[PATCH tip/core/rcu 10/10] context_tracking: Add comments on interface and internals
From: Frederic Weisbecker
This subsystem lacks many explanations on its purpose and
design. Add these missing comments.
v4: Document function parameter to be more kernel-doc
friendly, as per Namhyung suggestion.
Reported-by: Andrew Morton
Signed-off-by: Frederic Weisbecker
Cc: Alessio Igor Bogani
Cc: Andrew Morton
Cc: Chris Metcalf
Cc: Christoph Lameter
Cc: Geoff Levand
Cc: Gilad Ben Yossef
Cc: Hakan Akkan
Cc: Ingo Molnar
Cc: Li Zhong
Cc: Namhyung Kim
Cc: Paul E. McKenney
Cc: Paul Gortmaker
Cc: Peter Zijlstra
Cc: Steven Rostedt
Cc: Thomas Gleixner
Signed-off-by: Paul E. McKenney
---
kernel/context_tracking.c | 75 +++--
1 files changed, 65 insertions(+), 10 deletions(-)
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index e0e07fd..d566aba 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
+ *
+ * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
+ * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
+ *
+ */
+
#include
#include
#include
@@ -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, probes 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 +64,70 @@ 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 callbacks
+ * @prev: the task that is being switched out
+ * @next: the task that is being switched in
+ *
+ * The
[PATCH tip/core/rcu 10/10] context_tracking: Add comments on interface and internals
From: Frederic Weisbecker fweis...@gmail.com
This subsystem lacks many explanations on its purpose and
design. Add these missing comments.
v4: Document function parameter to be more kernel-doc
friendly, as per Namhyung suggestion.
Reported-by: Andrew Morton a...@linux-foundation.org
Signed-off-by: Frederic Weisbecker fweis...@gmail.com
Cc: Alessio Igor Bogani abog...@kernel.org
Cc: Andrew Morton a...@linux-foundation.org
Cc: Chris Metcalf cmetc...@tilera.com
Cc: Christoph Lameter c...@linux.com
Cc: Geoff Levand ge...@infradead.org
Cc: Gilad Ben Yossef gi...@benyossef.com
Cc: Hakan Akkan hakanak...@gmail.com
Cc: Ingo Molnar mi...@kernel.org
Cc: Li Zhong zh...@linux.vnet.ibm.com
Cc: Namhyung Kim namhyung@lge.com
Cc: Paul E. McKenney paul...@linux.vnet.ibm.com
Cc: Paul Gortmaker paul.gortma...@windriver.com
Cc: Peter Zijlstra pet...@infradead.org
Cc: Steven Rostedt rost...@goodmis.org
Cc: Thomas Gleixner t...@linutronix.de
Signed-off-by: Paul E. McKenney paul...@linux.vnet.ibm.com
---
kernel/context_tracking.c | 75 +++--
1 files changed, 65 insertions(+), 10 deletions(-)
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index e0e07fd..d566aba 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 fweis...@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, probes 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 +64,70 @@ 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,
