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Message-ID: <CAKfTPtB1J+AXi4ZViuxPRxag3O4C9O2ZUeFza1HUFeAiYUy+yg@mail.gmail.com> Date: Fri, 1 Mar 2013 13:05:04 +0100 From: Vincent Guittot <vincent.guittot@...aro.org> To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> Cc: tglx@...utronix.de, peterz@...radead.org, tj@...nel.org, oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au, mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org, rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com, rjw@...k.pl, sbw@....edu, fweisbec@...il.com, linux@....linux.org.uk, nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org, linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, walken@...gle.com Subject: Re: [PATCH v6 00/46] CPU hotplug: stop_machine()-free CPU hotplug 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@...ux.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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-pm" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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