use of tracing and RCU read side critical sections in these
> > areas have been explicitly annotated.
> > 
> > Hence the exception_enter()/exception_exit() couple doesn't need to be
> > implemented in this case.
> 
> Could you please explain what exception_{enter,exit}() actually do, then
> explain what is required to make it superfluous? Because as is, I don't
> have enough information to verify the claims made.

So, the typical steps with context tracking are:

1) Task runs in userspace
2) Task enters the kernel (syscall/exception/IRQ)
3) Task switches from context tracking state CONTEXT_USER to CONTEXT_KERNEL (user_exit())
4) Task does stuff in kernel
5) Task switches from context tracking state CONTEXT_KERNEL to CONTEXT_USER (user_enter())
6) Task exits the kernel

If an exception fires between 5) and 6), the pt_regs and the context tracking disagree
on the context of the origin. We must set CONTEXT_KERNEL before the exception handler,
that's unconditional for those that want to call into schedule, but we must restore
CONTEXT_USER when we exit the exception while pt_regs tells us that we are resuming to
kernel space.

We can't fix that with storing the context tracking state in a per-cpu or per-task variable
since another exception may fire onto the current one and overwrite the saved state. Also
the task can schedule. So it has to be stored in a per task stack.

This is how exception_enter()/exception_exit() papers over the problem:

5) Task switches from context tracking state CONTEXT_KERNEL to CONTEXT_USER (user_enter())
5.1) Exception fires
5.2) prev_state = exception_enter() // save CONTEXT_USER to prev_state and set CONTEXT_KERNEL
5.3) Exception handler
5.4) exception_enter(prev_state) // restore CONTEXT_USER
5.5) Exception resumes
6) Task exits the kernel

What is required to make it superfluous is to forbid exceptions and IRQs between 2) and 3)
and between 5) and 6), or if we allow any, they won't call into context tracking, eg: NMIs,
and they won't schedule.

I can add that in the changelog if that makes it any clearer.

Thanks.

^ permalink raw reply	[flat|nested] 10+ messages in thread

* Re: [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs
  2020-11-11 14:34   ` Peter Zijlstra
@ 2020-11-11 22:22     ` Frederic Weisbecker
  0 siblings, 0 replies; 10+ messages in thread
From: Frederic Weisbecker @ 2020-11-11 22:22 UTC (permalink / raw)
  To: Peter Zijlstra
  Cc: LKML, Paul E . McKenney, Thomas Gleixner, Phil Auld, Marcelo Tosatti

On Wed, Nov 11, 2020 at 03:34:58PM +0100, Peter Zijlstra wrote:
> On Tue, Oct 27, 2020 at 04:08:26PM +0100, Frederic Weisbecker wrote:
> > schedule_user() was traditionally used by the entry code's tail to
> > preempt userspace after the call to user_enter(). Indeed the call to
> > user_enter() used to be performed upon syscall exit slow path which was
> > right before the last opportunity to schedule() while resuming to
> > userspace. The context tracking state had to be saved on the task stack
> > and set back to CONTEXT_KERNEL temporarily in order to safely switch to
> > another task.
> > 
> > Only a few archs use it now and those implementing
> > HAVE_CONTEXT_TRACKING_OFFSTACK definetly can't rely on it.
> 
> powerpc64 and sparc64 only, right?

Exactly! I can precise that in the changelog.

Thanks.

> 
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Phil Auld <pauld@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > ---
> >  kernel/sched/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index c23d7cb5aee3..44426e5acde2 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4631,7 +4631,7 @@ void __sched schedule_idle(void)
> >  	} while (need_resched());
> >  }
> >  
> > -#ifdef CONFIG_CONTEXT_TRACKING
> > +#if defined(CONFIG_CONTEXT_TRACKING) && !defined(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK)
> >  asmlinkage __visible void __sched schedule_user(void)
> >  {
> >  	/*
> > -- 
> > 2.25.1
> > 

^ permalink raw reply	[flat|nested] 10+ messages in thread

end of thread, other threads:[~2020-11-11 22:22 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-10-27 15:08 [PATCH 0/5] context_tracking: Flatter archs not using exception_enter/exit() v2 Frederic Weisbecker
2020-10-27 15:08 ` [PATCH 1/5] context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
2020-10-27 15:08 ` [PATCH 2/5] context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker
2020-11-11 14:32   ` Peter Zijlstra
2020-11-11 22:20     ` Frederic Weisbecker
2020-10-27 15:08 ` [PATCH 3/5] sched: Detect call to schedule from critical entry code Frederic Weisbecker
2020-10-27 15:08 ` [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs Frederic Weisbecker
2020-11-11 14:34   ` Peter Zijlstra
2020-11-11 22:22     ` Frederic Weisbecker
2020-10-27 15:08 ` [PATCH 5/5] x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK Frederic Weisbecker

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