On Fri, Nov 04, 2005 at 02:30:35PM -0800, Andrew Morton wrote:
> Ashok Raj <[email protected]> wrote:
> >
> >
> > ...
> >
> > seems ugly, but i dont find a better looking cure...
> >
>
> Could you take another look, please? It really is pretty gross.
>
So here it is again... thanks to Andrew suggesting to take the PF_ avenue.
Sure enough the old one was gross, and there was an even groosier patch
that met infant mortality...
since the last patch in -git8 broke some implementations, this is
relative to that earlier patch.
It would be nice to have this in base sooner so cpufreq's dont break.
Thanks a ton.
--------------
When calling target drivers to set frequency, we take cpucontrol lock.
When we modified the code to accomodate CPU hotplug, there was an
attempt to take a double lock of cpucontrol leading to a deadlock.
Since the current thread context is already holding the cpucontrol lock,
we dont need to make another attempt to acquire it.
Now we leave a trace in current->flags indicating current thread already
is under cpucontrol lock held, so we dont attempt to do this another time.
Thanks to Andrew Morton for the beating:-)
Signed-off-by: Ashok Raj <[email protected]>
-----------------------------------------------------------------
drivers/cpufreq/cpufreq.c | 24 ++++++++++--------------
include/linux/cpu.h | 1 +
include/linux/sched.h | 1 +
kernel/cpu.c | 33 +++++++++++++++++++++++++++++++++
4 files changed, 45 insertions(+), 14 deletions(-)
Index: linux-2.6.14-rc4-mm1/drivers/cpufreq/cpufreq.c
===================================================================
--- linux-2.6.14-rc4-mm1.orig/drivers/cpufreq/cpufreq.c
+++ linux-2.6.14-rc4-mm1/drivers/cpufreq/cpufreq.c
@@ -38,7 +38,6 @@ static struct cpufreq_driver *cpufreq
static struct cpufreq_policy *cpufreq_cpu_data[NR_CPUS];
static DEFINE_SPINLOCK(cpufreq_driver_lock);
-
/* internal prototypes */
static int __cpufreq_governor(struct cpufreq_policy *policy, unsigned int event);
static void handle_update(void *data);
@@ -1115,24 +1114,21 @@ int __cpufreq_driver_target(struct cpufr
int retval = -EINVAL;
/*
- * Converted the lock_cpu_hotplug to preempt_disable()
- * and preempt enable. This is a bit kludgy and relies on
- * how cpu hotplug works. All we need is a gaurantee that cpu hotplug
- * wont make progress on any cpu. Once we do preempt_disable(), this
- * would ensure hotplug threads dont get on this cpu, thereby delaying
- * the cpu remove process.
- *
- * we removed the lock_cpu_hotplug since we need to call this function via
- * cpu hotplug callbacks, which result in locking the cpu hotplug
- * thread itself. Agree this is not very clean, cpufreq community
- * could improve this if required. - Ashok Raj <[email protected]>
+ * If we are already in context of hotplug thread, we dont need to
+ * acquire the hotplug lock. Otherwise acquire cpucontrol to prevent
+ * hotplug from removing this cpu that we are working on.
*/
- preempt_disable();
+ if (!current_in_cpu_hotplug())
+ lock_cpu_hotplug();
+
dprintk("target for CPU %u: %u kHz, relation %u\n", policy->cpu,
target_freq, relation);
if (cpu_online(policy->cpu) && cpufreq_driver->target)
retval = cpufreq_driver->target(policy, target_freq, relation);
- preempt_enable();
+
+ if (!current_in_cpu_hotplug())
+ unlock_cpu_hotplug();
+
return retval;
}
EXPORT_SYMBOL_GPL(__cpufreq_driver_target);
Index: linux-2.6.14-rc4-mm1/include/linux/cpu.h
===================================================================
--- linux-2.6.14-rc4-mm1.orig/include/linux/cpu.h
+++ linux-2.6.14-rc4-mm1/include/linux/cpu.h
@@ -33,6 +33,7 @@ struct cpu {
extern int register_cpu(struct cpu *, int, struct node *);
extern struct sys_device *get_cpu_sysdev(int cpu);
+extern int current_in_cpu_hotplug(void);
#ifdef CONFIG_HOTPLUG_CPU
extern void unregister_cpu(struct cpu *, struct node *);
#endif
Index: linux-2.6.14-rc4-mm1/include/linux/sched.h
===================================================================
--- linux-2.6.14-rc4-mm1.orig/include/linux/sched.h
+++ linux-2.6.14-rc4-mm1/include/linux/sched.h
@@ -880,6 +880,7 @@ do { if (atomic_dec_and_test(&(tsk)->usa
#define PF_SYNCWRITE 0x00200000 /* I am doing a sync write */
#define PF_BORROWED_MM 0x00400000 /* I am a kthread doing use_mm */
#define PF_RANDOMIZE 0x00800000 /* randomize virtual address space */
+#define PF_HOTPLUG_CPU 0x01000000 /* Currently performing CPU hotplug */
/*
* Only the _current_ task can read/write to tsk->flags, but other
Index: linux-2.6.14-rc4-mm1/kernel/cpu.c
===================================================================
--- linux-2.6.14-rc4-mm1.orig/kernel/cpu.c
+++ linux-2.6.14-rc4-mm1/kernel/cpu.c
@@ -20,6 +20,24 @@ DECLARE_MUTEX(cpucontrol);
static struct notifier_block *cpu_chain;
+/*
+ * Used to check by callers if they need to acquire the cpucontrol
+ * or not to protect a cpu from being removed. Its sometimes required to
+ * call these functions both for normal operations, and in response to
+ * a cpu being added/removed. If the context of the call is in the same
+ * thread context as a CPU hotplug thread, we dont need to take the lock
+ * since its already protected
+ * check drivers/cpufreq/cpufreq.c for its usage - Ashok Raj
+ */
+
+int current_in_cpu_hotplug(void)
+{
+ return (current->flags & PF_HOTPLUG_CPU);
+}
+
+EXPORT_SYMBOL_GPL(current_in_cpu_hotplug);
+
+
/* Need to know about CPUs going up/down? */
int register_cpu_notifier(struct notifier_block *nb)
{
@@ -93,6 +111,13 @@ int cpu_down(unsigned int cpu)
goto out;
}
+ /*
+ * Leave a trace in current->flags indicating we are already in
+ * process of performing CPU hotplug. Callers can check if cpucontrol
+ * is already acquired by current thread, and if so not cause
+ * a dead lock by not acquiring the lock
+ */
+ current->flags |= PF_HOTPLUG_CPU;
err = notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
(void *)(long)cpu);
if (err == NOTIFY_BAD) {
@@ -145,6 +170,7 @@ out_thread:
out_allowed:
set_cpus_allowed(current, old_allowed);
out:
+ current->flags &= ~PF_HOTPLUG_CPU;
unlock_cpu_hotplug();
return err;
}
@@ -162,6 +188,12 @@ int __devinit cpu_up(unsigned int cpu)
ret = -EINVAL;
goto out;
}
+
+ /*
+ * Leave a trace in current->flags indicating we are already in
+ * process of performing CPU hotplug.
+ */
+ current->flags |= PF_HOTPLUG_CPU;
ret = notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu);
if (ret == NOTIFY_BAD) {
printk("%s: attempt to bring up CPU %u failed\n",
@@ -184,6 +216,7 @@ out_notify:
if (ret != 0)
notifier_call_chain(&cpu_chain, CPU_UP_CANCELED, hcpu);
out:
+ current->flags &= ~PF_HOTPLUG_CPU;
up(&cpucontrol);
return ret;
}
-
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