* [PATCH v2] context_tracking: Add comments on interface and internals
@ 2012-12-14 15:27 Frederic Weisbecker
2012-12-16 9:22 ` Ingo Molnar
0 siblings, 1 reply; 3+ messages in thread
From: Frederic Weisbecker @ 2012-12-14 15:27 UTC (permalink / raw)
To: Paul E. McKenney, Ingo Molnar, Andrew Morton
Cc: LKML, Frederic Weisbecker, Gilad Ben-Yossef, Thomas Gleixner,
Steven Rostedt, Peter Zijlstra, Li Zhong
This subsystem lacks many explanations on its purpose and
design. Add these missing comments.
v2: Address comments from Andrew
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
---
kernel/context_tracking.c | 73 ++++++++++++++++++++++++++++++++++++++------
1 files changed, 63 insertions(+), 10 deletions(-)
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index e0e07fd..9f6c38f 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -1,3 +1,19 @@
+/*
+ * Context tracking: Probe on high level context boundaries such as kernel
+ * and userspace. This includes syscalls and exceptions entry/exit.
+ *
+ * This is used by RCU to remove its dependency on the timer tick while a CPU
+ * runs in userspace.
+ *
+ * Started by Frederic Weisbecker:
+ *
+ * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
+ *
+ * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
+ * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
+ *
+ */
+
#include <linux/context_tracking.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
@@ -6,8 +22,8 @@
struct context_tracking {
/*
- * When active is false, hooks are not set to
- * minimize overhead: TIF flags are cleared
+ * When active is false, hooks are unset in order
+ * to minimize overhead: TIF flags are cleared
* and calls to user_enter/exit are ignored. This
* may be further optimized using static keys.
*/
@@ -24,6 +40,15 @@ static DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
#endif
};
+/**
+ * user_enter - Inform the context tracking that the CPU is going to
+ * enter userspace mode.
+ *
+ * This function must be called right before we switch from the kernel
+ * to userspace, when it's guaranteed the remaining kernel instructions
+ * to execute won't use any RCU read side critical section because this
+ * function sets RCU in extended quiescent state.
+ */
void user_enter(void)
{
unsigned long flags;
@@ -39,40 +64,68 @@ void user_enter(void)
if (in_interrupt())
return;
+ /* Kernel threads aren't supposed to go to userspace */
WARN_ON_ONCE(!current->mm);
local_irq_save(flags);
if (__this_cpu_read(context_tracking.active) &&
__this_cpu_read(context_tracking.state) != IN_USER) {
__this_cpu_write(context_tracking.state, IN_USER);
+ /*
+ * At this stage, only low level arch entry code remains and
+ * then we'll run in userspace. We can assume there won't be
+ * any RCU read-side critical section until the next call to
+ * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
+ * on the tick.
+ */
rcu_user_enter();
}
local_irq_restore(flags);
}
+
+/**
+ * user_exit - Inform the context tracking that the CPU is
+ * exiting userspace mode and entering the kernel.
+ *
+ * This function must be called after we entered the kernel from userspace
+ * before any use of RCU read side critical section. This potentially include
+ * any high level kernel code like syscalls, exceptions, signal handling, etc...
+ *
+ * This call supports re-entrancy. This way it can be called from any exception
+ * handler without needing to know if we came from userspace or not.
+ */
void user_exit(void)
{
unsigned long flags;
- /*
- * Some contexts may involve an exception occuring in an irq,
- * leading to that nesting:
- * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
- * This would mess up the dyntick_nesting count though. And rcu_irq_*()
- * helpers are enough to protect RCU uses inside the exception. So
- * just return immediately if we detect we are in an IRQ.
- */
if (in_interrupt())
return;
local_irq_save(flags);
if (__this_cpu_read(context_tracking.state) == IN_USER) {
__this_cpu_write(context_tracking.state, IN_KERNEL);
+ /*
+ * We are going to run code that may use RCU. Inform
+ * RCU core about that (ie: we may need the tick again).
+ */
rcu_user_exit();
}
local_irq_restore(flags);
}
+
+/**
+ * context_tracking_task_switch - context switch the syscall hooks
+ *
+ * The context tracking uses the syscall slow path to implement its user-kernel
+ * boundaries hooks on syscalls. This way it doesn't impact the syscall fast
+ * path on CPUs that don't do context tracking.
+ *
+ * But we need to clear the flag on the previous task because it may later
+ * migrate to some CPU that doesn't do the context tracking. As such the TIF
+ * flag may not be desired there.
+ */
void context_tracking_task_switch(struct task_struct *prev,
struct task_struct *next)
{
--
1.7.5.4
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH v2] context_tracking: Add comments on interface and internals
2012-12-14 15:27 [PATCH v2] context_tracking: Add comments on interface and internals Frederic Weisbecker
@ 2012-12-16 9:22 ` Ingo Molnar
2013-01-07 21:34 ` Frederic Weisbecker
0 siblings, 1 reply; 3+ messages in thread
From: Ingo Molnar @ 2012-12-16 9:22 UTC (permalink / raw)
To: Frederic Weisbecker
Cc: Paul E. McKenney, Andrew Morton, LKML, Gilad Ben-Yossef,
Thomas Gleixner, Steven Rostedt, Peter Zijlstra, Li Zhong
* Frederic Weisbecker <fweisbec@gmail.com> wrote:
> This subsystem lacks many explanations on its purpose and
> design. Add these missing comments.
>
> v2: Address comments from Andrew
>
> Reported-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Gilad Ben-Yossef <gilad@benyossef.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Li Zhong <zhong@linux.vnet.ibm.com>
> ---
> kernel/context_tracking.c | 73 ++++++++++++++++++++++++++++++++++++++------
> 1 files changed, 63 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index e0e07fd..9f6c38f 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -1,3 +1,19 @@
> +/*
> + * Context tracking: Probe on high level context boundaries such as kernel
> + * and userspace. This includes syscalls and exceptions entry/exit.
> + *
> + * This is used by RCU to remove its dependency on the timer tick while a CPU
> + * runs in userspace.
> + *
> + * Started by Frederic Weisbecker:
> + *
> + * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
> + *
> + * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
> + * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
> + *
> + */
> +
> #include <linux/context_tracking.h>
> #include <linux/rcupdate.h>
> #include <linux/sched.h>
> @@ -6,8 +22,8 @@
>
> struct context_tracking {
> /*
> - * When active is false, hooks are not set to
> - * minimize overhead: TIF flags are cleared
> + * When active is false, hooks are unset in order
> + * to minimize overhead: TIF flags are cleared
> * and calls to user_enter/exit are ignored. This
> * may be further optimized using static keys.
> */
> @@ -24,6 +40,15 @@ static DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
> #endif
> };
>
> +/**
> + * user_enter - Inform the context tracking that the CPU is going to
> + * enter userspace mode.
> + *
> + * This function must be called right before we switch from the kernel
> + * to userspace, when it's guaranteed the remaining kernel instructions
> + * to execute won't use any RCU read side critical section because this
> + * function sets RCU in extended quiescent state.
> + */
> void user_enter(void)
> {
> unsigned long flags;
> @@ -39,40 +64,68 @@ void user_enter(void)
> if (in_interrupt())
> return;
>
> + /* Kernel threads aren't supposed to go to userspace */
> WARN_ON_ONCE(!current->mm);
>
> local_irq_save(flags);
> if (__this_cpu_read(context_tracking.active) &&
> __this_cpu_read(context_tracking.state) != IN_USER) {
> __this_cpu_write(context_tracking.state, IN_USER);
> + /*
> + * At this stage, only low level arch entry code remains and
> + * then we'll run in userspace. We can assume there won't be
> + * any RCU read-side critical section until the next call to
> + * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
> + * on the tick.
> + */
> rcu_user_enter();
> }
> local_irq_restore(flags);
> }
>
> +
> +/**
> + * user_exit - Inform the context tracking that the CPU is
> + * exiting userspace mode and entering the kernel.
> + *
> + * This function must be called after we entered the kernel from userspace
> + * before any use of RCU read side critical section. This potentially include
> + * any high level kernel code like syscalls, exceptions, signal handling, etc...
> + *
> + * This call supports re-entrancy. This way it can be called from any exception
> + * handler without needing to know if we came from userspace or not.
> + */
> void user_exit(void)
> {
> unsigned long flags;
>
> - /*
> - * Some contexts may involve an exception occuring in an irq,
> - * leading to that nesting:
> - * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
> - * This would mess up the dyntick_nesting count though. And rcu_irq_*()
> - * helpers are enough to protect RCU uses inside the exception. So
> - * just return immediately if we detect we are in an IRQ.
> - */
> if (in_interrupt())
> return;
>
> local_irq_save(flags);
> if (__this_cpu_read(context_tracking.state) == IN_USER) {
> __this_cpu_write(context_tracking.state, IN_KERNEL);
> + /*
> + * We are going to run code that may use RCU. Inform
> + * RCU core about that (ie: we may need the tick again).
> + */
> rcu_user_exit();
> }
> local_irq_restore(flags);
> }
>
> +
> +/**
> + * context_tracking_task_switch - context switch the syscall hooks
> + *
> + * The context tracking uses the syscall slow path to implement its user-kernel
> + * boundaries hooks on syscalls. This way it doesn't impact the syscall fast
> + * path on CPUs that don't do context tracking.
> + *
> + * But we need to clear the flag on the previous task because it may later
> + * migrate to some CPU that doesn't do the context tracking. As such the TIF
> + * flag may not be desired there.
If possible: s/hooks/callbacks
'hook' gives me the visual of a box match. YMMV.
Thanks,
Ingo
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH v2] context_tracking: Add comments on interface and internals
2012-12-16 9:22 ` Ingo Molnar
@ 2013-01-07 21:34 ` Frederic Weisbecker
0 siblings, 0 replies; 3+ messages in thread
From: Frederic Weisbecker @ 2013-01-07 21:34 UTC (permalink / raw)
To: Ingo Molnar
Cc: Paul E. McKenney, Andrew Morton, LKML, Gilad Ben-Yossef,
Thomas Gleixner, Steven Rostedt, Peter Zijlstra, Li Zhong
2012/12/16 Ingo Molnar <mingo@kernel.org>:
>
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
>> +
>> +/**
>> + * context_tracking_task_switch - context switch the syscall hooks
>> + *
>> + * The context tracking uses the syscall slow path to implement its user-kernel
>> + * boundaries hooks on syscalls. This way it doesn't impact the syscall fast
>> + * path on CPUs that don't do context tracking.
>> + *
>> + * But we need to clear the flag on the previous task because it may later
>> + * migrate to some CPU that doesn't do the context tracking. As such the TIF
>> + * flag may not be desired there.
>
> If possible: s/hooks/callbacks
>
> 'hook' gives me the visual of a box match. YMMV.
Ok, I'm fixing this.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2012-12-16 9:22 ` Ingo Molnar
2013-01-07 21:34 ` Frederic Weisbecker
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