[RFC PATCH v2 00/10][RESEND] CPU hotplug: stop_machine()-free CPU hotplug

[RFC PATCH v2 00/10][RESEND] CPU hotplug: stop_machine()-free CPU hotplug

[Srivatsa S. Bhat]

[Resending because I forgot to CC lkml last time. Please send your replies to
this thread instead of the previous (non-public) one. Sorry for the trouble!]

Hi,

This patchset removes CPU hotplug's dependence on stop_machine() from the CPU
offline path and provides an alternative (set of APIs) to preempt_disable() to
prevent CPUs from going offline, which can be invoked from atomic context.

This is an RFC patchset with only a few call-sites of preempt_disable()
converted to the new APIs for now, and the main goal is to get feedback on the
design of the new atomic APIs and see if it serves as a viable replacement for
stop_machine()-free CPU hotplug.

Overview of the patches:
-----------------------

Patch 1 introduces the new APIs that can be used from atomic context, to
prevent CPUs from going offline. This set of APIs is for "light" atomic
hotplug readers (the term "light" is explained in the changelog/comments).

Patch 2 introduces the new APIs for "full" atomic hotplug readers.

Patch 3 is a cleanup; it converts preprocessor macros to static inline
functions.

Patches 4 to 9 convert various call-sites to use the new APIs.
(Patches 4 to 8 deal with "light" readers, patch 9 deals with a "full"
reader. Together, they demonstrate how to use these APIs effectively).

Patch 10 is the one which actually removes stop_machine() from the CPU
offline path.


Changes in v2:
-------------
* Completely redesigned the synchronization scheme to avoid using any extra
  cpumasks.

* Provided APIs for 2 types of atomic hotplug readers: "light" (for
  light-weight) and "full". We wish to have more "light" readers than
  the "full" ones, to avoid indirectly inducing the "stop_machine effect"
  without even actually using stop_machine().

  And the patches show that it _is_ generally true: 5 patches deal with
  "light" readers, whereas only 1 patch deals with a "full" reader.

  Also, the "light" readers happen to be in very hot paths. So it makes a
  lot of sense to have such a distinction and a corresponding light-weight
  API.

v1: https://lkml.org/lkml/2012/12/4/88

Comments and suggestions welcome!

--
 Paul E. McKenney (1):
      cpu: No more __stop_machine() in _cpu_down()

Srivatsa S. Bhat (9):
      CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline
      CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline
      CPU hotplug: Convert preprocessor macros to static inline functions
      smp, cpu hotplug: Fix smp_call_function_*() to prevent CPU offline properly
      smp, cpu hotplug: Fix on_each_cpu_*() to prevent CPU offline properly
      sched, cpu hotplug: Use stable online cpus in try_to_wake_up() & select_task_rq()
      kick_process(), cpu-hotplug: Prevent offlining of target CPU properly
      yield_to(), cpu-hotplug: Prevent offlining of other CPUs properly
      kvm, vmx: Add full atomic synchronization with CPU Hotplug


  arch/x86/kvm/vmx.c  |    8 ++-
 include/linux/cpu.h |   12 ++++-
 kernel/cpu.c        |  136 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 kernel/sched/core.c |   22 +++++++-
 kernel/smp.c        |   64 +++++++++++++++---------
 5 files changed, 210 insertions(+), 32 deletions(-)



Thanks,
Srivatsa S. Bhat
IBM Linux Technology Center

[RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

There are places where preempt_disable() is used to prevent any CPU from
going offline during the critical section. Let us call them as "atomic
hotplug readers" (atomic because they run in atomic contexts).

Often, these atomic hotplug readers have a simple need : they want the cpu
online mask that they work with (inside their critical section), to be
stable, i.e., it should be guaranteed that CPUs in that mask won't go
offline during the critical section. The important point here is that
they don't really need to synchronize with the actual CPU tear-down
sequence. All they need is synchronization with the updates to the
cpu_online_mask. (Hence the term "light", for light-weight).

The intent of this patch is to provide synchronization APIs for such
"light" atomic hotplug readers. [ get/put_online_cpus_atomic_light() ]

Fundamental idea behind the design:
-----------------------------------

Simply put, in the hotplug writer path, have appropriate locking around the
update to the cpu_online_mask in the CPU tear-down sequence. And once the
update is done, release the lock and allow the "light" atomic hotplug readers
to go ahead. Meanwhile, the hotplug writer can safely continue the actual
CPU tear-down sequence (running CPU_DYING notifiers etc) since the "light"
atomic readers don't really care about those operations (and hence don't
need to synchronize with them).

Also, once the hotplug writer completes taking the CPU offline, it should
not start any new cpu_down() operations until all existing "light" atomic
hotplug readers have completed.

Some important design requirements and considerations:
-----------------------------------------------------

1. The "light" atomic hotplug readers should ideally *never* have to wait
   for the hotplug writer (cpu_down()) for too long (like entire duration
   of CPU offline, for example). Because, these atomic hotplug readers can be
   in very hot-paths like interrupt handling/IPI and hence, if they have to
   wait for an ongoing cpu_down() to complete, it would pretty much
   introduce the same performance/latency problems as stop_machine().

2. Any synchronization at the atomic hotplug readers side must be highly
   scalable - avoid global single-holder locks/counters etc. Because, these
   paths currently use the extremely fast preempt_disable(); our replacement
   to preempt_disable() should not become ridiculously costly and also should
   not serialize the readers among themselves needlessly.

3. preempt_disable() was recursive. The replacement should also be recursive.

Implementation of the design:
----------------------------

At the core, we use a reader-writer lock to synchronize the update to the
cpu_online_mask. That way, multiple "light" atomic hotplug readers can co-exist
and the writer can acquire the lock only when all the readers have completed.

Once acquired, the writer holds the "light" lock only during the duration of
the update to the cpu_online_mask. That way, the readers don't have to spin
for too long (ie., the write-hold-time for the "light" lock is tiny), which
keeps the readers in good shape.

Reader-writer lock are recursive, so they can be used in a nested fashion in
the reader-path. Together, these satisfy all the 3 requirements mentioned above.

Also, since we don't use per-cpu locks (because rwlocks themselves are quite
scalable for readers), we don't end up in any lock ordering problems that can
occur if we try to use per-cpu locks.

I'm indebted to Michael Wang and Xiao Guangrong for their numerous thoughtful
suggestions and ideas, which inspired and influenced many of the decisions in
this as well as previous designs. Thanks a lot Michael and Xiao!

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/cpu.h |    4 ++
 kernel/cpu.c        |   87 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 90 insertions(+), 1 deletion(-)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index ce7a074..dd0a3ee 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -175,6 +175,8 @@ extern struct bus_type cpu_subsys;
 
 extern void get_online_cpus(void);
 extern void put_online_cpus(void);
+extern void get_online_cpus_atomic_light(void);
+extern void put_online_cpus_atomic_light(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
@@ -198,6 +200,8 @@ static inline void cpu_hotplug_driver_unlock(void)
 
 #define get_online_cpus()	do { } while (0)
 #define put_online_cpus()	do { } while (0)
+#define get_online_cpus_atomic_light()	do { } while (0)
+#define put_online_cpus_atomic_light()	do { } while (0)
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 42bd331..381593c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -49,6 +49,69 @@ static int cpu_hotplug_disabled;
 
 #ifdef CONFIG_HOTPLUG_CPU
 
+/*
+ * Reader-writer lock to synchronize between "light" atomic hotplug readers
+ * and the hotplug writer while updating cpu_online_mask.
+ * "Light" atomic hotplug readers are those who don't really need to
+ * synchronize with the actual CPU bring-up/take-down sequence, but only
+ * need to synchronize with the updates to the cpu_online_mask.
+ */
+static DEFINE_RWLOCK(light_hotplug_rwlock);
+
+/*
+ * Hotplug readers (those that want to prevent CPUs from coming online or
+ * going offline ) sometimes run from atomic contexts, and hence can't use
+ * get/put_online_cpus() because they can sleep. And often-times, all
+ * they really want is that the cpu_online_mask remain unchanged while
+ * they are executing in their critical section. They also don't really
+ * need to synchronize with the actual CPU tear-down sequence. Such atomic
+ * hotplug readers are called "light" readers (light for light-weight).
+ *
+ * These "light" atomic hotplug readers can use the APIs
+ * get/put_online_atomic_light() around their critical sections to
+ * ensure that the cpu_online_mask remains unaltered throughout that
+ * critical section.
+ *
+ * Caution!: While the readers are in their critical section, a CPU offline
+ * operation can actually happen under the covers; its just that the bit flip
+ * in the cpu_online_mask will be synchronized properly if you use these APIs.
+ * If you really want full synchronization with the entire CPU tear-down
+ * sequence, then you are not a "light" hotplug reader. So don't use these
+ * APIs!
+ *
+ * Eg:
+ *
+ * "Light" atomic hotplug read-side critical section:
+ * --------------------------------------------------
+ *
+ * get_online_cpus_atomic_light();
+ *
+ * for_each_online_cpu(cpu) {
+ *         ... Do something...
+ * }
+ *    ...
+ *
+ * if (cpu_online(other_cpu))
+ *         do_something();
+ *
+ * put_online_cpus_atomic_light();
+ *
+ * You can call this function recursively.
+ */
+void get_online_cpus_atomic_light(void)
+{
+	preempt_disable();
+	read_lock(&light_hotplug_rwlock);
+}
+EXPORT_SYMBOL_GPL(get_online_cpus_atomic_light);
+
+void put_online_cpus_atomic_light(void)
+{
+	read_unlock(&light_hotplug_rwlock);
+	preempt_enable();
+}
+EXPORT_SYMBOL_GPL(put_online_cpus_atomic_light);
+
 static struct {
 	struct task_struct *active_writer;
 	struct mutex lock; /* Synchronizes accesses to refcount, */
@@ -246,14 +309,36 @@ struct take_cpu_down_param {
 static int __ref take_cpu_down(void *_param)
 {
 	struct take_cpu_down_param *param = _param;
+	unsigned long flags;
 	int err;
 
+	/*
+	 *  __cpu_disable() is the step where the CPU is removed from the
+	 *  cpu_online_mask. Protect it with the light-lock held for write.
+	 */
+	write_lock_irqsave(&light_hotplug_rwlock, flags);
+
 	/* Ensure this CPU doesn't handle any more interrupts. */
 	err = __cpu_disable();
-	if (err < 0)
+	if (err < 0) {
+		write_unlock_irqrestore(&light_hotplug_rwlock, flags);
 		return err;
+	}
+
+	/*
+	 * We have successfully removed the CPU from the cpu_online_mask.
+	 * So release the light-lock, so that the light-weight atomic readers
+	 * (who care only about the cpu_online_mask updates, and not really
+	 * about the actual cpu-take-down operation) can continue.
+	 *
+	 * But don't enable interrupts yet, because we still have work left to
+	 * do, to actually bring the CPU down.
+	 */
+	write_unlock(&light_hotplug_rwlock);
 
 	cpu_notify(CPU_DYING | param->mod, param->hcpu);
+
+	local_irq_restore(flags);
 	return 0;
 }
 

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

Replaying what Tejun wrote:

(cc'ing Oleg)

Hello, Srivatsa.

On 12/06/2012 12:13 AM, Srivatsa S. Bhat wrote:
> Also, since we don't use per-cpu locks (because rwlocks themselves are quite
> scalable for readers), we don't end up in any lock ordering problems that can
> occur if we try to use per-cpu locks.
> 

Read-lock really isn't that scalable when you compare it to
preempt_disable/enable().  When used on hot paths, it's gonna generate
a lot of cacheline pingpongs.  This patch is essentially creating a
new big lock which has potential for being very hot.

preempt_disable/enable() + stop_machine() essentially works as percpu
rwlock with very heavy penalty on the writer side.  Because the reader
side doesn't even implement spinning while writer is in progress, the
writer side has to preempt the readers before entering critical
section and that's what the "stopping machine" is about.

Note that the resolution on the reader side is very low.  Any section
w/ preemption disabled is protected against stop_machine().  Also, the
stop_machine() itself is extremely heavy involving essentially locking
up the machine until all CPUs can reach the same condition via
scheduling the stop_machine tasks.  So, I *think* all you need to do
here is making cpu online locking finer grained (separated from
preemption) and lighten the writer side a bit.  I'm quite doubtful
that you would need to go hunting donw all get_online_cpus().  They
aren't used that often anyway.

Anyways, so, separating out cpu hotplug locking from preemption is the
right thing to do but I think rwlock is likely to be too heavy on the
reader side.  I think percpu reader accounting + reader spinning while
writer in progress should be a good combination.  It's a bit heavier
than preempt_disable() - it'll have an extra conditional jump on the
hot path, but there won't be any cacheline bouncing.  The writer side
would need to synchronize against all CPUs but only against the ones
actually read locking cpu hotplug.  As long as reader side critical
sections don't go crazy, it should be okay.

So, we basically need percpu_rwlock.  We already have percpu_rwsem.
We used to have some different variants of writer-heavy locks.  Dunno
what happened to them.  Maybe we still have it somewhere.  Oleg has
been working on the area lately and should know more.  Oleg, it seems
CPU hotplug needs big-reader rwlock, ideas on how to proceed?

Thanks.

-- tejun 

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

Replaying what Oleg wrote:

Hi,

Sorry I don't understand the context and I can't find this thread
anywhere, so I am not sure I understand...

> Replaying what Tejun wrote:
> So, we basically need percpu_rwlock.  We already have percpu_rwsem.

Yes, and with -mm patches it becomes reader-friendly. In particular
see http://marc.info/?l=linux-mm-commits&m=135240650828875

> Oleg, it seems
> CPU hotplug needs big-reader rwlock, ideas on how to proceed?
> 

I am going to convert get_online_cpus() to use percpu_down_read(),
this looks simple.

We already discussed this with Paul, see

	http://marc.info/?l=linux-kernel&m=135248463226031

and the whole thread.

In short, all we need is percpu_down_write_recursive_readers() and
afaics the only complication is lockdep, we need down_read_no_lockdep()
which (like __up_read) doesn't do rwsem_acquire_read().

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

Replaying what Tejun wrote:

Hello, Oleg.

> Replaying what Oleg wrote:
> 
> Hi,
> 
> Sorry I don't understand the context and I can't find this thread
> anywhere, so I am not sure I understand...
> 

Weird, lkml cc is missing.  Srivatsa?

[Now fixed. This thread has lkml CC]

>> Replaying what Tejun wrote:
>> So, we basically need percpu_rwlock.  We already have percpu_rwsem.
> 
> Yes, and with -mm patches it becomes reader-friendly. In particular
> see http://marc.info/?l=linux-mm-commits&m=135240650828875
> 
>> Oleg, it seems
>> CPU hotplug needs big-reader rwlock, ideas on how to proceed?
>>
> 
> I am going to convert get_online_cpus() to use percpu_down_read(),
> this looks simple.
> 
> We already discussed this with Paul, see
> 
> 	http://marc.info/?l=linux-kernel&m=135248463226031
> 
> and the whole thread.
> 
> In short, all we need is percpu_down_write_recursive_readers() and
> afaics the only complication is lockdep, we need down_read_no_lockdep()
> which (like __up_read) doesn't do rwsem_acquire_read().
> 

So, it's a different thing.  There are two mechanism protecting
against cpu hotplug - get_online_cpus() and preempt_disable().  The
former can be used by ones which can sleep and need to protect against
the whole up/down process (DOWN_PREPARE and so on).  The latter
protects the last step and can be used when the caller can't sleep.
Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
is about replacing preempt_disable with something finer grained and
less heavy on the writer side - IOW, percpu_rwlock as opposed to
percpu_rwsem, so, I think the end result would be that CPU hotplug
will be protected by percpu_rwsem for the whole part and by
percpu_rwlock for the last commit stage.

The problem seems that we don't have percpu_rwlock yet.  It shouldn't
be too difficult to implement, right?

Thanks.

-- tejun 

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

Replaying what Oleg wrote:

(add lkml)

> Replaying what Tejun wrote:

> Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
> is about replacing preempt_disable with something finer grained and
> less heavy on the writer side

If only I understood why preempt_disable() is bad ;-)

OK, I guess "less heavy on the writer side" is the hint, and in the
previous email you mentioned that "stop_machine() itself is extremely
heavy".

Looks like, you are going to remove stop_machine() from cpu_down ???
 
> The problem seems that we don't have percpu_rwlock yet.  It shouldn't
> be too difficult to implement, right?
> 

Oh, I am not sure... unless you simply copy-and-paste the lglock code
and replace spinlock_t with rwlock_t.

We probably want something more efficient, but I bet we can't avoid
the barriers on the read side.

And somehow we should avoid the livelocks. Say, we can't simply add
the per_cpu_reader_counter, _read_lock should spin if the writer is
active. But at the same time _read_lock should be recursive.

Tejun, could you please send me mbox with this thread offlist?

[That should now be unnecessary, since the discussion can continue
on-list on this thread].

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

Replaying what Tejun wrote:

Hello, Oleg.

> Replaying what Oleg wrote:
>> Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
>> is about replacing preempt_disable with something finer grained and
>> less heavy on the writer side
> 
> If only I understood why preempt_disable() is bad ;-)
> 
> OK, I guess "less heavy on the writer side" is the hint, and in the
> previous email you mentioned that "stop_machine() itself is extremely
> heavy".
> 
> Looks like, you are going to remove stop_machine() from cpu_down ???
>

Yeah, that's what Srivatsa is trying to do.  The problem seems to be
that cpu up/down is very frequent on certain mobile platforms for
power management and as currently implemented cpu hotplug is too heavy
and latency-inducing.
  
>> The problem seems that we don't have percpu_rwlock yet.  It shouldn't
>> be too difficult to implement, right?
>>
> 
> Oh, I am not sure... unless you simply copy-and-paste the lglock code
> and replace spinlock_t with rwlock_t.
> 

Ah... right, so that's where brlock ended up.  So, lglock is the new
thing and brlock is a wrapper around it.

> We probably want something more efficient, but I bet we can't avoid
> the barriers on the read side.
> 
> And somehow we should avoid the livelocks. Say, we can't simply add
> the per_cpu_reader_counter, _read_lock should spin if the writer is
> active. But at the same time _read_lock should be recursive.
> 

I think we should just go with lglock.  It does involve local atomic
ops but atomic ops themselves aren't that expensive and it's not like
we can avoid memory barriers.  Also, that's the non-sleeping
counterpart of percpu_rwsem.  If it's not good enough for some reason,
we should improve it rather than introducing something else.

Thanks.

-- tejun

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

> Replaying what Tejun wrote:
> 
> Hello, Oleg.
> 
>> Replaying what Oleg wrote:
>>> Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
>>> is about replacing preempt_disable with something finer grained and
>>> less heavy on the writer side
>>
>> If only I understood why preempt_disable() is bad ;-)
>>
>> OK, I guess "less heavy on the writer side" is the hint, and in the
>> previous email you mentioned that "stop_machine() itself is extremely
>> heavy".
>>
>> Looks like, you are going to remove stop_machine() from cpu_down ???
>>
> 
> Yeah, that's what Srivatsa is trying to do.  The problem seems to be
> that cpu up/down is very frequent on certain mobile platforms for
> power management and as currently implemented cpu hotplug is too heavy
> and latency-inducing.
>   
>>> The problem seems that we don't have percpu_rwlock yet.  It shouldn't
>>> be too difficult to implement, right?
>>>
>>
>> Oh, I am not sure... unless you simply copy-and-paste the lglock code
>> and replace spinlock_t with rwlock_t.
>>
> 
> Ah... right, so that's where brlock ended up.  So, lglock is the new
> thing and brlock is a wrapper around it.
> 
>> We probably want something more efficient, but I bet we can't avoid
>> the barriers on the read side.
>>
>> And somehow we should avoid the livelocks. Say, we can't simply add
>> the per_cpu_reader_counter, _read_lock should spin if the writer is
>> active. But at the same time _read_lock should be recursive.
>>
> 
> I think we should just go with lglock.  It does involve local atomic
> ops but atomic ops themselves aren't that expensive and it's not like
> we can avoid memory barriers.  Also, that's the non-sleeping
> counterpart of percpu_rwsem.  If it's not good enough for some reason,
> we should improve it rather than introducing something else.
> 

While working on the v2 yesterday, I had actually used rwlocks for
the light readers and atomic ops for the full-readers. (Later I changed
both to rwlocks while posting this v2). Anyway, the atomic ops version
looked something like shown below.

I'll take a look at lglocks and see if that helps in our case.

Regards,
Srivatsa S. Bhat


---

 include/linux/cpu.h |    4 ++
 kernel/cpu.c        |   98 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+)


diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index c64b6ed..5011c7d 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -177,6 +177,8 @@ extern void get_online_cpus(void);
 extern void put_online_cpus(void);
 extern void get_online_cpus_stable_atomic(void);
 extern void put_online_cpus_stable_atomic(void);
+extern void get_online_cpus_atomic(void);
+extern void put_online_cpus_atomic(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
@@ -202,6 +204,8 @@ static inline void cpu_hotplug_driver_unlock(void)
 #define put_online_cpus()	do { } while (0)
 #define get_online_cpus_stable_atomic()	do { } while (0)
 #define put_online_cpus_stable_atomic()	do { } while (0)
+#define get_online_cpus_atomic()	do { } while (0)
+#define put_online_cpus_atomic()	do { } while (0)
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 8c9eecc..76b07f7 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -19,6 +19,7 @@
 #include <linux/mutex.h>
 #include <linux/gfp.h>
 #include <linux/suspend.h>
+#include <linux/atomic.h>
 
 #include "smpboot.h"
 
@@ -104,6 +105,58 @@ void put_online_cpus_stable_atomic(void)
 }
 EXPORT_SYMBOL_GPL(put_online_cpus_stable_atomic);
 
+static DEFINE_PER_CPU(atomic_t, atomic_reader_refcount);
+
+#define writer_active(v)	((v) < 0)
+#define reader_active(v)	((v) > 0)
+
+/*
+ * Invoked by hotplug reader, to prevent CPUs from going offline.
+ * Increments its per-cpu 'atomic_reader_refcount' to mark itself as being
+ * active.
+ *
+ * If 'atomic_reader_refcount' is negative, it means that a CPU offline
+ * operation is in progress (hotplug writer). Wait for it to complete
+ * and then mark your presence (increment the count) and return.
+ *
+ * You can call this recursively, because it doesn't hold any locks.
+ *
+ * Returns with preemption disabled.
+ */
+void get_online_cpus_atomic(void)
+{
+	int c, old;
+
+	preempt_disable();
+	read_lock(&hotplug_rwlock);
+
+	for (;;) {
+		c = atomic_read(&__get_cpu_var(atomic_reader_refcount));
+		if (unlikely(writer_active(c))) {
+			cpu_relax();
+			continue;
+		}
+
+		old = atomic_cmpxchg(&__get_cpu_var(atomic_reader_refcount),
+				     c, c + 1);
+
+		if (likely(old == c))
+			break;
+
+		c = old;
+	}
+}
+EXPORT_SYMBOL_GPL(get_online_cpus_atomic);
+
+void put_online_cpus_atomic(void)
+{
+	atomic_dec(&__get_cpu_var(atomic_reader_refcount));
+	smp_mb__after_atomic_dec();
+	read_unlock(&hotplug_rwlock);
+	preempt_enable();
+}
+EXPORT_SYMBOL_GPL(put_online_cpus_atomic);
+
 static struct {
 	struct task_struct *active_writer;
 	struct mutex lock; /* Synchronizes accesses to refcount, */
@@ -292,6 +345,42 @@ static inline void check_for_tasks(int cpu)
 	write_unlock_irq(&tasklist_lock);
 }
 
+/*
+ * Invoked by hotplug writer, in preparation to take a CPU offline.
+ * Decrements the per-cpu 'atomic_reader_refcount' to mark itself as being
+ * active.
+ *
+ * If 'atomic_reader_refcount' is positive, it means that there are active
+ * hotplug readers (those that prevent hot-unplug). Wait for them to complete
+ * and then mark your presence (decrement the count) and return.
+ */
+static void disable_atomic_reader(unsigned int cpu)
+{
+	int c, old;
+
+	for (;;) {
+		c = atomic_read(&per_cpu(atomic_reader_refcount, cpu));
+		if (likely(reader_active(c))) {
+			cpu_relax();
+			continue;
+		}
+
+		old = atomic_cmpxchg(&per_cpu(atomic_reader_refcount, cpu),
+				     c, c - 1);
+
+		if (likely(old == c))
+			break;
+
+		c = old;
+	}
+}
+
+static void enable_atomic_reader(unsigned int cpu)
+{
+	atomic_inc(&per_cpu(atomic_reader_refcount, cpu));
+	smp_mb__after_atomic_inc();
+}
+
 struct take_cpu_down_param {
 	unsigned long mod;
 	void *hcpu;
@@ -302,6 +391,7 @@ static int __ref take_cpu_down(void *_param)
 {
 	struct take_cpu_down_param *param = _param;
 	unsigned long flags;
+	unsigned int cpu;
 	int err;
 
 	/*
@@ -317,6 +407,10 @@ static int __ref take_cpu_down(void *_param)
 		return err;
 	}
 
+	/* Disable the atomic hotplug readers who need full synchronization */
+	for_each_online_cpu(cpu)
+		disable_atomic_reader(cpu);
+
 	/*
 	 * We have successfully removed the CPU from the cpu_online_mask.
 	 * So release the lock, so that the light-weight atomic readers (who care
@@ -330,6 +424,10 @@ static int __ref take_cpu_down(void *_param)
 
 	cpu_notify(CPU_DYING | param->mod, param->hcpu);
 
+	/* Enable the atomic hotplug readers who need full synchronization */
+	for_each_online_cpu(cpu)
+		enable_atomic_reader(cpu);
+
 	local_irq_restore(flags);
 	return 0;
 }

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov]

On 12/06, Srivatsa S. Bhat wrote:
>
> +void get_online_cpus_atomic(void)
> +{
> +	int c, old;
> +
> +	preempt_disable();
> +	read_lock(&hotplug_rwlock);

Confused... Why it also takes hotplug_rwlock?

> +
> +	for (;;) {
> +		c = atomic_read(&__get_cpu_var(atomic_reader_refcount));
> +		if (unlikely(writer_active(c))) {
> +			cpu_relax();
> +			continue;
> +		}
> +
> +		old = atomic_cmpxchg(&__get_cpu_var(atomic_reader_refcount),
> +				     c, c + 1);
> +
> +		if (likely(old == c))
> +			break;
> +
> +		c = old;
> +	}
> +}

	while (!atomic_inc_unless_negative(...))
		cpu_relax();
	
and atomic_dec_unless_positive() in disable_atomic_reader().


Obviously you can't use get_online_cpus_atomic() under rq->lock or
task->pi_lock or any other lock CPU_DYING can take. Probably this is
fine, but perhaps it makes sense to add the lockdep annotations.

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
> On 12/06, Srivatsa S. Bhat wrote:
>>
>> +void get_online_cpus_atomic(void)
>> +{
>> +	int c, old;
>> +
>> +	preempt_disable();
>> +	read_lock(&hotplug_rwlock);
> 
> Confused... Why it also takes hotplug_rwlock?

To avoid ABBA deadlocks.

hotplug_rwlock was meant for the "light" readers.
The atomic counters were meant for the "heavy/full" readers.
I wanted them to be able to nest in any manner they wanted,
such as:

Full inside light:

get_online_cpus_atomic_light()
	...
	get_online_cpus_atomic_full()
	...
	put_online_cpus_atomic_full()
	...
put_online_cpus_atomic_light()

Or, light inside full:

get_online_cpus_atomic_full()
	...
	get_online_cpus_atomic_light()
	...
	put_online_cpus_atomic_light()
	...
put_online_cpus_atomic_full()

To allow this, I made the two sets of APIs take the locks
in the same order internally.

(I had some more description of this logic in the changelog
of 2/10; the only difference there is that instead of atomic
counters, I used rwlocks for the full-readers as well.
https://lkml.org/lkml/2012/12/5/320)

> 
>> +
>> +	for (;;) {
>> +		c = atomic_read(&__get_cpu_var(atomic_reader_refcount));
>> +		if (unlikely(writer_active(c))) {
>> +			cpu_relax();
>> +			continue;
>> +		}
>> +
>> +		old = atomic_cmpxchg(&__get_cpu_var(atomic_reader_refcount),
>> +				     c, c + 1);
>> +
>> +		if (likely(old == c))
>> +			break;
>> +
>> +		c = old;
>> +	}
>> +}
> 
> 	while (!atomic_inc_unless_negative(...))
> 		cpu_relax();
> 	
> and atomic_dec_unless_positive() in disable_atomic_reader().
> 

Ah, great! I was searching for them while writing the code, but somehow
overlooked them and rolled out my own. ;-)

> 
> Obviously you can't use get_online_cpus_atomic() under rq->lock or
> task->pi_lock or any other lock CPU_DYING can take. Probably this is
> fine, but perhaps it makes sense to add the lockdep annotations.
> 

Hmm, you are right. We can't use _atomic() in the CPU_DYING path.
So how about altering it to _allow_ that, instead of teaching lockdep
that we don't allow it? I mean, just like how the existing
get_online_cpus() allows such calls in the writer?

(I haven't thought it through; just thinking aloud...)

Regards,
Srivatsa S. Bhat

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/07/2012 12:18 AM, Srivatsa S. Bhat wrote:
> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
>> On 12/06, Srivatsa S. Bhat wrote:
>>>
>>> +void get_online_cpus_atomic(void)
>>> +{
>>> +	int c, old;
>>> +
>>> +	preempt_disable();
>>> +	read_lock(&hotplug_rwlock);
>>
>> Confused... Why it also takes hotplug_rwlock?
> 
> To avoid ABBA deadlocks.
> 
> hotplug_rwlock was meant for the "light" readers.
> The atomic counters were meant for the "heavy/full" readers.
> I wanted them to be able to nest in any manner they wanted,
> such as:
> 
> Full inside light:
> 
> get_online_cpus_atomic_light()
> 	...
> 	get_online_cpus_atomic_full()
> 	...
> 	put_online_cpus_atomic_full()
> 	...
> put_online_cpus_atomic_light()
> 
> Or, light inside full:
> 
> get_online_cpus_atomic_full()
> 	...
> 	get_online_cpus_atomic_light()
> 	...
> 	put_online_cpus_atomic_light()
> 	...
> put_online_cpus_atomic_full()
> 
> To allow this, I made the two sets of APIs take the locks
> in the same order internally.
> 
> (I had some more description of this logic in the changelog
> of 2/10; the only difference there is that instead of atomic
> counters, I used rwlocks for the full-readers as well.
> https://lkml.org/lkml/2012/12/5/320)
> 

One of the reasons why I changed everybody to global rwlocks
instead of per-cpu atomic counters was to avoid lock ordering
related deadlocks associated with per-cpu locking.

Eg:

CPU 0                         CPU 1
------                        ------

1. Acquire lock A             Increment CPU1's
                              atomic counter



2. Increment CPU0's            Try to acquire lock A
   atomic counter


Now consider what happens if a hotplug writer (cpu_down) begins,
and starts looking at CPU0, to try to decrement its atomic counter,
in between steps 1 and 2.

The hotplug writer will be successful in CPU0 because CPU0 hasn't
yet incremented its counter. So, now CPU0 spins waiting for the
hotplug writer to reset the atomic counter again.

When the hotplug writer looks at CPU1, it can't decrement the
atomic counter because CPU1 has already incremented it. So the
hotplug writer waits. CPU1 goes ahead, only to start spinning on
lock A which was acquired by CPU0. So we end up in a deadlock due
to circular locking dependency between the 3 entities.

One way to deal with this would be that the writer should abort
its loop of trying to atomic_dec per-cpu counters, whenever it has
to wait. But that might prove to be too wasteful (and overly
paranoid) in practice.

So, instead, if we have global locks (like global rwlocks that I
finally used when posting out this v2), then we won't end up in
such messy issues.

And why exactly was I concerned about all this?
Because, the current code uses the extremely flexible preempt_disable()
/preempt_enable() pair which impose absolutely no ordering
restrictions. And probably the existing code _depends_ on that.
So if our new APIs that replace preempt_disable/enable start
imposing ordering restrictions, I feared that they might become
unusable. Hence I used global rwlocks, thereby trading cache-line
bouncing (due to global rwlocks) for lock-safety and flexibility.

Regards,
Srivatsa S. Bhat

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov]

On 12/07, Srivatsa S. Bhat wrote:
>
> CPU 0                         CPU 1
> ------                        ------
>
> 1. Acquire lock A             Increment CPU1's
>                               atomic counter
>
>
>
> 2. Increment CPU0's            Try to acquire lock A
>    atomic counter
>
>
> Now consider what happens if a hotplug writer (cpu_down) begins,

Exactly. So the fake lockdep_map should be per-cpu as well.

lglock doesn't need this because lg_local_lock() is not recursive
and lockdep can catch the bug like this. So it can look as single
lock for lockdep.

IOW. If you use the global lockdep_map and want lockdep to notice
this deadlock you need rwlock_acquire_read(map, 0, 0, IP). But then
lockdep will complain if the task does "read lock" under "read lock".

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Steven Rostedt]

On Fri, 2012-12-07 at 00:18 +0530, Srivatsa S. Bhat wrote:
> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
> > On 12/06, Srivatsa S. Bhat wrote:
> >>
> >> +void get_online_cpus_atomic(void)
> >> +{
> >> +	int c, old;
> >> +
> >> +	preempt_disable();
> >> +	read_lock(&hotplug_rwlock);
> > 
> > Confused... Why it also takes hotplug_rwlock?
> 
> To avoid ABBA deadlocks.
> 
> hotplug_rwlock was meant for the "light" readers.
> The atomic counters were meant for the "heavy/full" readers.
> I wanted them to be able to nest in any manner they wanted,
> such as:
> 
> Full inside light:
> 
> get_online_cpus_atomic_light()
> 	...
> 	get_online_cpus_atomic_full()
> 	...
> 	put_online_cpus_atomic_full()
> 	...
> put_online_cpus_atomic_light()
> 
> Or, light inside full:
> 
> get_online_cpus_atomic_full()
> 	...
> 	get_online_cpus_atomic_light()
> 	...
> 	put_online_cpus_atomic_light()
> 	...
> put_online_cpus_atomic_full()
> 
> To allow this, I made the two sets of APIs take the locks
> in the same order internally.
> 
> (I had some more description of this logic in the changelog
> of 2/10; the only difference there is that instead of atomic
> counters, I used rwlocks for the full-readers as well.
> https://lkml.org/lkml/2012/12/5/320)
> 

You know reader locks can deadlock with each other, right? And this
isn't caught be lockdep yet. This is because rwlocks have been made to
be fair with writers. Before writers could be starved if a CPU always
let a reader in. Now if a writer is waiting, a reader will block behind
the writer. But this has introduced new issues with the kernel as
follows:


   CPU0			   CPU1	 	   CPU2		   CPU3
   ----			   ----		   ----		   ----
read_lock(A);
			read_lock(B)
					write_lock(A) <- block
							write_lock(B) <- block
read_lock(B) <-block

			read_lock(A) <- block

DEADLOCK!

-- Steve

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/07/2012 12:58 AM, Steven Rostedt wrote:
> On Fri, 2012-12-07 at 00:18 +0530, Srivatsa S. Bhat wrote:
>> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
>>> On 12/06, Srivatsa S. Bhat wrote:
>>>>
>>>> +void get_online_cpus_atomic(void)
>>>> +{
>>>> +	int c, old;
>>>> +
>>>> +	preempt_disable();
>>>> +	read_lock(&hotplug_rwlock);
>>>
>>> Confused... Why it also takes hotplug_rwlock?
>>
>> To avoid ABBA deadlocks.
>>
>> hotplug_rwlock was meant for the "light" readers.
>> The atomic counters were meant for the "heavy/full" readers.
>> I wanted them to be able to nest in any manner they wanted,
>> such as:
>>
>> Full inside light:
>>
>> get_online_cpus_atomic_light()
>> 	...
>> 	get_online_cpus_atomic_full()
>> 	...
>> 	put_online_cpus_atomic_full()
>> 	...
>> put_online_cpus_atomic_light()
>>
>> Or, light inside full:
>>
>> get_online_cpus_atomic_full()
>> 	...
>> 	get_online_cpus_atomic_light()
>> 	...
>> 	put_online_cpus_atomic_light()
>> 	...
>> put_online_cpus_atomic_full()
>>
>> To allow this, I made the two sets of APIs take the locks
>> in the same order internally.
>>
>> (I had some more description of this logic in the changelog
>> of 2/10; the only difference there is that instead of atomic
>> counters, I used rwlocks for the full-readers as well.
>> https://lkml.org/lkml/2012/12/5/320)
>>
> 
> You know reader locks can deadlock with each other, right? And this
> isn't caught be lockdep yet. This is because rwlocks have been made to
> be fair with writers. Before writers could be starved if a CPU always
> let a reader in. Now if a writer is waiting, a reader will block behind
> the writer. But this has introduced new issues with the kernel as
> follows:
> 
> 
>    CPU0			   CPU1	 	   CPU2		   CPU3
>    ----			   ----		   ----		   ----
> read_lock(A);
> 			read_lock(B)
> 					write_lock(A) <- block
> 							write_lock(B) <- block
> read_lock(B) <-block
> 
> 			read_lock(A) <- block
> 
> DEADLOCK!
> 

The root-cause of this deadlock is again lock-ordering mismatch right?
CPU0 takes locks in order A, B
CPU1 takes locks in order B, A

And the writer facilitates in actually getting deadlocked.

I avoid this in this patchset by always taking the locks in the same
order. So we won't be deadlocking like this.

Regards,
Srivatsa S. Bhat

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Steven Rostedt]

On Fri, 2012-12-07 at 01:06 +0530, Srivatsa S. Bhat wrote:
> On 12/07/2012 12:58 AM, Steven Rostedt wrote:
> > On Fri, 2012-12-07 at 00:18 +0530, Srivatsa S. Bhat wrote:
> >> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
> >>> On 12/06, Srivatsa S. Bhat wrote:
> >>>>
> >>>> +void get_online_cpus_atomic(void)
> >>>> +{
> >>>> +	int c, old;
> >>>> +
> >>>> +	preempt_disable();
> >>>> +	read_lock(&hotplug_rwlock);
> >>>
> >>> Confused... Why it also takes hotplug_rwlock?
> >>
> >> To avoid ABBA deadlocks.
> >>
> >> hotplug_rwlock was meant for the "light" readers.
> >> The atomic counters were meant for the "heavy/full" readers.
> >> I wanted them to be able to nest in any manner they wanted,
> >> such as:
> >>
> >> Full inside light:
> >>
> >> get_online_cpus_atomic_light()
> >> 	...
> >> 	get_online_cpus_atomic_full()
> >> 	...
> >> 	put_online_cpus_atomic_full()
> >> 	...
> >> put_online_cpus_atomic_light()
> >>
> >> Or, light inside full:
> >>
> >> get_online_cpus_atomic_full()
> >> 	...
> >> 	get_online_cpus_atomic_light()
> >> 	...
> >> 	put_online_cpus_atomic_light()
> >> 	...
> >> put_online_cpus_atomic_full()
> >>
> >> 



> The root-cause of this deadlock is again lock-ordering mismatch right?
> CPU0 takes locks in order A, B
> CPU1 takes locks in order B, A
> 
> And the writer facilitates in actually getting deadlocked.
> 
> I avoid this in this patchset by always taking the locks in the same
> order. So we won't be deadlocking like this.

OK, I haven't looked closely at the patch yet. I'm currently hacking on
my own problems. But just from the description above, it looked like you
were using rw_locks() to be able to inverse the order of the locks.

-- Steve

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/07/2012 03:32 AM, Steven Rostedt wrote:
> On Fri, 2012-12-07 at 01:06 +0530, Srivatsa S. Bhat wrote:
>> The root-cause of this deadlock is again lock-ordering mismatch right?
>> CPU0 takes locks in order A, B
>> CPU1 takes locks in order B, A
>>
>> And the writer facilitates in actually getting deadlocked.
>>
>> I avoid this in this patchset by always taking the locks in the same
>> order. So we won't be deadlocking like this.
> 
> OK, I haven't looked closely at the patch yet. I'm currently hacking on
> my own problems. But just from the description above, it looked like you
> were using rw_locks() to be able to inverse the order of the locks.
> 

Ah, ok, no problem! I'd be grateful if you could take a look when you
are free :-) I'll post a v3 soon, which has a completely redesigned
synchronization scheme, to address the performance concerns related to
global rwlocks that Tejun raised.

Regards,
Srivatsa S. Bhat

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov]

On 12/07, Oleg Nesterov wrote:
>
> On 12/06, Steven Rostedt wrote:
> >
> > You know reader locks can deadlock with each other, right? And this
> > isn't caught be lockdep yet. This is because rwlocks have been made to
> > be fair with writers. Before writers could be starved if a CPU always
> > let a reader in. Now if a writer is waiting, a reader will block behind
> > the writer. But this has introduced new issues with the kernel as
> > follows:
> >
> >
> >    CPU0			   CPU1	 	   CPU2		   CPU3
> >    ----			   ----		   ----		   ----
> > read_lock(A);
> > 			read_lock(B)
> > 					write_lock(A) <- block
> > 							write_lock(B) <- block
> > read_lock(B) <-block
> >
> > 			read_lock(A) <- block
> >
> > DEADLOCK!
>
> Really??? Oh I didn't know...
>
> Yes this was always true for rwsem, but rwlock_t?

Sorry, please ignore my email. I misread your email.

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Steven Rostedt]

On Mon, 2012-12-10 at 19:21 +0100, Oleg Nesterov wrote:
> On 12/07, Oleg Nesterov wrote:
> >
> > On 12/06, Steven Rostedt wrote:
> > >
> > > You know reader locks can deadlock with each other, right? And this
> > > isn't caught be lockdep yet. This is because rwlocks have been made to
> > > be fair with writers. Before writers could be starved if a CPU always
> > > let a reader in. Now if a writer is waiting, a reader will block behind
> > > the writer. But this has introduced new issues with the kernel as
> > > follows:
> > >
> > >
> > >    CPU0			   CPU1	 	   CPU2		   CPU3
> > >    ----			   ----		   ----		   ----
> > > read_lock(A);
> > > 			read_lock(B)
> > > 					write_lock(A) <- block
> > > 							write_lock(B) <- block
> > > read_lock(B) <-block
> > >
> > > 			read_lock(A) <- block
> > >
> > > DEADLOCK!
> >
> > Really??? Oh I didn't know...
> >
> > Yes this was always true for rwsem, but rwlock_t?
> 
> Sorry, please ignore my email. I misread your email.
> 

No prob, looking at what I wrote, I should have explicitly stated two
different rwlocks. The only hint that I gave about two locks was (A) and
(B). Even what I started with: "reader locks can deadlock with each
other" is a bit ambiguous. So I can easily see the confusion.

-- Steve

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov]

On 12/07, Srivatsa S. Bhat wrote:
>
> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
> > On 12/06, Srivatsa S. Bhat wrote:
> >>
> >> +void get_online_cpus_atomic(void)
> >> +{
> >> +	int c, old;
> >> +
> >> +	preempt_disable();
> >> +	read_lock(&hotplug_rwlock);
> >
> > Confused... Why it also takes hotplug_rwlock?
>
> To avoid ABBA deadlocks.
>
> hotplug_rwlock was meant for the "light" readers.
> The atomic counters were meant for the "heavy/full" readers.

OK, I got lost a bit. I'll try to read v3 tomorrow.

> > Obviously you can't use get_online_cpus_atomic() under rq->lock or
> > task->pi_lock or any other lock CPU_DYING can take. Probably this is
> > fine, but perhaps it makes sense to add the lockdep annotations.
>
> Hmm, you are right. We can't use _atomic() in the CPU_DYING path.

Not sure I undestand... I simply meant that, say,
get_online_cpus_atomic() under task->pi_lock can obviously deadlock
with take_cpu_down() which can want the same task->pi_lock after
disable_atomic_reader().

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/08/2012 01:26 AM, Oleg Nesterov wrote:
> On 12/07, Srivatsa S. Bhat wrote:
>>
>> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
>>> On 12/06, Srivatsa S. Bhat wrote:
>>>>
>>>> +void get_online_cpus_atomic(void)
>>>> +{
>>>> +	int c, old;
>>>> +
>>>> +	preempt_disable();
>>>> +	read_lock(&hotplug_rwlock);
>>>
>>> Confused... Why it also takes hotplug_rwlock?
>>
>> To avoid ABBA deadlocks.
>>
>> hotplug_rwlock was meant for the "light" readers.
>> The atomic counters were meant for the "heavy/full" readers.
> 
> OK, I got lost a bit. I'll try to read v3 tomorrow.
> 

OK, thanks! But note that v3 is completely different from v2.

>>> Obviously you can't use get_online_cpus_atomic() under rq->lock or
>>> task->pi_lock or any other lock CPU_DYING can take. Probably this is
>>> fine, but perhaps it makes sense to add the lockdep annotations.
>>
>> Hmm, you are right. We can't use _atomic() in the CPU_DYING path.
> 
> Not sure I undestand... I simply meant that, say,
> get_online_cpus_atomic() under task->pi_lock can obviously deadlock
> with take_cpu_down() which can want the same task->pi_lock after
> disable_atomic_reader().
> 

Right, I mistook your point for something else (i.e., ability for
the writer to do get_online_cpus_atomic() safely, which I fixed in
v3).

So, your point above is very valid. And yes, we can't do much
about it, we'll just have to teach lockdep to catch such usages.

Regards,
Srivatsa S. Bhat

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov]

On 12/08, Srivatsa S. Bhat wrote:
>
> On 12/08/2012 01:26 AM, Oleg Nesterov wrote:
> > Not sure I undestand... I simply meant that, say,
> > get_online_cpus_atomic() under task->pi_lock can obviously deadlock
> > with take_cpu_down() which can want the same task->pi_lock after
> > disable_atomic_reader().
>
> Right, I mistook your point for something else (i.e., ability for
> the writer to do get_online_cpus_atomic() safely, which I fixed in
> v3).
>
> So, your point above is very valid. And yes, we can't do much
> about it, we'll just have to teach lockdep to catch such usages.

Afaics, this is simple. Just add the "fake" lockdep_map as, say,
lglock does. Except, you need rwlock_acquire_read(map, 0, 1, IP)
because this lock is recursive.

But. There is another email from you about the possible deadlock.
I'll write the reply in a minute...

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov]

I'll try to read this series later,

one minor and almost offtopic nit.

On 12/06, Srivatsa S. Bhat wrote:
>
>  static int __ref take_cpu_down(void *_param)
>  {
>  	struct take_cpu_down_param *param = _param;
> +	unsigned long flags;
>  	int err;
>
> +	/*
> +	 *  __cpu_disable() is the step where the CPU is removed from the
> +	 *  cpu_online_mask. Protect it with the light-lock held for write.
> +	 */
> +	write_lock_irqsave(&light_hotplug_rwlock, flags);
> +
>  	/* Ensure this CPU doesn't handle any more interrupts. */
>  	err = __cpu_disable();
> -	if (err < 0)
> +	if (err < 0) {
> +		write_unlock_irqrestore(&light_hotplug_rwlock, flags);
>  		return err;
> +	}
> +
> +	/*
> +	 * We have successfully removed the CPU from the cpu_online_mask.
> +	 * So release the light-lock, so that the light-weight atomic readers
> +	 * (who care only about the cpu_online_mask updates, and not really
> +	 * about the actual cpu-take-down operation) can continue.
> +	 *
> +	 * But don't enable interrupts yet, because we still have work left to
> +	 * do, to actually bring the CPU down.
> +	 */
> +	write_unlock(&light_hotplug_rwlock);
>
>  	cpu_notify(CPU_DYING | param->mod, param->hcpu);
> +
> +	local_irq_restore(flags);
>  	return 0;

This is subjective, but imho _irqsave and the fat comment look confusing.

Currently take_cpu_down() is always called with irqs disabled, so you
do not need to play with interrupts.

10/10 does s/__stop_machine/stop_cpus/ and that patch could simply add
local_irq_disable/enable into take_cpu_down().

But again this is minor and subjective, I won't insist.

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/06/2012 12:37 AM, Oleg Nesterov wrote:
> I'll try to read this series later,
> 
> one minor and almost offtopic nit.
> 
> On 12/06, Srivatsa S. Bhat wrote:
>>
>>  static int __ref take_cpu_down(void *_param)
>>  {
>>  	struct take_cpu_down_param *param = _param;
>> +	unsigned long flags;
>>  	int err;
>>
>> +	/*
>> +	 *  __cpu_disable() is the step where the CPU is removed from the
>> +	 *  cpu_online_mask. Protect it with the light-lock held for write.
>> +	 */
>> +	write_lock_irqsave(&light_hotplug_rwlock, flags);
>> +
>>  	/* Ensure this CPU doesn't handle any more interrupts. */
>>  	err = __cpu_disable();
>> -	if (err < 0)
>> +	if (err < 0) {
>> +		write_unlock_irqrestore(&light_hotplug_rwlock, flags);
>>  		return err;
>> +	}
>> +
>> +	/*
>> +	 * We have successfully removed the CPU from the cpu_online_mask.
>> +	 * So release the light-lock, so that the light-weight atomic readers
>> +	 * (who care only about the cpu_online_mask updates, and not really
>> +	 * about the actual cpu-take-down operation) can continue.
>> +	 *
>> +	 * But don't enable interrupts yet, because we still have work left to
>> +	 * do, to actually bring the CPU down.
>> +	 */
>> +	write_unlock(&light_hotplug_rwlock);
>>
>>  	cpu_notify(CPU_DYING | param->mod, param->hcpu);
>> +
>> +	local_irq_restore(flags);
>>  	return 0;
> 
> This is subjective, but imho _irqsave and the fat comment look confusing.
> 
> Currently take_cpu_down() is always called with irqs disabled, so you
> do not need to play with interrupts.
> 
> 10/10 does s/__stop_machine/stop_cpus/ and that patch could simply add
> local_irq_disable/enable into take_cpu_down().
> 

Hmm, we could certainly do that, but somehow I felt it would be easier to
read if we tinker and fix up the take_cpu_down() logic at one place, as a
whole, instead of breaking up into pieces in different patches. And that
also makes the last patch look really cute: it just replaces stop_machine()
with stop_cpus(), as the changelog intended.

I'll see if doing like what you suggested improves the readability, and
if yes, I'll change it. Thank you!

Regards,
Srivatsa S. Bhat

[RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

Some of the atomic hotplug readers cannot tolerate CPUs going offline while
they are in their critical section. That is, they can't get away with just
synchronizing with the updates to the cpu_online_mask; they really need to
synchronize with the entire CPU tear-down sequence, because they are very
much involved in the hotplug related code paths.

Such "full" atomic hotplug readers need a way to *actually* and *truly*
prevent CPUs from going offline while they are active.

The intent of this patch is to provide synchronization APIs for such "full"
atomic hotplug readers. [ get/put_online_cpus_atomic_full()]

Some important design requirements and considerations:
-----------------------------------------------------

1. Scalable synchronization

Any synchronization at the atomic hotplug readers side must be highly
scalable - avoid global single-holder locks/counters etc. Because, these
paths currently use the extremely fast preempt_disable(); our replacement
to preempt_disable() should not become ridiculously costly and also should
not serialize the readers among themselves needlessly.

2. Should not have ABBA deadlock possibilities between the 2 types of atomic
readers ("light" vs "full")

Atomic readers who can get away with a stable online mask ("light" readers)
and atomic readers who need full synchronization with CPU offline ("full"
readers) must not end up in situations leading to ABBA deadlocks because of
the APIs they use respectively.

Also, we should not impose any ordering restrictions on how the 2 types of
readers can nest. They should be allowed to nest freely in any way they want,
and we should provide the guarantee that they won't deadlock.
(preempt_disable() posed no ordering restrictions before. Neither should we).

3. preempt_disable() was recursive. The replacement should also be recursive.

Implementation of the design:
----------------------------

Basically, we use another reader-writer lock for synchronizing the "full"
hotplug readers with the writer. But since we want to avoid ABBA deadlock
possibilities, we need to be careful as well as clever while designing this
"full" reader APIs.

Simplification: All "full" readers are also "light" readers
-----------------------------------------------------------

This simplification helps us get rid of ABBA deadlock possibilites, because
the lock ordering remains consistent to both types of readers, and looks
something like this:

Light reader:
------------
    Take light-lock for read

        /* Critical section */

    Release the light-lock

Full reader:
-----------
    Take light-lock for read

        Take full-lock for read

            /* Critical section */

        Release the full-lock

    Release the light-lock


But then, the writer path should be cleverly designed in such a way that
after the update to cpu_online_mask, only the light-readers can continue, but
the full-readers continue to spin until entire CPU offline operation is
complete.

So the lock ordering in the writer should look like this:

Writer:
------

    Take light-lock for write

        Take the full-lock for write

            Update cpu_online_mask (flip the bit)


    /*
     * Now allow only the light-readers to continue, while keeping the
     * full-readers spinning (ie., release the light-lock alone).
     */
    Release the light-lock

    /* Continue CPU tear-down, calling CPU_DYING notifiers */

    /* Finally, allow the full-readers to continue */
    Release the full-lock


It can be verified that, with this scheme, there is no possibility of any
ABBA deadlocks, and that the 2 types of atomic readers can nest in any
way they want, without fear.


We expect that the atomic hotplug readers who need full synchronization
with CPU offline (and cannot just get away with a stable online mask), be
rare. Otherwise, we could end up creating a similar effect as
stop_machine() without even using stop_machine()! [That is, if too many
readers are of this kind, everybody will wait for the entire CPU offline to
finish, which is almost like having stop_machine() itself.]

So we hope that most atomic hotplug readers are of the "light" type.
That would keeps things fast and scalable and make CPU offline operations
painless.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/cpu.h |    4 ++++
 kernel/cpu.c        |   47 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index dd0a3ee..e2a9c49 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -177,6 +177,8 @@ extern void get_online_cpus(void);
 extern void put_online_cpus(void);
 extern void get_online_cpus_atomic_light(void);
 extern void put_online_cpus_atomic_light(void);
+extern void get_online_cpus_atomic_full(void);
+extern void put_online_cpus_atomic_full(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
@@ -202,6 +204,8 @@ static inline void cpu_hotplug_driver_unlock(void)
 #define put_online_cpus()	do { } while (0)
 #define get_online_cpus_atomic_light()	do { } while (0)
 #define put_online_cpus_atomic_light()	do { } while (0)
+#define get_online_cpus_atomic_full()	do { } while (0)
+#define put_online_cpus_atomic_full()	do { } while (0)
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 381593c..c71c723 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -112,6 +112,46 @@ void put_online_cpus_atomic_light(void)
 }
 EXPORT_SYMBOL_GPL(put_online_cpus_atomic_light);
 
+/*
+ * Reader-writer lock to synchronize between "full/heavy" atomic hotplug
+ * readers and the hotplug writer while doing CPU offline operation.
+ * "Full/heavy" atomic hotplug readers are those who need to synchronize
+ * with the full CPU take-down sequence, and not just the bit flip in the
+ * cpu_online_mask.
+ */
+static DEFINE_RWLOCK(full_hotplug_rwlock);
+
+/*
+ * Some atomic hotplug readers need to synchronize with the entire CPU
+ * tear-down sequence, and not just with the update of the cpu_online_mask.
+ * Such readers are called "full" atomic hotplug readers.
+ *
+ * The following APIs help them synchronize fully with the CPU offline
+ * operation.
+ *
+ * You can call this function recursively.
+ *
+ * Also, you can mix and match (nest) "full" and "light" atomic hotplug
+ * readers in any way you want (without worrying about their ordering).
+ * The respective APIs have been designed in such a way as to provide
+ * the guarantee that you won't end up in a deadlock.
+ */
+void get_online_cpus_atomic_full(void)
+{
+	preempt_disable();
+	read_lock(&light_hotplug_rwlock);
+	read_lock(&full_hotplug_rwlock);
+}
+EXPORT_SYMBOL_GPL(get_online_cpus_atomic_full);
+
+void put_online_cpus_atomic_full(void)
+{
+	read_unlock(&full_hotplug_rwlock);
+	read_unlock(&light_hotplug_rwlock);
+	preempt_enable();
+}
+EXPORT_SYMBOL_GPL(put_online_cpus_atomic_full);
+
 static struct {
 	struct task_struct *active_writer;
 	struct mutex lock; /* Synchronizes accesses to refcount, */
@@ -318,9 +358,13 @@ static int __ref take_cpu_down(void *_param)
 	 */
 	write_lock_irqsave(&light_hotplug_rwlock, flags);
 
+	/* Disable the atomic hotplug readers who need full synchronization */
+	write_lock(&full_hotplug_rwlock);
+
 	/* Ensure this CPU doesn't handle any more interrupts. */
 	err = __cpu_disable();
 	if (err < 0) {
+		write_unlock(&full_hotplug_rwlock);
 		write_unlock_irqrestore(&light_hotplug_rwlock, flags);
 		return err;
 	}
@@ -338,6 +382,9 @@ static int __ref take_cpu_down(void *_param)
 
 	cpu_notify(CPU_DYING | param->mod, param->hcpu);
 
+	/* Enable the atomic hotplug readers who need full synchronization */
+	write_unlock(&full_hotplug_rwlock);
+
 	local_irq_restore(flags);
 	return 0;
 }

Re: [RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

Replaying what Tejun wrote:

On 12/06/2012 12:13 AM, Srivatsa S. Bhat wrote:
> Some of the atomic hotplug readers cannot tolerate CPUs going offline while
> they are in their critical section. That is, they can't get away with just
> synchronizing with the updates to the cpu_online_mask; they really need to
> synchronize with the entire CPU tear-down sequence, because they are very
> much involved in the hotplug related code paths.
> 
> Such "full" atomic hotplug readers need a way to *actually* and *truly*
> prevent CPUs from going offline while they are active.
> 

I don't think this is a good idea.  You really should just need
get/put_online_cpus() and get/put_online_cpus_atomic().  The former
the same as they are.  The latter replacing what
preempt_disable/enable() was protecting.  Let's please not go
overboard unless we know they're necessary.  I strongly suspect that
breaking up reader side from preempt_disable and making writer side a
bit lighter should be enough.  Conceptually, it really should be a
simple conversion - convert preempt_disable/enable() pairs protecting
CPU on/offlining w/ get/put_cpu_online_atomic() and wrap the
stop_machine() section with the matching write lock.

Thanks.

-- tejun 

Re: [RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

> Replaying what Tejun wrote:
> 
> On 12/06/2012 12:13 AM, Srivatsa S. Bhat wrote:
>> Some of the atomic hotplug readers cannot tolerate CPUs going offline while
>> they are in their critical section. That is, they can't get away with just
>> synchronizing with the updates to the cpu_online_mask; they really need to
>> synchronize with the entire CPU tear-down sequence, because they are very
>> much involved in the hotplug related code paths.
>>
>> Such "full" atomic hotplug readers need a way to *actually* and *truly*
>> prevent CPUs from going offline while they are active.
>>
> 
> I don't think this is a good idea.  You really should just need
> get/put_online_cpus() and get/put_online_cpus_atomic().  The former
> the same as they are.  The latter replacing what
> preempt_disable/enable() was protecting.  Let's please not go
> overboard unless we know they're necessary.  I strongly suspect that
> breaking up reader side from preempt_disable and making writer side a
> bit lighter should be enough.  Conceptually, it really should be a
> simple conversion - convert preempt_disable/enable() pairs protecting
> CPU on/offlining w/ get/put_cpu_online_atomic() and wrap the
> stop_machine() section with the matching write lock.
> 

Yes, that _sounds_ sufficient, but IMHO it won't be, in practice. The
*number* of call-sites that you need to convert from preempt_disable/enable
to get/put_online_cpus_atomic() won't be too many, however the *frequency*
of usage of those call-sites can potentially be very high.

For example, the IPI path (smp_call_function_*) needs to use the new APIs
instead of preempt_disable(); and this is quite a hot path. So if we replace
preempt_disable/enable() with a synchronization mechanism that spins
the reader *throughout* the CPU offline operation, and provide no light-weight
alternative API, then even such very hot readers will have to bear the wrath.

And IPIs and interrupts are the work-generators in a system. Since they
can be hotplug readers, if we spin them like this, we effectively end up
recreating the stop_machine() "effect", without even using stop_machine().

This is what I meant in my yesterday's reply too:
https://lkml.org/lkml/2012/12/4/349

That's why we need a light-weight variant IMHO, so that we can use them
atleast where feasible, like IPI path (smp_call_function_*) for example.
That'll help us avoid the "stop_machine effect", hoping that most readers
are of the light-type. As I mentioned in the cover-letter, most readers
_are_ of the light-type (eg: 5 patches in this series deal with light
readers, only 1 patch deals with a heavy/full reader). I don't see why
we should unnecessarily slow down every reader just because a minority of
readers actually need full synchronization with CPU offline.

Regards,
Srivatsa S. Bhat

Re: [RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline

[Tejun Heo]

Hello,

On Thu, Dec 06, 2012 at 02:01:35AM +0530, Srivatsa S. Bhat wrote:
> Yes, that _sounds_ sufficient, but IMHO it won't be, in practice. The
> *number* of call-sites that you need to convert from preempt_disable/enable
> to get/put_online_cpus_atomic() won't be too many, however the *frequency*
> of usage of those call-sites can potentially be very high.

I don't think that will be the case and, even if it is, doing it this
way would make it difficult to tell.  The right thing to do is
replacing stop_machine with finer grained percpu locking first.
Refining it further should happen iff that isn't enough and there
isn't an simpler solution.  So, let's please do the simple conversion
first.

Thanks.

-- 
tejun

Re: [RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/06/2012 02:27 AM, Tejun Heo wrote:
> Hello,
> 
> On Thu, Dec 06, 2012 at 02:01:35AM +0530, Srivatsa S. Bhat wrote:
>> Yes, that _sounds_ sufficient, but IMHO it won't be, in practice. The
>> *number* of call-sites that you need to convert from preempt_disable/enable
>> to get/put_online_cpus_atomic() won't be too many, however the *frequency*
>> of usage of those call-sites can potentially be very high.
> 
> I don't think that will be the case and, even if it is, doing it this
> way would make it difficult to tell.  The right thing to do is
> replacing stop_machine with finer grained percpu locking first.
> Refining it further should happen iff that isn't enough and there
> isn't an simpler solution.  So, let's please do the simple conversion
> first.
> 

Hmm, OK, that sounds like a good plan. So I'll drop the "light" and
"full" variants for now and work on providing a straight-forward
get/put_online_cpus_atomic() APIs.

Thank you!
 
Regards,
Srivatsa S. Bhat

[RFC PATCH v2 03/10] CPU hotplug: Convert preprocessor macros to static inline functions

[Srivatsa S. Bhat]

On 12/05/2012 06:10 AM, Andrew Morton wrote:
"static inline C functions would be preferred if possible.  Feel free to
fix up the wrong crufty surrounding code as well ;-)"

Convert the macros in the CPU hotplug code to static inline C functions.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/cpu.h |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index e2a9c49..599b376 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -200,12 +200,12 @@ static inline void cpu_hotplug_driver_unlock(void)
 
 #else		/* CONFIG_HOTPLUG_CPU */
 
-#define get_online_cpus()	do { } while (0)
-#define put_online_cpus()	do { } while (0)
-#define get_online_cpus_atomic_light()	do { } while (0)
-#define put_online_cpus_atomic_light()	do { } while (0)
-#define get_online_cpus_atomic_full()	do { } while (0)
-#define put_online_cpus_atomic_full()	do { } while (0)
+static inline void get_online_cpus(void) {}
+static inline void put_online_cpus(void) {}
+static inline void get_online_cpus_atomic_light(void) {}
+static inline void put_online_cpus_atomic_light(void) {}
+static inline void get_online_cpus_atomic_full(void) {}
+static inline void put_online_cpus_atomic_full(void) {}
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })

[RFC PATCH v2 04/10] smp, cpu hotplug: Fix smp_call_function_*() to prevent CPU offline properly

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able to
depend on preempt_disable() to prevent CPUs from going offline from under us.

Use the get/put_online_cpus_atomic_light() APIs to prevent changes to the
cpu_online_mask, while invoking from atomic context.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/smp.c |   38 +++++++++++++++++++++++++-------------
 1 file changed, 25 insertions(+), 13 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 29dd40a..abcc4d2 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -310,7 +310,8 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 	 * prevent preemption and reschedule on another processor,
 	 * as well as CPU removal
 	 */
-	this_cpu = get_cpu();
+	get_online_cpus_atomic_light();
+	this_cpu = smp_processor_id();
 
 	/*
 	 * Can deadlock when called with interrupts disabled.
@@ -342,7 +343,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 		}
 	}
 
-	put_cpu();
+	put_online_cpus_atomic_light();
 
 	return err;
 }
@@ -371,8 +372,10 @@ int smp_call_function_any(const struct cpumask *mask,
 	const struct cpumask *nodemask;
 	int ret;
 
+	get_online_cpus_atomic_light();
 	/* Try for same CPU (cheapest) */
-	cpu = get_cpu();
+	cpu = smp_processor_id();
+
 	if (cpumask_test_cpu(cpu, mask))
 		goto call;
 
@@ -388,7 +391,7 @@ int smp_call_function_any(const struct cpumask *mask,
 	cpu = cpumask_any_and(mask, cpu_online_mask);
 call:
 	ret = smp_call_function_single(cpu, func, info, wait);
-	put_cpu();
+	put_online_cpus_atomic_light();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(smp_call_function_any);
@@ -409,14 +412,17 @@ void __smp_call_function_single(int cpu, struct call_single_data *data,
 	unsigned int this_cpu;
 	unsigned long flags;
 
-	this_cpu = get_cpu();
+	get_online_cpus_atomic_light();
+
+	this_cpu = smp_processor_id();
+
 	/*
 	 * Can deadlock when called with interrupts disabled.
 	 * We allow cpu's that are not yet online though, as no one else can
 	 * send smp call function interrupt to this cpu and as such deadlocks
 	 * can't happen.
 	 */
-	WARN_ON_ONCE(cpu_online(smp_processor_id()) && wait && irqs_disabled()
+	WARN_ON_ONCE(cpu_online(this_cpu) && wait && irqs_disabled()
 		     && !oops_in_progress);
 
 	if (cpu == this_cpu) {
@@ -427,7 +433,7 @@ void __smp_call_function_single(int cpu, struct call_single_data *data,
 		csd_lock(data);
 		generic_exec_single(cpu, data, wait);
 	}
-	put_cpu();
+	put_online_cpus_atomic_light();
 }
 
 /**
@@ -451,6 +457,8 @@ void smp_call_function_many(const struct cpumask *mask,
 	unsigned long flags;
 	int refs, cpu, next_cpu, this_cpu = smp_processor_id();
 
+	get_online_cpus_atomic_light();
+
 	/*
 	 * Can deadlock when called with interrupts disabled.
 	 * We allow cpu's that are not yet online though, as no one else can
@@ -467,17 +475,18 @@ void smp_call_function_many(const struct cpumask *mask,
 
 	/* No online cpus?  We're done. */
 	if (cpu >= nr_cpu_ids)
-		return;
+		goto out_unlock;
 
 	/* Do we have another CPU which isn't us? */
 	next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
 	if (next_cpu == this_cpu)
-		next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
+		next_cpu = cpumask_next_and(next_cpu, mask,
+						cpu_online_mask);
 
 	/* Fastpath: do that cpu by itself. */
 	if (next_cpu >= nr_cpu_ids) {
 		smp_call_function_single(cpu, func, info, wait);
-		return;
+		goto out_unlock;
 	}
 
 	data = &__get_cpu_var(cfd_data);
@@ -523,7 +532,7 @@ void smp_call_function_many(const struct cpumask *mask,
 	/* Some callers race with other cpus changing the passed mask */
 	if (unlikely(!refs)) {
 		csd_unlock(&data->csd);
-		return;
+		goto out_unlock;
 	}
 
 	raw_spin_lock_irqsave(&call_function.lock, flags);
@@ -554,6 +563,9 @@ void smp_call_function_many(const struct cpumask *mask,
 	/* Optionally wait for the CPUs to complete */
 	if (wait)
 		csd_lock_wait(&data->csd);
+
+out_unlock:
+	put_online_cpus_atomic_light();
 }
 EXPORT_SYMBOL(smp_call_function_many);
 
@@ -574,9 +586,9 @@ EXPORT_SYMBOL(smp_call_function_many);
  */
 int smp_call_function(smp_call_func_t func, void *info, int wait)
 {
-	preempt_disable();
+	get_online_cpus_atomic_light();
 	smp_call_function_many(cpu_online_mask, func, info, wait);
-	preempt_enable();
+	put_online_cpus_atomic_light();
 
 	return 0;
 }

[RFC PATCH v2 05/10] smp, cpu hotplug: Fix on_each_cpu_*() to prevent CPU offline properly

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able to
depend on preempt_disable() to prevent CPUs from going offline from under us.

Use the get/put_online_cpus_atomic_light() APIs to prevent changes to the
cpu_online_mask, while invoking from atomic context.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/smp.c |   26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index abcc4d2..b258a92 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -688,12 +688,12 @@ int on_each_cpu(void (*func) (void *info), void *info, int wait)
 	unsigned long flags;
 	int ret = 0;
 
-	preempt_disable();
+	get_online_cpus_atomic_light();
 	ret = smp_call_function(func, info, wait);
 	local_irq_save(flags);
 	func(info);
 	local_irq_restore(flags);
-	preempt_enable();
+	put_online_cpus_atomic_light();
 	return ret;
 }
 EXPORT_SYMBOL(on_each_cpu);
@@ -715,7 +715,11 @@ EXPORT_SYMBOL(on_each_cpu);
 void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
 			void *info, bool wait)
 {
-	int cpu = get_cpu();
+	int cpu;
+
+	get_online_cpus_atomic_light();
+
+	cpu = smp_processor_id();
 
 	smp_call_function_many(mask, func, info, wait);
 	if (cpumask_test_cpu(cpu, mask)) {
@@ -723,7 +727,7 @@ void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
 		func(info);
 		local_irq_enable();
 	}
-	put_cpu();
+	put_online_cpus_atomic_light();
 }
 EXPORT_SYMBOL(on_each_cpu_mask);
 
@@ -748,8 +752,10 @@ EXPORT_SYMBOL(on_each_cpu_mask);
  * The function might sleep if the GFP flags indicates a non
  * atomic allocation is allowed.
  *
- * Preemption is disabled to protect against CPUs going offline but not online.
- * CPUs going online during the call will not be seen or sent an IPI.
+ * We use get/put_online_cpus_atomic_light() to have a stable online mask
+ * to work with, whose CPUs won't go offline in-between our operation.
+ * And we will skip those CPUs which have already begun their offline journey.
+ * CPUs coming online during the call will not be seen or sent an IPI.
  *
  * You must not call this function with disabled interrupts or
  * from a hardware interrupt handler or from a bottom half handler.
@@ -764,26 +770,26 @@ void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
 	might_sleep_if(gfp_flags & __GFP_WAIT);
 
 	if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
-		preempt_disable();
+		get_online_cpus_atomic_light();
 		for_each_online_cpu(cpu)
 			if (cond_func(cpu, info))
 				cpumask_set_cpu(cpu, cpus);
 		on_each_cpu_mask(cpus, func, info, wait);
-		preempt_enable();
+		put_online_cpus_atomic_light();
 		free_cpumask_var(cpus);
 	} else {
 		/*
 		 * No free cpumask, bother. No matter, we'll
 		 * just have to IPI them one by one.
 		 */
-		preempt_disable();
+		get_online_cpus_atomic_light();
 		for_each_online_cpu(cpu)
 			if (cond_func(cpu, info)) {
 				ret = smp_call_function_single(cpu, func,
 								info, wait);
 				WARN_ON_ONCE(!ret);
 			}
-		preempt_enable();
+		put_online_cpus_atomic_light();
 	}
 }
 EXPORT_SYMBOL(on_each_cpu_cond);

[RFC PATCH v2 06/10] sched, cpu hotplug: Use stable online cpus in try_to_wake_up() & select_task_rq()

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able to
depend on preempt_disable() to prevent CPUs from going offline from under us.

Use the get/put_online_cpus_atomic_light() APIs to prevent changes to the
cpu_online_mask, while invoking from atomic context.

Scheduler functions such as try_to_wake_up() and select_task_rq() (and even
select_fallback_rq()) deal with picking new CPUs to run tasks. They care only
about the cpu_online_mask and not really about the actual CPU tear-down. So
they qualify as "light" atomic readers. So use the _light() variant of the APIs.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/sched/core.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d8927f..c9a331e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1103,6 +1103,10 @@ EXPORT_SYMBOL_GPL(kick_process);
 #ifdef CONFIG_SMP
 /*
  * ->cpus_allowed is protected by both rq->lock and p->pi_lock
+ *
+ *  Must be called under get/put_online_cpus_atomic_light() or
+ *  equivalent, to avoid CPUs from going offline from underneath
+ *  us.
  */
 static int select_fallback_rq(int cpu, struct task_struct *p)
 {
@@ -1166,6 +1170,9 @@ out:
 
 /*
  * The caller (fork, wakeup) owns p->pi_lock, ->cpus_allowed is stable.
+ *
+ * Must be called under get/put_online_cpus_atomic_light(), to prevent
+ * CPUs from going offline from underneath us.
  */
 static inline
 int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
@@ -1406,6 +1413,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	int cpu, success = 0;
 
 	smp_wmb();
+	get_online_cpus_atomic_light();
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
 	if (!(p->state & state))
 		goto out;
@@ -1446,6 +1454,7 @@ stat:
 	ttwu_stat(p, cpu, wake_flags);
 out:
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+	put_online_cpus_atomic_light();
 
 	return success;
 }
@@ -1624,6 +1633,7 @@ void wake_up_new_task(struct task_struct *p)
 	unsigned long flags;
 	struct rq *rq;
 
+	get_online_cpus_atomic_light();
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
 #ifdef CONFIG_SMP
 	/*
@@ -1644,6 +1654,7 @@ void wake_up_new_task(struct task_struct *p)
 		p->sched_class->task_woken(rq, p);
 #endif
 	task_rq_unlock(rq, p, &flags);
+	put_online_cpus_atomic_light();
 }
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS
@@ -2541,6 +2552,7 @@ void sched_exec(void)
 	unsigned long flags;
 	int dest_cpu;
 
+	get_online_cpus_atomic_light();
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
 	dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
 	if (dest_cpu == smp_processor_id())
@@ -2550,11 +2562,13 @@ void sched_exec(void)
 		struct migration_arg arg = { p, dest_cpu };
 
 		raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+		put_online_cpus_atomic_light();
 		stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
 		return;
 	}
 unlock:
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+	put_online_cpus_atomic_light();
 }
 
 #endif

[RFC PATCH v2 07/10] kick_process(), cpu-hotplug: Prevent offlining of target CPU properly

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able to
depend on preempt_disable() to prevent CPUs from going offline from under us.

Use the get/put_online_cpus_atomic_light() APIs to prevent changes to the
cpu_online_mask, while invoking from atomic context.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/sched/core.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c9a331e..84a8579 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1091,11 +1091,11 @@ void kick_process(struct task_struct *p)
 {
 	int cpu;
 
-	preempt_disable();
+	get_online_cpus_atomic_light();
 	cpu = task_cpu(p);
 	if ((cpu != smp_processor_id()) && task_curr(p))
 		smp_send_reschedule(cpu);
-	preempt_enable();
+	put_online_cpus_atomic_light();
 }
 EXPORT_SYMBOL_GPL(kick_process);
 #endif /* CONFIG_SMP */

[RFC PATCH v2 08/10] yield_to(), cpu-hotplug: Prevent offlining of other CPUs properly

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able to
depend on local_irq_save() to prevent CPUs from going offline from under us.

Use the get/put_online_cpus_atomic_light() APIs to prevent changes to the
cpu_online_mask, while invoking from atomic context.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/sched/core.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 84a8579..1ef595a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4312,6 +4312,7 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
 	unsigned long flags;
 	bool yielded = 0;
 
+	get_online_cpus_atomic_light();
 	local_irq_save(flags);
 	rq = this_rq();
 
@@ -4339,13 +4340,14 @@ again:
 		 * Make p's CPU reschedule; pick_next_entity takes care of
 		 * fairness.
 		 */
-		if (preempt && rq != p_rq)
+		if (preempt && rq != p_rq && cpu_online(task_cpu(p)))
 			resched_task(p_rq->curr);
 	}
 
 out:
 	double_rq_unlock(rq, p_rq);
 	local_irq_restore(flags);
+	put_online_cpus_atomic_light();
 
 	if (yielded)
 		schedule();

[RFC PATCH v2 09/10] kvm, vmx: Add full atomic synchronization with CPU Hotplug

[Srivatsa S. Bhat]

preempt_disable() will no longer help prevent CPUs from going offline, once
stop_machine() gets removed from the CPU offline path. So use
get/put_online_cpus_atomic_full() in vmx_vcpu_load() to prevent CPUs from
going offline while clearing vmcs. Here we truly need full-synchronization
with CPU hotplug (and not just an unchanging cpu_online_mask), because we
want to prevent race with the CPU_DYING callback from kvm.

Reported-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Debugged-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/kvm/vmx.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index f858159..23c1063 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1519,10 +1519,14 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	u64 phys_addr = __pa(per_cpu(vmxarea, cpu));
 
-	if (!vmm_exclusive)
+	if (!vmm_exclusive) {
 		kvm_cpu_vmxon(phys_addr);
-	else if (vmx->loaded_vmcs->cpu != cpu)
+	} else if (vmx->loaded_vmcs->cpu != cpu) {
+		/* Prevent any CPU from going offline */
+		get_online_cpus_atomic_full();
 		loaded_vmcs_clear(vmx->loaded_vmcs);
+		put_online_cpus_atomic_full();
+	}
 
 	if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {
 		per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;

[RFC PATCH v2 10/10] cpu: No more __stop_machine() in _cpu_down()

[Srivatsa S. Bhat]

From: Paul E. McKenney <paul.mckenney@linaro.org>

The _cpu_down() function invoked as part of the CPU-hotplug offlining
process currently invokes __stop_machine(), which is slow and inflicts
substantial real-time latencies on the entire system.  This patch
substitutes stop_cpus() for __stop_machine() in order to improve
both performance and real-time latency.

This is currently unsafe, because there are a number of uses of
preempt_disable() that are intended to block CPU-hotplug offlining.
These will be fixed by using get/put_online_cpus_atomic_light(), or
get/put_online_cpus_atomic_full(), but in the meantime, this commit is one
way to help locate them.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ Srivatsa: Refer to the new sync primitives for readers, in the changelog ]
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/cpu.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index c71c723..00a1edc 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -418,7 +418,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 	}
 	smpboot_park_threads(cpu);
 
-	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
+	err = stop_cpus(cpumask_of(cpu), take_cpu_down, &tcd_param);
 	if (err) {
 		/* CPU didn't die: tell everyone.  Can't complain. */
 		smpboot_unpark_threads(cpu);

Re: [RFC PATCH v2 10/10] cpu: No more __stop_machine() in _cpu_down()

[Oleg Nesterov]

On 12/06, Srivatsa S. Bhat wrote:
>
> @@ -418,7 +418,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
>  	}
>  	smpboot_park_threads(cpu);
>
> -	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
> +	err = stop_cpus(cpumask_of(cpu), take_cpu_down, &tcd_param);

stop_one_cpu(cpu) ?

Oleg.

Re: [RFC PATCH v2 10/10] cpu: No more __stop_machine() in _cpu_down()

[Srivatsa S. Bhat]

On 12/06/2012 12:38 AM, Oleg Nesterov wrote:
> On 12/06, Srivatsa S. Bhat wrote:
>>
>> @@ -418,7 +418,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
>>  	}
>>  	smpboot_park_threads(cpu);
>>
>> -	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
>> +	err = stop_cpus(cpumask_of(cpu), take_cpu_down, &tcd_param);
> 
> stop_one_cpu(cpu) ?
> 

Even I was thinking of using that. Paul, any particular reason you chose
stop_cpus() over stop_one_cpu() in [1]?

[1]. https://lkml.org/lkml/2012/10/30/359

Regards,
Srivatsa S. Bhat

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Oleg Nesterov]

(add lkml)

On 12/05, Tejun Heo wrote:
>
> Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
> is about replacing preempt_disable with something finer grained and
> less heavy on the writer side

If only I understood why preempt_disable() is bad ;)

OK, I guess "less heavy on the writer side" is the hint, and in the
previous email you mentioned that "stop_machine() itself is extremely
heavy".

Looks like, you are going to remove stop_machine() from cpu_down ???

> The problem seems that we don't have percpu_rwlock yet.  It shouldn't
> be too difficult to implement, right?

Oh, I am not sure... unless you simply copy-and-paste the lglock code
and replace spinlock_t with rwlock_t.

We probably want something more efficient, but I bet we can't avoid
the barriers on the read side.

And somehow we should avoid the livelocks. Say, we can't simply add
the per_cpu_reader_counter, _read_lock should spin if the writer is
active. But at the same time _read_lock should be recursive.

Tejun, could you please send me mbox with this thread offlist?

Oleg.

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Tejun Heo]

Hello, Oleg.

On Wed, Dec 05, 2012 at 07:15:24PM +0100, Oleg Nesterov wrote:
> On 12/05, Tejun Heo wrote:
> > Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
> > is about replacing preempt_disable with something finer grained and
> > less heavy on the writer side
> 
> If only I understood why preempt_disable() is bad ;)
> 
> OK, I guess "less heavy on the writer side" is the hint, and in the
> previous email you mentioned that "stop_machine() itself is extremely
> heavy".
> 
> Looks like, you are going to remove stop_machine() from cpu_down ???

Yeah, that's what Srivatsa is trying to do.  The problem seems to be
that cpu up/down is very frequent on certain mobile platforms for
power management and as currently implemented cpu hotplug is too heavy
and latency-inducing.

> > The problem seems that we don't have percpu_rwlock yet.  It shouldn't
> > be too difficult to implement, right?
> 
> Oh, I am not sure... unless you simply copy-and-paste the lglock code
> and replace spinlock_t with rwlock_t.

Ah... right, so that's where brlock ended up.  So, lglock is the new
thing and brlock is a wrapper around it.

> We probably want something more efficient, but I bet we can't avoid
> the barriers on the read side.
> 
> And somehow we should avoid the livelocks. Say, we can't simply add
> the per_cpu_reader_counter, _read_lock should spin if the writer is
> active. But at the same time _read_lock should be recursive.

I think we should just go with lglock.  It does involve local atomic
ops but atomic ops themselves aren't that expensive and it's not like
we can avoid memory barriers.  Also, that's the non-sleeping
counterpart of percpu_rwsem.  If it's not good enough for some reason,
we should improve it rather than introducing something else.

Thanks.

-- 
tejun

Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomic readers to prevent CPU offline

[Srivatsa S. Bhat]

On 12/05/2012 10:23 PM, Tejun Heo wrote:
> Hello, Oleg.
> 
> On Wed, Dec 05, 2012 at 05:46:40PM +0100, Oleg Nesterov wrote:
>> Sorry I don't understand the context and I can't find this thread
>> anywhere, so I am not sure I understand...
> 
> Weird, lkml cc is missing.  Srivatsa?
> 

Argh! I forgot to CC mailing lists :-(
Sorry about that! I'll repost the patchset (and I'll probably even
try to post the comments that you and Oleg have already given, for
the benefit of other readers).

Regards,
Srivatsa S. Bhat

>> On 12/05, Tejun Heo wrote:
>>>
>>> So, we basically need percpu_rwlock.  We already have percpu_rwsem.
>>
>> Yes, and with -mm patches it becomes reader-friendly. In particular
>> see http://marc.info/?l=linux-mm-commits&m=135240650828875
>>
>>> Oleg, it seems
>>> CPU hotplug needs big-reader rwlock, ideas on how to proceed?
>>
>> I am going to convert get_online_cpus() to use percpu_down_read(),
>> this looks simple.
>>
>> We already discussed this with Paul, see
>>
>> 	http://marc.info/?l=linux-kernel&m=135248463226031
>>
>> and the whole thread.
>>
>> In short, all we need is percpu_down_write_recursive_readers() and
>> afaics the only complication is lockdep, we need down_read_no_lockdep()
>> which (like __up_read) doesn't do rwsem_acquire_read().
> 
> So, it's a different thing.  There are two mechanism protecting
> against cpu hotplug - get_online_cpus() and preempt_disable().  The
> former can be used by ones which can sleep and need to protect against
> the whole up/down process (DOWN_PREPARE and so on).  The latter
> protects the last step and can be used when the caller can't sleep.
> Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
> is about replacing preempt_disable with something finer grained and
> less heavy on the writer side - IOW, percpu_rwlock as opposed to
> percpu_rwsem, so, I think the end result would be that CPU hotplug
> will be protected by percpu_rwsem for the whole part and by
> percpu_rwlock for the last commit stage.
> 
> The problem seems that we don't have percpu_rwlock yet.  It shouldn't
> be too difficult to implement, right?
> 
> Thanks.
>