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SubjectRe: [PATCH v6 00/46] CPU hotplug: stop_machine()-free CPU hotplug
From
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@linux.vnet.ibm.com> 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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