[PATCH 1/2] powerpc/pseries: remove cede offline state for CPUs
Gautham R Shenoy
ego at linux.vnet.ibm.com
Wed Jun 3 20:30:44 AEST 2020
Hello Nathan,
On Mon, Jun 01, 2020 at 11:31:39PM -0500, Nathan Lynch wrote:
> This effectively reverts commit 3aa565f53c39 ("powerpc/pseries: Add
> hooks to put the CPU into an appropriate offline state"), which added
> an offline mode for CPUs which uses the H_CEDE hcall instead of the
> architected stop-self RTAS function in order to facilitate "folding"
> of dedicated mode processors on PowerVM platforms to achieve energy
> savings. This has been the default offline mode since its
> introduction.
>
> There's nothing about stop-self that would prevent the hypervisor from
> achieving the energy savings available via H_CEDE, so the original
> premise of this change appears to be flawed.
IIRC, back in 2009, when the Extended-CEDE was introduced, it couldn't
be exposed via the cpuidle subsystem since this state needs an
explicit H_PROD as opposed to the H_IPI which wakes up the regular
CEDE call. So, the alterative was to use the CPU-Hotplug way by having
a userspace daemon fold the cores which weren't needed currently and
bring them back online when they were needed. Back then, Long Term
CEDE was definitely faster compared to stop-self call (It is a pity
that I didn't post the numbers when I wrote the patch) and the
time-taken to unfold a core was definitely one of the concerns.
(https://lkml.org/lkml/2009/9/23/522).
However, on current systems, this is not the case as you rightly
mention below. Based on the measurements that I did last year, there
is no significant difference in the time taken for a CPU-Hotplug
operation while using stop-self vs Extended-CEDE CPU-Hotplug.
Time taken to offline a core (in seconds)
With stop-self
N Min Max Median Avg Stddev
20 0.93 1.31 1.161 1.1308 0.11776721
With Extended cede
N Min Max Median Avg Stddev
20 1.12 1.65 1.39 1.3929 0.13363379
Time taken to online a core (in seconds)
With stop-self
N Min Max Median Avg Stddev
20 1.82 2.22 1.96 1.97605 0.093984587
With Extended cede
N Min Max Median Avg Stddev
20 1.65 2.1 1.9 1.8916 0.13074902
> I also have encountered the claim that the transition to and from
> ceded state is much faster than stop-self/start-cpu. Certainly we
> would not want to use stop-self as an *idle* mode. That is what H_CEDE
> is for.
True. However the regular H_CEDE doesn't get mapped to a deeper
platform idle state, while Extended H_CEDE does. And the deeper
platform idle states have associated performance benefits, such as the
online cores being able to execute Turbo frequencies. However, the
right thing to do would be for the hypervisor provide us with a new
latency-hint for a extended H_CEDE which can wakeup on H_IPI, instead
of needing an explicit H_PROD.
> However, this difference is insignificant in the context of
> Linux CPU hotplug, where the latency of an offline or online operation
> on current systems is on the order of 100ms, mainly attributable to
> all the various subsystems' cpuhp callbacks.
>
> The cede offline mode also prevents accurate accounting, as discussed
> before:
> https://lore.kernel.org/linuxppc-dev/1571740391-3251-1-git-send-email-ego@linux.vnet.ibm.com/
>
> Unconditionally use stop-self to offline processor threads. This is
> the architected method for offlining CPUs on PAPR systems.
>
> The "cede_offline" boot parameter is rendered obsolete.
>
> Removing this code enables the removal of the partition suspend code
> which temporarily onlines all present CPUs.
>
> Fixes: 3aa565f53c39 ("powerpc/pseries: Add hooks to put the CPU into an appropriate offline state")
>
> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
The patch looks good to me.
Reviewed-by: Gautham R. Shenoy <ego at linux.vnet.ibm.com>
> ---
> Documentation/core-api/cpu_hotplug.rst | 7 -
> arch/powerpc/platforms/pseries/hotplug-cpu.c | 170 ++----------------
> .../platforms/pseries/offline_states.h | 38 ----
> arch/powerpc/platforms/pseries/pmem.c | 1 -
> arch/powerpc/platforms/pseries/smp.c | 28 +--
> 5 files changed, 15 insertions(+), 229 deletions(-)
> delete mode 100644 arch/powerpc/platforms/pseries/offline_states.h
>
> diff --git a/Documentation/core-api/cpu_hotplug.rst b/Documentation/core-api/cpu_hotplug.rst
> index 4a50ab7817f7..b1ae1ac159cf 100644
> --- a/Documentation/core-api/cpu_hotplug.rst
> +++ b/Documentation/core-api/cpu_hotplug.rst
> @@ -50,13 +50,6 @@ Command Line Switches
>
> This option is limited to the X86 and S390 architecture.
>
> -``cede_offline={"off","on"}``
> - Use this option to disable/enable putting offlined processors to an extended
> - ``H_CEDE`` state on supported pseries platforms. If nothing is specified,
> - ``cede_offline`` is set to "on".
> -
> - This option is limited to the PowerPC architecture.
> -
> ``cpu0_hotplug``
> Allow to shutdown CPU0.
>
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index 3e8cbfe7a80f..d4b346355bb9 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -35,54 +35,10 @@
> #include <asm/topology.h>
>
> #include "pseries.h"
> -#include "offline_states.h"
>
> /* This version can't take the spinlock, because it never returns */
> static int rtas_stop_self_token = RTAS_UNKNOWN_SERVICE;
>
> -static DEFINE_PER_CPU(enum cpu_state_vals, preferred_offline_state) =
> - CPU_STATE_OFFLINE;
> -static DEFINE_PER_CPU(enum cpu_state_vals, current_state) = CPU_STATE_OFFLINE;
> -
> -static enum cpu_state_vals default_offline_state = CPU_STATE_OFFLINE;
> -
> -static bool cede_offline_enabled __read_mostly = true;
> -
> -/*
> - * Enable/disable cede_offline when available.
> - */
> -static int __init setup_cede_offline(char *str)
> -{
> - return (kstrtobool(str, &cede_offline_enabled) == 0);
> -}
> -
> -__setup("cede_offline=", setup_cede_offline);
> -
> -enum cpu_state_vals get_cpu_current_state(int cpu)
> -{
> - return per_cpu(current_state, cpu);
> -}
> -
> -void set_cpu_current_state(int cpu, enum cpu_state_vals state)
> -{
> - per_cpu(current_state, cpu) = state;
> -}
> -
> -enum cpu_state_vals get_preferred_offline_state(int cpu)
> -{
> - return per_cpu(preferred_offline_state, cpu);
> -}
> -
> -void set_preferred_offline_state(int cpu, enum cpu_state_vals state)
> -{
> - per_cpu(preferred_offline_state, cpu) = state;
> -}
> -
> -void set_default_offline_state(int cpu)
> -{
> - per_cpu(preferred_offline_state, cpu) = default_offline_state;
> -}
> -
> static void rtas_stop_self(void)
> {
> static struct rtas_args args;
> @@ -101,9 +57,7 @@ static void rtas_stop_self(void)
>
> static void pseries_mach_cpu_die(void)
> {
> - unsigned int cpu = smp_processor_id();
> unsigned int hwcpu = hard_smp_processor_id();
> - u8 cede_latency_hint = 0;
>
> local_irq_disable();
> idle_task_exit();
> @@ -112,49 +66,6 @@ static void pseries_mach_cpu_die(void)
> else
> xics_teardown_cpu();
>
> - if (get_preferred_offline_state(cpu) == CPU_STATE_INACTIVE) {
> - set_cpu_current_state(cpu, CPU_STATE_INACTIVE);
> - if (ppc_md.suspend_disable_cpu)
> - ppc_md.suspend_disable_cpu();
> -
> - cede_latency_hint = 2;
> -
> - get_lppaca()->idle = 1;
> - if (!lppaca_shared_proc(get_lppaca()))
> - get_lppaca()->donate_dedicated_cpu = 1;
> -
> - while (get_preferred_offline_state(cpu) == CPU_STATE_INACTIVE) {
> - while (!prep_irq_for_idle()) {
> - local_irq_enable();
> - local_irq_disable();
> - }
> -
> - extended_cede_processor(cede_latency_hint);
> - }
> -
> - local_irq_disable();
> -
> - if (!lppaca_shared_proc(get_lppaca()))
> - get_lppaca()->donate_dedicated_cpu = 0;
> - get_lppaca()->idle = 0;
> -
> - if (get_preferred_offline_state(cpu) == CPU_STATE_ONLINE) {
> - unregister_slb_shadow(hwcpu);
> -
> - hard_irq_disable();
> - /*
> - * Call to start_secondary_resume() will not return.
> - * Kernel stack will be reset and start_secondary()
> - * will be called to continue the online operation.
> - */
> - start_secondary_resume();
> - }
> - }
> -
> - /* Requested state is CPU_STATE_OFFLINE at this point */
> - WARN_ON(get_preferred_offline_state(cpu) != CPU_STATE_OFFLINE);
> -
> - set_cpu_current_state(cpu, CPU_STATE_OFFLINE);
> unregister_slb_shadow(hwcpu);
> rtas_stop_self();
>
> @@ -200,24 +111,13 @@ static void pseries_cpu_die(unsigned int cpu)
> int cpu_status = 1;
> unsigned int pcpu = get_hard_smp_processor_id(cpu);
>
> - if (get_preferred_offline_state(cpu) == CPU_STATE_INACTIVE) {
> - cpu_status = 1;
> - for (tries = 0; tries < 5000; tries++) {
> - if (get_cpu_current_state(cpu) == CPU_STATE_INACTIVE) {
> - cpu_status = 0;
> - break;
> - }
> - msleep(1);
> - }
> - } else if (get_preferred_offline_state(cpu) == CPU_STATE_OFFLINE) {
> + for (tries = 0; tries < 25; tries++) {
> + cpu_status = smp_query_cpu_stopped(pcpu);
> + if (cpu_status == QCSS_STOPPED ||
> + cpu_status == QCSS_HARDWARE_ERROR)
> + break;
> + cpu_relax();
>
> - for (tries = 0; tries < 25; tries++) {
> - cpu_status = smp_query_cpu_stopped(pcpu);
> - if (cpu_status == QCSS_STOPPED ||
> - cpu_status == QCSS_HARDWARE_ERROR)
> - break;
> - cpu_relax();
> - }
> }
>
> if (cpu_status != 0) {
> @@ -359,28 +259,15 @@ static int dlpar_offline_cpu(struct device_node *dn)
> if (get_hard_smp_processor_id(cpu) != thread)
> continue;
>
> - if (get_cpu_current_state(cpu) == CPU_STATE_OFFLINE)
> + if (!cpu_online(cpu))
> break;
>
> - if (get_cpu_current_state(cpu) == CPU_STATE_ONLINE) {
> - set_preferred_offline_state(cpu,
> - CPU_STATE_OFFLINE);
> - cpu_maps_update_done();
> - timed_topology_update(1);
> - rc = device_offline(get_cpu_device(cpu));
> - if (rc)
> - goto out;
> - cpu_maps_update_begin();
> - break;
> - }
> -
> - /*
> - * The cpu is in CPU_STATE_INACTIVE.
> - * Upgrade it's state to CPU_STATE_OFFLINE.
> - */
> - set_preferred_offline_state(cpu, CPU_STATE_OFFLINE);
> - WARN_ON(plpar_hcall_norets(H_PROD, thread) != H_SUCCESS);
> - __cpu_die(cpu);
> + cpu_maps_update_done();
> + timed_topology_update(1);
> + rc = device_offline(get_cpu_device(cpu));
> + if (rc)
> + goto out;
> + cpu_maps_update_begin();
> break;
> }
> if (cpu == num_possible_cpus()) {
> @@ -414,8 +301,6 @@ static int dlpar_online_cpu(struct device_node *dn)
> for_each_present_cpu(cpu) {
> if (get_hard_smp_processor_id(cpu) != thread)
> continue;
> - BUG_ON(get_cpu_current_state(cpu)
> - != CPU_STATE_OFFLINE);
> cpu_maps_update_done();
> timed_topology_update(1);
> find_and_online_cpu_nid(cpu);
> @@ -1013,27 +898,8 @@ static struct notifier_block pseries_smp_nb = {
> .notifier_call = pseries_smp_notifier,
> };
>
> -#define MAX_CEDE_LATENCY_LEVELS 4
> -#define CEDE_LATENCY_PARAM_LENGTH 10
> -#define CEDE_LATENCY_PARAM_MAX_LENGTH \
> - (MAX_CEDE_LATENCY_LEVELS * CEDE_LATENCY_PARAM_LENGTH * sizeof(char))
> -#define CEDE_LATENCY_TOKEN 45
> -
> -static char cede_parameters[CEDE_LATENCY_PARAM_MAX_LENGTH];
> -
> -static int parse_cede_parameters(void)
> -{
> - memset(cede_parameters, 0, CEDE_LATENCY_PARAM_MAX_LENGTH);
> - return rtas_call(rtas_token("ibm,get-system-parameter"), 3, 1,
> - NULL,
> - CEDE_LATENCY_TOKEN,
> - __pa(cede_parameters),
> - CEDE_LATENCY_PARAM_MAX_LENGTH);
> -}
> -
> static int __init pseries_cpu_hotplug_init(void)
> {
> - int cpu;
> int qcss_tok;
>
> #ifdef CONFIG_ARCH_CPU_PROBE_RELEASE
> @@ -1056,16 +922,8 @@ static int __init pseries_cpu_hotplug_init(void)
> smp_ops->cpu_die = pseries_cpu_die;
>
> /* Processors can be added/removed only on LPAR */
> - if (firmware_has_feature(FW_FEATURE_LPAR)) {
> + if (firmware_has_feature(FW_FEATURE_LPAR))
> of_reconfig_notifier_register(&pseries_smp_nb);
> - cpu_maps_update_begin();
> - if (cede_offline_enabled && parse_cede_parameters() == 0) {
> - default_offline_state = CPU_STATE_INACTIVE;
> - for_each_online_cpu(cpu)
> - set_default_offline_state(cpu);
> - }
> - cpu_maps_update_done();
> - }
>
> return 0;
> }
> diff --git a/arch/powerpc/platforms/pseries/offline_states.h b/arch/powerpc/platforms/pseries/offline_states.h
> deleted file mode 100644
> index 51414aee2862..000000000000
> --- a/arch/powerpc/platforms/pseries/offline_states.h
> +++ /dev/null
> @@ -1,38 +0,0 @@
> -/* SPDX-License-Identifier: GPL-2.0 */
> -#ifndef _OFFLINE_STATES_H_
> -#define _OFFLINE_STATES_H_
> -
> -/* Cpu offline states go here */
> -enum cpu_state_vals {
> - CPU_STATE_OFFLINE,
> - CPU_STATE_INACTIVE,
> - CPU_STATE_ONLINE,
> - CPU_MAX_OFFLINE_STATES
> -};
> -
> -#ifdef CONFIG_HOTPLUG_CPU
> -extern enum cpu_state_vals get_cpu_current_state(int cpu);
> -extern void set_cpu_current_state(int cpu, enum cpu_state_vals state);
> -extern void set_preferred_offline_state(int cpu, enum cpu_state_vals state);
> -extern void set_default_offline_state(int cpu);
> -#else
> -static inline enum cpu_state_vals get_cpu_current_state(int cpu)
> -{
> - return CPU_STATE_ONLINE;
> -}
> -
> -static inline void set_cpu_current_state(int cpu, enum cpu_state_vals state)
> -{
> -}
> -
> -static inline void set_preferred_offline_state(int cpu, enum cpu_state_vals state)
> -{
> -}
> -
> -static inline void set_default_offline_state(int cpu)
> -{
> -}
> -#endif
> -
> -extern enum cpu_state_vals get_preferred_offline_state(int cpu);
> -#endif
> diff --git a/arch/powerpc/platforms/pseries/pmem.c b/arch/powerpc/platforms/pseries/pmem.c
> index f860a897a9e0..f827de7087e9 100644
> --- a/arch/powerpc/platforms/pseries/pmem.c
> +++ b/arch/powerpc/platforms/pseries/pmem.c
> @@ -24,7 +24,6 @@
> #include <asm/topology.h>
>
> #include "pseries.h"
> -#include "offline_states.h"
>
> static struct device_node *pmem_node;
>
> diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
> index ad61e90032da..a8a070269151 100644
> --- a/arch/powerpc/platforms/pseries/smp.c
> +++ b/arch/powerpc/platforms/pseries/smp.c
> @@ -44,8 +44,6 @@
> #include <asm/svm.h>
>
> #include "pseries.h"
> -#include "offline_states.h"
> -
>
> /*
> * The Primary thread of each non-boot processor was started from the OF client
> @@ -108,10 +106,7 @@ static inline int smp_startup_cpu(unsigned int lcpu)
>
> /* Fixup atomic count: it exited inside IRQ handler. */
> task_thread_info(paca_ptrs[lcpu]->__current)->preempt_count = 0;
> -#ifdef CONFIG_HOTPLUG_CPU
> - if (get_cpu_current_state(lcpu) == CPU_STATE_INACTIVE)
> - goto out;
> -#endif
> +
> /*
> * If the RTAS start-cpu token does not exist then presume the
> * cpu is already spinning.
> @@ -126,9 +121,6 @@ static inline int smp_startup_cpu(unsigned int lcpu)
> return 0;
> }
>
> -#ifdef CONFIG_HOTPLUG_CPU
> -out:
> -#endif
> return 1;
> }
>
> @@ -143,10 +135,6 @@ static void smp_setup_cpu(int cpu)
> vpa_init(cpu);
>
> cpumask_clear_cpu(cpu, of_spin_mask);
> -#ifdef CONFIG_HOTPLUG_CPU
> - set_cpu_current_state(cpu, CPU_STATE_ONLINE);
> - set_default_offline_state(cpu);
> -#endif
> }
>
> static int smp_pSeries_kick_cpu(int nr)
> @@ -163,20 +151,6 @@ static int smp_pSeries_kick_cpu(int nr)
> * the processor will continue on to secondary_start
> */
> paca_ptrs[nr]->cpu_start = 1;
> -#ifdef CONFIG_HOTPLUG_CPU
> - set_preferred_offline_state(nr, CPU_STATE_ONLINE);
> -
> - if (get_cpu_current_state(nr) == CPU_STATE_INACTIVE) {
> - long rc;
> - unsigned long hcpuid;
> -
> - hcpuid = get_hard_smp_processor_id(nr);
> - rc = plpar_hcall_norets(H_PROD, hcpuid);
> - if (rc != H_SUCCESS)
> - printk(KERN_ERR "Error: Prod to wake up processor %d "
> - "Ret= %ld\n", nr, rc);
> - }
> -#endif
>
> return 0;
> }
> --
> 2.25.4
>
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