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Subject[PATCH v6 00/46] CPU hotplug: stop_machine()-free CPU hotplug
Date
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.
The motivation behind the removal of stop_machine() is to avoid its ill-effects
and thus improve the design of CPU hotplug. (More description regarding this
is available in the patches).

All the users of preempt_disable()/local_irq_disable() who used to use it to
prevent CPU offline, have been converted to the new primitives introduced in the
patchset. Also, the CPU_DYING notifiers have been audited to check whether
they can cope up with the removal of stop_machine() or whether they need to
use new locks for synchronization (all CPU_DYING notifiers looked OK, without
the need for any new locks).

Applies on current mainline (v3.8-rc7+).

This patchset is available in the following git branch:

git://github.com/srivatsabhat/linux.git stop-machine-free-cpu-hotplug-v6


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

Patches 1 to 7 introduce a generic, flexible Per-CPU Reader-Writer Locking
scheme.

Patch 8 uses this synchronization mechanism to build the
get/put_online_cpus_atomic() APIs which can be used from atomic context, to
prevent CPUs from going offline.

Patch 9 is a cleanup; it converts preprocessor macros to static inline
functions.

Patches 10 to 43 convert various call-sites to use the new APIs.

Patch 44 is the one which actually removes stop_machine() from the CPU
offline path.

Patch 45 decouples stop_machine() and CPU hotplug from Kconfig.

Patch 46 updates the documentation to reflect the new APIs.


Changes in v6:
--------------

* Fixed issues related to memory barriers, as pointed out by Paul and Oleg.
* Fixed the locking issue related to clockevents_lock, which was being
triggered when cpu idle was enabled.
* Some code restructuring to improve readability and to enhance some fastpath
optimizations.
* Randconfig build-fixes, reported by Fengguang Wu.


Changes in v5:
--------------
Exposed a new generic locking scheme: Flexible Per-CPU Reader-Writer locks,
based on the synchronization schemes already discussed in the previous
versions, and used it in CPU hotplug, to implement the new APIs.

Audited the CPU_DYING notifiers in the kernel source tree and replaced
usages of preempt_disable() with the new get/put_online_cpus_atomic() APIs
where necessary.


Changes in v4:
--------------
The synchronization scheme has been simplified quite a bit, which makes it
look a lot less complex than before. Some highlights:

* Implicit ACKs:

The earlier design required the readers to explicitly ACK the writer's
signal. The new design uses implicit ACKs instead. The reader switching
over to rwlock implicitly tells the writer to stop waiting for that reader.

* No atomic operations:

Since we got rid of explicit ACKs, we no longer have the need for a reader
and a writer to update the same counter. So we can get rid of atomic ops
too.

Changes in v3:
--------------
* Dropped the _light() and _full() variants of the APIs. Provided a single
interface: get/put_online_cpus_atomic().

* Completely redesigned the synchronization mechanism again, to make it
fast and scalable at the reader-side in the fast-path (when no hotplug
writers are active). This new scheme also ensures that there is no
possibility of deadlocks due to circular locking dependency.
In summary, this provides the scalability and speed of per-cpu rwlocks
(without actually using them), while avoiding the downside (deadlock
possibilities) which is inherent in any per-cpu locking scheme that is
meant to compete with preempt_disable()/enable() in terms of flexibility.

The problem with using per-cpu locking to replace preempt_disable()/enable
was explained here:
https://lkml.org/lkml/2012/12/6/290

Basically we use per-cpu counters (for scalability) when no writers are
active, and then switch to global rwlocks (for lock-safety) when a writer
becomes active. It is a slightly complex scheme, but it is based on
standard principles of distributed algorithms.

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.

Links to previous versions:
v5: http://lwn.net/Articles/533553/
v4: https://lkml.org/lkml/2012/12/11/209
v3: https://lkml.org/lkml/2012/12/7/287
v2: https://lkml.org/lkml/2012/12/5/322
v1: https://lkml.org/lkml/2012/12/4/88

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

Srivatsa S. Bhat (45):
percpu_rwlock: Introduce the global reader-writer lock backend
percpu_rwlock: Introduce per-CPU variables for the reader and the writer
percpu_rwlock: Provide a way to define and init percpu-rwlocks at compile time
percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks
percpu_rwlock: Make percpu-rwlocks IRQ-safe, optimally
percpu_rwlock: Rearrange the read-lock code to fastpath nested percpu readers
percpu_rwlock: Allow writers to be readers, and add lockdep annotations
CPU hotplug: Provide APIs to prevent CPU offline from atomic context
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/timer: Use get/put_online_cpus_atomic() to prevent CPU offline
sched/migration: Use raw_spin_lock/unlock since interrupts are already disabled
sched/rt: Use get/put_online_cpus_atomic() to prevent CPU offline
tick: Use get/put_online_cpus_atomic() to prevent CPU offline
time/clocksource: Use get/put_online_cpus_atomic() to prevent CPU offline
clockevents: Use get/put_online_cpus_atomic() in clockevents_notify()
softirq: Use get/put_online_cpus_atomic() to prevent CPU offline
irq: Use get/put_online_cpus_atomic() to prevent CPU offline
net: Use get/put_online_cpus_atomic() to prevent CPU offline
block: Use get/put_online_cpus_atomic() to prevent CPU offline
crypto: pcrypt - Protect access to cpu_online_mask with get/put_online_cpus()
infiniband: ehca: Use get/put_online_cpus_atomic() to prevent CPU offline
[SCSI] fcoe: Use get/put_online_cpus_atomic() to prevent CPU offline
staging: octeon: Use get/put_online_cpus_atomic() to prevent CPU offline
x86: Use get/put_online_cpus_atomic() to prevent CPU offline
perf/x86: Use get/put_online_cpus_atomic() to prevent CPU offline
KVM: Use get/put_online_cpus_atomic() to prevent CPU offline from atomic context
kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline
x86/xen: Use get/put_online_cpus_atomic() to prevent CPU offline
alpha/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
blackfin/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
cris/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
hexagon/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
ia64: Use get/put_online_cpus_atomic() to prevent CPU offline
m32r: Use get/put_online_cpus_atomic() to prevent CPU offline
MIPS: Use get/put_online_cpus_atomic() to prevent CPU offline
mn10300: Use get/put_online_cpus_atomic() to prevent CPU offline
parisc: Use get/put_online_cpus_atomic() to prevent CPU offline
powerpc: Use get/put_online_cpus_atomic() to prevent CPU offline
sh: Use get/put_online_cpus_atomic() to prevent CPU offline
sparc: Use get/put_online_cpus_atomic() to prevent CPU offline
tile: Use get/put_online_cpus_atomic() to prevent CPU offline
CPU hotplug, stop_machine: Decouple CPU hotplug from stop_machine() in Kconfig
Documentation/cpu-hotplug: Remove references to stop_machine()

Documentation/cpu-hotplug.txt | 17 +-
arch/alpha/kernel/smp.c | 19 +-
arch/arm/Kconfig | 1
arch/blackfin/Kconfig | 1
arch/blackfin/mach-common/smp.c | 6 -
arch/cris/arch-v32/kernel/smp.c | 8 +
arch/hexagon/kernel/smp.c | 5
arch/ia64/Kconfig | 1
arch/ia64/kernel/irq_ia64.c | 13 +
arch/ia64/kernel/perfmon.c | 6 +
arch/ia64/kernel/smp.c | 23 ++
arch/ia64/mm/tlb.c | 6 -
arch/m32r/kernel/smp.c | 12 +
arch/mips/Kconfig | 1
arch/mips/kernel/cevt-smtc.c | 8 +
arch/mips/kernel/smp.c | 16 +-
arch/mips/kernel/smtc.c | 3
arch/mips/mm/c-octeon.c | 4
arch/mn10300/Kconfig | 1
arch/mn10300/kernel/smp.c | 2
arch/mn10300/mm/cache-smp.c | 5
arch/mn10300/mm/tlb-smp.c | 15 +
arch/parisc/Kconfig | 1
arch/parisc/kernel/smp.c | 4
arch/powerpc/Kconfig | 1
arch/powerpc/mm/mmu_context_nohash.c | 2
arch/s390/Kconfig | 1
arch/sh/Kconfig | 1
arch/sh/kernel/smp.c | 12 +
arch/sparc/Kconfig | 1
arch/sparc/kernel/leon_smp.c | 2
arch/sparc/kernel/smp_64.c | 9 -
arch/sparc/kernel/sun4d_smp.c | 2
arch/sparc/kernel/sun4m_smp.c | 3
arch/tile/kernel/smp.c | 4
arch/x86/Kconfig | 1
arch/x86/include/asm/ipi.h | 5
arch/x86/kernel/apic/apic_flat_64.c | 10 +
arch/x86/kernel/apic/apic_numachip.c | 5
arch/x86/kernel/apic/es7000_32.c | 5
arch/x86/kernel/apic/io_apic.c | 7 -
arch/x86/kernel/apic/ipi.c | 10 +
arch/x86/kernel/apic/x2apic_cluster.c | 4
arch/x86/kernel/apic/x2apic_uv_x.c | 4
arch/x86/kernel/cpu/mcheck/therm_throt.c | 4
arch/x86/kernel/cpu/perf_event_intel_uncore.c | 5
arch/x86/kvm/vmx.c | 8 +
arch/x86/mm/tlb.c | 14 +
arch/x86/xen/mmu.c | 11 +
arch/x86/xen/smp.c | 9 +
block/blk-softirq.c | 4
crypto/pcrypt.c | 4
drivers/infiniband/hw/ehca/ehca_irq.c | 8 +
drivers/scsi/fcoe/fcoe.c | 7 +
drivers/staging/octeon/ethernet-rx.c | 3
include/linux/cpu.h | 8 +
include/linux/percpu-rwlock.h | 74 +++++++
include/linux/stop_machine.h | 2
init/Kconfig | 2
kernel/cpu.c | 59 +++++-
kernel/irq/manage.c | 7 +
kernel/sched/core.c | 36 +++-
kernel/sched/fair.c | 5
kernel/sched/rt.c | 3
kernel/smp.c | 65 ++++--
kernel/softirq.c | 3
kernel/time/clockevents.c | 3
kernel/time/clocksource.c | 5
kernel/time/tick-broadcast.c | 2
kernel/timer.c | 2
lib/Kconfig | 3
lib/Makefile | 1
lib/percpu-rwlock.c | 256 +++++++++++++++++++++++++
net/core/dev.c | 9 +
virt/kvm/kvm_main.c | 10 +
75 files changed, 776 insertions(+), 123 deletions(-)
create mode 100644 include/linux/percpu-rwlock.h
create mode 100644 lib/percpu-rwlock.c



Regards,
Srivatsa S. Bhat
IBM Linux Technology Center



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