Re: [PATCH v6 00/46] CPU hotplug: stop_machine()-free CPU hotplug

[Vincent Guittot]

Hi Srivatsa,

I have run some tests with genload on my ARM platform but even with
the mainline the cpu_down is quite short and stable ( around  4ms )
with 5 or 2 online cores. The duration is similar with your patches

I have maybe not used the right option for genload ? I have used
genload -m 10 which seems to generate the most system time. Which
command have you used for your tests ?

Vincent

On 18 February 2013 13:38, Srivatsa S. Bhat
<srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
> 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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