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SubjectRe: CPU Hotplug rework

Here is my attempt at a summary of the discussion.

Srivatsa, I left out the preempt_disable() pieces, but would be happy
to add them in when you let me know what you are thinking to do for
de-stop_machine()ing CPU hotplug.

Thanx, Paul


CPU-hotplug work breakout:

1. Read and understand the current generic code.
Srivatsa Bhat has done this, as have Paul E. McKenney and
Peter Zijlstra to a lesser extent.

2. Read and understand the architecture-specific code, looking
for opportunities to consolidate additional function into
core code.

a. Carry out any indicated consolidation.

b. Convert all architectures to make use of the
consolidated implementation.

Not started. Low priority from a big.LITTLE perspective.

3. Address the current kthread creation/teardown/migration
performance issues. (More details below.)

Highest priority from a big.LITTLE perspective.

4. Wean CPU-hotplug offlining from stop_machine().
(More details below.)

Moderate priority from a big.LITTLE perspective.


1. Evaluate approaches. Approaches currently under
consideration include:

a. Park the kthreads rather than tearing them down or
migrating them. RCU currently takes this sort of
approach. Note that RCU currently relies on both
preempt_disable() and local_bh_disable() blocking the
current CPU from going offline.

b. Allow in-kernel kthreads to avoid the delay
required to work around a bug in old versions of
bash. (This bug is a failure to expect receiving
a SIGCHILD signal corresponding to a child
created by a fork() system call that has not yet

This might be implemented using an additional
CLONE_ flag. This should allow kthreads to
be created and torn down much more quickly.

c. Have some other TBD way to "freeze" a kthread.
(As in "your clever idea here".)

2. Implement the chosen approach or approaches. (Different
kernel subsystems might have different constraints, possibly
requiring different kthread handling.)


1. CPU_DYING notifier fixes needed as of 3.2:

o vfp_hotplug(): I believe that this works as-is.
o s390_nohz_notify(): I believe that this works as-is.
o x86_pmu_notifier(): I believe that this works as-is.
o perf_ibs_cpu_notifier(): I don't know enough about
APIC to say.
o tboot_cpu_callback(): I believe that this works as-is,
but this one returns NOTIFY_BAD to a CPU_DYING notifier,
which is badness. But it looks like that case is a
"cannot happen" case. Still needs to be fixed.
o clockevents_notify(): This one acquires a global lock,
so it should be safe as-is.
o console_cpu_notify(): This one takes the same action
CPU_UP_CANCELLED that it does for CPU_DYING, so it
should be OK.
* rcu_cpu_notify(): This one needs adjustment as noted
above, but nothing major. Patch has been posted,
probably needs a bit of debugging.
o migration_call(): I defer to Peter on this one.
It looks to me like it is written to handle other
CPUs, but...
* workqueue_cpu_callback(): Might need help, does a
non-atomic OR.
o kvm_cpu_hotplug(): Uses a global spinlock, so should
be OK as-is.

2. Evaluate designs for stop_machine()-free CPU hotplug.
Implement the chosen design. An outline for a particular
design is shown below, but the actual design might be
quite different.

3. Fix issues with CPU Hotplug callback registration. Currently
there is no totally-race-free way to register callbacks and do
setup for already online cpus.

Srivatsa had posted an incomplete patchset some time ago
regarding this, which gives an idea of the direction he had
in mind.

4. There is a mismatch between the code and the documentation around
the difference between [un/register]_hotcpu_notifier and
[un/register]_cpu_notifier. And I remember seeing several places
in the code that uses them inconsistently. Not terribly important,
but good to fix it up while we are at it.

5. There was another thread where stuff related to CPU hotplug had
been discussed. It had exposed some new challenges to CPU hotplug,
if we were to support asynchronous smp booting.

6. If preempt_disable() no longer blocks CPU offlining, then
uses of preempt_disable() in the kernel need to be inspected
to see which are relying on blocking offlining, and any
identified will need adjustment.


1. preempt_disable() or something similarly lightweight and
unconditional must block removal of any CPU that was
in cpu_online_map at the start of the "critical section".
(I will identify these as hotplug read-side critical sections.)

I don't believe that there is any prohibition against a CPU
appearing suddenly, but some auditing would be required to
confirm this. But see below.

2. A subsystem not involved in the CPU-hotplug process must be able
to test if a CPU is online and be guaranteed that this test
remains valid (the CPU remains fully functional) for the duration
of the hotplug read-side critical section.

3. If a subsystem needs to operate on all currently online CPUs,
then it must participate in the CPU-hotplug process. My
belief is that if some code needs to test whether a CPU is
present, and needs an "offline" indication to persist, then
that code's subsystem must participate in CPU-hotplug operations.

4. There must be a way to register/unregister for CPU-hotplug events.
This is currently cpu_notifier(), register_cpu_notifier(),
and unregister_cpu_notifier().

n-1. CPU-hotplug operations should be reasonably fast. A few
milliseconds is OK, multiple seconds not so much.

n. (Your additional constraints here.)


a. Maintain the cpu_online_map, as currently, but the meaning
of a set bit is that the CPU is fully functional. If there
is any service that the CPU no longer offers, its bit is

b. Continue to use preempt_enable()/preempt_disable() to mark
hotplug read-side critical sections.

c. Instead of using __stop_machine(), use a per-CPU variable that
is checked in the idle loop. Possibly another TIF_ bit.

d. The CPU notifiers are like today, except that CPU_DYING() is
invoked by the CPU after it sees that its per-CPU variable
telling it to go offline. As today, the CPU_DYING notifiers
are invoked with interrupts disabled, but other CPUs are still
running. Of course, the CPU_DYING notifiers need to be audited
and repaired. There are fewer than 20 of them, so not so bad.
RCU's is an easy fix: Just re-introduce locking and the global
RCU callback orphanage. My guesses for the others at the end.

e. Getting rid of __stop_machine() means that the final step of the
CPU going offline will no longer be seen as atomic by other CPUs.
This will require more careful tracking of dependencies among
different subsystems. The required tracking can be reduced
by invoking notifiers in registration order for CPU-online
operations and invoking them in the reverse of registration
order for CPU-offline operations.

For example, the scheduler uses RCU. If notifiers are invoked in
the same order for all CPU-hotplug operations, then on CPU-offline
operations, during the time between when RCU's notifier is invoked
and when the scheduler's notifier is invoked, the scheduler must
deal with a CPU on which RCU isn't working. (RCU currently
works around this by allowing a one-jiffy time period after
notification when it still pays attention to the CPU.)

In contrast, if notifiers are invoked in reverse-registration
order for CPU-offline operations, then any time the scheduler
sees a CPU as online, RCU also is treating it as online.

f. There will be some circular dependencies. For example, the
scheduler uses RCU, but in some configurations, RCU also uses
kthreads. These dependencies must be handled on a case-by-case
basis. For example, the scheduler could invoke an RCU API
to tell RCU when to shut down its per-CPU kthreads and when
to start them up. Or RCU could deal with its kthreads in the
CPU_DOWN_PREPARE and CPU_ONLINE notifiers. Either way, RCU
needs to correctly handle the interval when it cannot use
kthreads on a given CPU that it is still handling, for example,
by switching to running the RCU core code in softirq context.

g. Most subsystems participating in CPU-hotplug operations will need
to keep their own copy of CPU online/offline state. For example,
RCU uses the ->qsmaskinit fields in the rcu_node structure for
this purpose.

h. So CPU-offline handling looks something like the following:

i. Acquire the hotplug mutex.

ii. Invoke the CPU_DOWN_PREPARE notifiers. If there
are objections, invoke the CPU_DOWN_FAILED notifiers
and return an error.

iii. Clear the CPU's bit in cpu_online_map.

iv. Invoke synchronize_sched() to ensure that all future hotplug
read-side critical sections ignore the outgoing CPU.

v. Set a per-CPU variable telling the CPU to take itself
offline. There would need to be something here to
help the CPU get to idle quickly, possibly requiring
another round of notifiers. CPU_DOWN?

vi. When the dying CPU gets to the idle loop, it invokes the
CPU_DYING notifiers and updates its per-CPU variable to
indicate that it is ready to die. It then spins in a
tight loop (or does some other architecture-specified
operation to wait to be turned off).

Note that there is no need for RCU to guess how long the
CPU might be executing RCU read-side critical sections.

vii. When the task doing the offline operation sees the
updated per-CPU variable, it calls __cpu_die().

viii. The CPU_DEAD notifiers are invoked.

ix. Theeck_for_tasks() function is invoked.

x. Release the hotplug mutex.

xi. Invoke the CPU_POST_DEAD notifiers.

i. I do not believe that the CPU-offline handling needs to change

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