Re: [linux-pm] [RFC][PATCH 1/2] Freezer: Introduce PF_FREEZER_NOSIG

From: Rafael J. Wysocki
Date: Wed May 07 2008 - 08:12:58 EST

On Wednesday, 7 of May 2008, Gautham R Shenoy wrote:
> Hi Rafael,

Hi,

> On Wed, May 07, 2008 at 12:05:19AM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rjw@xxxxxxx>
> >
> > The freezer currently attempts to distinguish kernel threads from
> > user space tasks by checking if their mm pointer is unset and it
> > does not send fake signals to kernel threads. However, there are
> > kernel threads, mostly related to networking, that behave like
> > user space tasks and may want to be sent a fake signal to be frozen.
> >
> > Introduce the new process flag PF_FREEZER_NOSIG that will be set
> > by default for all kernel threads and make the freezer only send
> > fake signals to the tasks having PF_FREEZER_NOSIG unset. Provide
> > the set_freezable_with_signal() function to be called by the kernel
> > threads that want to be sent a fake signal for freezing.
> >
> > This patch should not change the freezer's observable behavior.
> >
> > Signed-off-by: Rafael J. Wysocki <rjw@xxxxxxx>
> > ---
> > include/linux/freezer.h | 10 ++++
> > include/linux/sched.h | 1
> > kernel/kthread.c | 2
> > kernel/power/process.c | 97 ++++++++++++++++++++----------------------------
> > 4 files changed, 54 insertions(+), 56 deletions(-)
> >
> > Index: linux-2.6/include/linux/freezer.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/freezer.h
> > +++ linux-2.6/include/linux/freezer.h
> > @@ -128,6 +128,15 @@ static inline void set_freezable(void)
> > }
> >
> > /*
> > + * Tell the freezer that the current task should be frozen by it and that it
> > + * should send a fake signal to the task to freeze it.
> > + */
> > +static inline void set_freezable_with_signal(void)
> > +{
> > + current->flags &= ~(PF_NOFREEZE | PF_FREEZER_NOSIG);
> > +}
> > +
> > +/*
> > * Freezer-friendly wrappers around wait_event_interruptible() and
> > * wait_event_interruptible_timeout(), originally defined in <linux/wait.h>
> > */
> > @@ -174,6 +183,7 @@ static inline void freezer_do_not_count(
> > static inline void freezer_count(void) {}
> > static inline int freezer_should_skip(struct task_struct *p) { return 0; }
> > static inline void set_freezable(void) {}
> > +static inline void set_freezable_with_signal(void) {}
> >
> > #define wait_event_freezable(wq, condition) \
> > wait_event_interruptible(wq, condition)
> > Index: linux-2.6/include/linux/sched.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/sched.h
> > +++ linux-2.6/include/linux/sched.h
> > @@ -1508,6 +1508,7 @@ static inline void put_task_struct(struc
> > #define PF_MEMPOLICY 0x10000000 /* Non-default NUMA mempolicy */
> > #define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */
> > #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezeable */
> > +#define PF_FREEZER_NOSIG 0x80000000 /* Freezer won't send signals to it */
> >
> > /*
> > * Only the _current_ task can read/write to tsk->flags, but other
> > Index: linux-2.6/kernel/power/process.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/power/process.c
> > +++ linux-2.6/kernel/power/process.c
> > @@ -19,9 +19,6 @@
> > */
> > #define TIMEOUT (20 * HZ)
> >
> > -#define FREEZER_KERNEL_THREADS 0
> > -#define FREEZER_USER_SPACE 1
> > -
> > static inline int freezeable(struct task_struct * p)
> > {
> > if ((p == current) ||
> > @@ -84,63 +81,53 @@ static void fake_signal_wake_up(struct t
> > spin_unlock_irqrestore(&p->sighand->siglock, flags);
> > }
> >
> > -static int has_mm(struct task_struct *p)
> > +static inline bool should_send_signal(struct task_struct *p)
> > {
> > - return (p->mm && !(p->flags & PF_BORROWED_MM));
> > + return !(current->flags & PF_FREEZER_NOSIG);
> ^^^^^^^^^^^^^^
> p->flags ?

Yes, nice catch, thanks!

I wonder how on Earth that wasn't caught by testing (current->flags is always
false here) ...

[--snip--]
> >
> > -static void thaw_tasks(int thaw_user_space)
> > +static void thaw_tasks(bool sig_only)
> > {
> > struct task_struct *g, *p;
> >
> > @@ -260,7 +247,7 @@ static void thaw_tasks(int thaw_user_spa
> > if (!freezeable(p))
> > continue;
> >
> > - if (!p->mm == thaw_user_space)
> > + if (sig_only && !should_send_signal(p))
> > continue;
> >
> > thaw_process(p);
> > @@ -271,8 +258,8 @@ static void thaw_tasks(int thaw_user_spa
> > void thaw_processes(void)
> > {
> > printk("Restarting tasks ... ");
> > - thaw_tasks(FREEZER_KERNEL_THREADS);
> > - thaw_tasks(FREEZER_USER_SPACE);
> > + thaw_tasks(true);
> > + thaw_tasks(false);
>
> Shouldn't the order be
> thaw_tasks(false);
> thaw_tasks(true);

Ouch, thanks again!

But, that's supposed to be "first thaw tasks we did not send fake signals to
and then thaw everybody else", which translates to something different from
what I wrote in thaw_tasks(), too.

[Well, frankly I'm not sure if there is a practical situation in which that
matters.]

Corrected patch is appended.

Thanks,
Rafael


---
include/linux/freezer.h | 10 ++++
include/linux/sched.h | 1
kernel/kthread.c | 2
kernel/power/process.c | 97 ++++++++++++++++++++----------------------------
4 files changed, 54 insertions(+), 56 deletions(-)

Index: linux-2.6/include/linux/freezer.h
===================================================================
--- linux-2.6.orig/include/linux/freezer.h
+++ linux-2.6/include/linux/freezer.h
@@ -128,6 +128,15 @@ static inline void set_freezable(void)
}

/*
+ * Tell the freezer that the current task should be frozen by it and that it
+ * should send a fake signal to the task to freeze it.
+ */
+static inline void set_freezable_with_signal(void)
+{
+ current->flags &= ~(PF_NOFREEZE | PF_FREEZER_NOSIG);
+}
+
+/*
* Freezer-friendly wrappers around wait_event_interruptible() and
* wait_event_interruptible_timeout(), originally defined in <linux/wait.h>
*/
@@ -174,6 +183,7 @@ static inline void freezer_do_not_count(
static inline void freezer_count(void) {}
static inline int freezer_should_skip(struct task_struct *p) { return 0; }
static inline void set_freezable(void) {}
+static inline void set_freezable_with_signal(void) {}

#define wait_event_freezable(wq, condition) \
wait_event_interruptible(wq, condition)
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1508,6 +1508,7 @@ static inline void put_task_struct(struc
#define PF_MEMPOLICY 0x10000000 /* Non-default NUMA mempolicy */
#define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */
#define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezeable */
+#define PF_FREEZER_NOSIG 0x80000000 /* Freezer won't send signals to it */

/*
* Only the _current_ task can read/write to tsk->flags, but other
Index: linux-2.6/kernel/power/process.c
===================================================================
--- linux-2.6.orig/kernel/power/process.c
+++ linux-2.6/kernel/power/process.c
@@ -19,9 +19,6 @@
*/
#define TIMEOUT (20 * HZ)

-#define FREEZER_KERNEL_THREADS 0
-#define FREEZER_USER_SPACE 1
-
static inline int freezeable(struct task_struct * p)
{
if ((p == current) ||
@@ -84,63 +81,53 @@ static void fake_signal_wake_up(struct t
spin_unlock_irqrestore(&p->sighand->siglock, flags);
}

-static int has_mm(struct task_struct *p)
+static inline bool should_send_signal(struct task_struct *p)
{
- return (p->mm && !(p->flags & PF_BORROWED_MM));
+ return !(p->flags & PF_FREEZER_NOSIG);
}

/**
* freeze_task - send a freeze request to given task
* @p: task to send the request to
- * @with_mm_only: if set, the request will only be sent if the task has its
- * own mm
- * Return value: 0, if @with_mm_only is set and the task has no mm of its
- * own or the task is frozen, 1, otherwise
+ * @sig_only: if set, the request will only be sent if the task has the
+ * PF_FREEZER_NOSIG flag unset
+ * Return value: 'false', if @sig_only is set and the task has
+ * PF_FREEZER_NOSIG set or the task is frozen, 'true', otherwise
*
- * The freeze request is sent by seting the tasks's TIF_FREEZE flag and
+ * The freeze request is sent by setting the tasks's TIF_FREEZE flag and
* either sending a fake signal to it or waking it up, depending on whether
- * or not it has its own mm (ie. it is a user land task). If @with_mm_only
- * is set and the task has no mm of its own (ie. it is a kernel thread),
- * its TIF_FREEZE flag should not be set.
- *
- * The task_lock() is necessary to prevent races with exit_mm() or
- * use_mm()/unuse_mm() from occuring.
+ * or not it has PF_FREEZER_NOSIG set. If @sig_only is set and the task
+ * has PF_FREEZER_NOSIG set (ie. it is a typical kernel thread), its
+ * TIF_FREEZE flag will not be set.
*/
-static int freeze_task(struct task_struct *p, int with_mm_only)
+static bool freeze_task(struct task_struct *p, bool sig_only)
{
- int ret = 1;
+ /*
+ * We first check if the task is freezing and next if it has already
+ * been frozen to avoid the race with frozen_process() which first marks
+ * the task as frozen and next clears its TIF_FREEZE.
+ */
+ if (!freezing(p)) {
+ rmb();
+ if (frozen(p))
+ return false;

- task_lock(p);
- if (freezing(p)) {
- if (has_mm(p)) {
- if (!signal_pending(p))
- fake_signal_wake_up(p);
- } else {
- if (with_mm_only)
- ret = 0;
- else
- wake_up_state(p, TASK_INTERRUPTIBLE);
- }
+ if (!sig_only || should_send_signal(p))
+ set_freeze_flag(p);
+ else
+ return false;
+ }
+
+ if (should_send_signal(p)) {
+ if (!signal_pending(p))
+ fake_signal_wake_up(p);
+ } else if (sig_only) {
+ return false;
} else {
- rmb();
- if (frozen(p)) {
- ret = 0;
- } else {
- if (has_mm(p)) {
- set_freeze_flag(p);
- fake_signal_wake_up(p);
- } else {
- if (with_mm_only) {
- ret = 0;
- } else {
- set_freeze_flag(p);
- wake_up_state(p, TASK_INTERRUPTIBLE);
- }
- }
- }
+ wake_up_state(p, TASK_INTERRUPTIBLE);
}
- task_unlock(p);
- return ret;
+
+ return true;
}

static void cancel_freezing(struct task_struct *p)
@@ -156,7 +143,7 @@ static void cancel_freezing(struct task_
}
}

-static int try_to_freeze_tasks(int freeze_user_space)
+static int try_to_freeze_tasks(bool sig_only)
{
struct task_struct *g, *p;
unsigned long end_time;
@@ -175,7 +162,7 @@ static int try_to_freeze_tasks(int freez
if (frozen(p) || !freezeable(p))
continue;

- if (!freeze_task(p, freeze_user_space))
+ if (!freeze_task(p, sig_only))
continue;

/*
@@ -235,13 +222,13 @@ int freeze_processes(void)
int error;

printk("Freezing user space processes ... ");
- error = try_to_freeze_tasks(FREEZER_USER_SPACE);
+ error = try_to_freeze_tasks(true);
if (error)
goto Exit;
printk("done.\n");

printk("Freezing remaining freezable tasks ... ");
- error = try_to_freeze_tasks(FREEZER_KERNEL_THREADS);
+ error = try_to_freeze_tasks(false);
if (error)
goto Exit;
printk("done.");
@@ -251,7 +238,7 @@ int freeze_processes(void)
return error;
}

-static void thaw_tasks(int thaw_user_space)
+static void thaw_tasks(bool nosig_only)
{
struct task_struct *g, *p;

@@ -260,7 +247,7 @@ static void thaw_tasks(int thaw_user_spa
if (!freezeable(p))
continue;

- if (!p->mm == thaw_user_space)
+ if (nosig_only && should_send_signal(p))
continue;

thaw_process(p);
@@ -271,8 +258,8 @@ static void thaw_tasks(int thaw_user_spa
void thaw_processes(void)
{
printk("Restarting tasks ... ");
- thaw_tasks(FREEZER_KERNEL_THREADS);
- thaw_tasks(FREEZER_USER_SPACE);
+ thaw_tasks(true);
+ thaw_tasks(false);
schedule();
printk("done.\n");
}
Index: linux-2.6/kernel/kthread.c
===================================================================
--- linux-2.6.orig/kernel/kthread.c
+++ linux-2.6/kernel/kthread.c
@@ -234,7 +234,7 @@ int kthreadd(void *unused)
set_user_nice(tsk, KTHREAD_NICE_LEVEL);
set_cpus_allowed(tsk, CPU_MASK_ALL);

- current->flags |= PF_NOFREEZE;
+ current->flags |= PF_NOFREEZE | PF_FREEZER_NOSIG;

for (;;) {
set_current_state(TASK_INTERRUPTIBLE);
--
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