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SubjectRE: [PATCH] cpufreq: Avoid leaving stale IRQ work items during CPU offline
Date
> Subject: [PATCH] cpufreq: Avoid leaving stale IRQ work items during CPU
> offline
>
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> The scheduler code calling cpufreq_update_util() may run during CPU offline
> on the target CPU after the IRQ work lists have been flushed for it, so the
> target CPU should be prevented from running code that may queue up an IRQ
> work item on it at that point.
>
> Unfortunately, that may not be the case if dvfs_possible_from_any_cpu is set
> for at least one cpufreq policy in the system, because that allows the CPU
> going offline to run the utilization update callback of the cpufreq governor on
> behalf of another (online) CPU in some cases.
>
> If that happens, the cpufreq governor callback may queue up an IRQ work on
> the CPU running it, which is going offline, and the IRQ work will not be flushed

"will" -> "might"

> after that point. Moreover, that IRQ work cannot be flushed until the
> "offlining" CPU goes back online, so if any other CPU calls irq_work_sync() to
> wait for the completion of that IRQ work, it will have to wait until the
> "offlining" CPU is back online and that may not happen forever. In particular,
> a system-wide deadlock may occur during CPU online as a result of that.
>
> The failing scenario is as follows. CPU0 is the boot CPU, so it creates a
> cpufreq policy and becomes the "leader" of it (policy->cpu). It cannot go
> offline, because it is the boot CPU.
> Next, other CPUs join the cpufreq policy as they go online and they leave it
> when they go offline. The last CPU to go offline, say CPU3, may queue up an
> IRQ work while running the governor callback on behalf of CPU0 after leaving
> the cpufreq policy because of the dvfs_possible_from_any_cpu effect
> described above. Then, CPU0 is the only online CPU in the system and the
> stale IRQ work is still queued on CPU3. When, say, CPU1 goes back online, it
> will run
> irq_work_sync() to wait for that IRQ work to complete and so it will wait for
> CPU3 to go back online (which may never happen even in principle), but
> (worse yet) CPU0 is waiting for CPU1 at that point too and a system-wide
> deadlock occurs.
>
> To address this problem notice that CPUs which cannot run cpufreq utilization
> update code for themselves (for example, because they have left the cpufreq
> policies that they belonged to), should also be prevented from running that
> code on behalf of the other CPUs that belong to a cpufreq policy with
> dvfs_possible_from_any_cpu set and so in that case the
> cpufreq_update_util_data pointer of the CPU running the code must not be
> NULL as well as for the CPU which is the target of the cpufreq utilization
> update in progress.
>
> Accordingly, change cpufreq_this_cpu_can_update() into a regular function in
> kernel/sched/cpufreq.c (instead of a static inline in a header file) and make it
> check the cpufreq_update_util_data pointer of the local CPU if
> dvfs_possible_from_any_cpu is set for the target cpufreq policy.
>
> Also update the schedutil governor to do the
> cpufreq_this_cpu_can_update() check in the non-fast-switch case too to avoid
> the stale IRQ work issues.
>
> Fixes: 99d14d0e16fa ("cpufreq: Process remote callbacks from any CPU if the
> platform permits")
> Link:
> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.ke
> rnel.org%2Flinux-pm%2F20191121093557.bycvdo4xyinbc5cb%40vireshk-i7%
> 2F&amp;data=02%7C01%7Cpeng.fan%40nxp.com%7C969872a0d7014a14b0
> 7b08d77e24e5b7%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C6
> 37116569251293496&amp;sdata=jEVM3d8btZpjqEhAKEF40PC8dxq4qjQbYAN
> Xi%2BA8a8o%3D&amp;reserved=0
> Reported-by: Anson Huang <anson.huang@nxp.com>
> Cc: 4.14+ <stable@vger.kernel.org> # 4.14+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Tested-by: Peng Fan <peng.fan@nxp.com> (i.MX8QXP-MEK)

> ---
>
> Peter,
>
> The reason why I want to address the issue this way is because IMO the right
> place to do the check is the cpufreq governor. Governors that don't use
> cpufreq_this_cpu_can_update() at all don't need to worry as well as
> governors that don't use IRQ works.
>
> The cpufreq governor is given an opportunity to update the frequency of the
> CPU and it needs to decide whether or not to really do that.
>
> Please let me know if you have any concerns.
>
> ---
> include/linux/cpufreq.h | 11 -----------
> include/linux/sched/cpufreq.h | 3 +++
> kernel/sched/cpufreq.c | 18 ++++++++++++++++++
> kernel/sched/cpufreq_schedutil.c | 8 +++-----
> 4 files changed, 24 insertions(+), 16 deletions(-)
>
> Index: linux-pm/include/linux/cpufreq.h
> ==============================================================
> =====
> --- linux-pm.orig/include/linux/cpufreq.h
> +++ linux-pm/include/linux/cpufreq.h
> @@ -595,17 +595,6 @@ struct governor_attr {
> size_t count);
> };
>
> -static inline bool cpufreq_this_cpu_can_update(struct cpufreq_policy *policy)
> -{
> - /*
> - * Allow remote callbacks if:
> - * - dvfs_possible_from_any_cpu flag is set
> - * - the local and remote CPUs share cpufreq policy
> - */
> - return policy->dvfs_possible_from_any_cpu ||
> - cpumask_test_cpu(smp_processor_id(), policy->cpus);
> -}
> -
>
> /*************************************************************
> ********
> * FREQUENCY TABLE HELPERS
> *
>
> **************************************************************
> *******/
> Index: linux-pm/kernel/sched/cpufreq.c
> ==============================================================
> =====
> --- linux-pm.orig/kernel/sched/cpufreq.c
> +++ linux-pm/kernel/sched/cpufreq.c
> @@ -5,6 +5,8 @@
> * Copyright (C) 2016, Intel Corporation
> * Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> */
> +#include <linux/cpufreq.h>
> +
> #include "sched.h"
>
> DEFINE_PER_CPU(struct update_util_data __rcu *,
> cpufreq_update_util_data); @@ -57,3 +59,19 @@ void
> cpufreq_remove_update_util_hook(int
> rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), NULL); }
> EXPORT_SYMBOL_GPL(cpufreq_remove_update_util_hook);
> +
> +/**
> + * cpufreq_this_cpu_can_update - Check if cpufreq policy can be updated.
> + * @policy: cpufreq policy to check.
> + *
> + * Return 'true' if:
> + * - the local and remote CPUs share @policy,
> + * - dvfs_possible_from_any_cpu is set in @policy and the local CPU is not
> going
> + * offline (in which case it is not expected to run cpufreq updates any
> more).
> + */
> +bool cpufreq_this_cpu_can_update(struct cpufreq_policy *policy) {
> + return cpumask_test_cpu(smp_processor_id(), policy->cpus) ||
> + (policy->dvfs_possible_from_any_cpu &&
> +
> rcu_dereference_sched(*this_cpu_ptr(&cpufreq_update_util_data)));
> +}
> Index: linux-pm/include/linux/sched/cpufreq.h
> ==============================================================
> =====
> --- linux-pm.orig/include/linux/sched/cpufreq.h
> +++ linux-pm/include/linux/sched/cpufreq.h
> @@ -12,6 +12,8 @@
> #define SCHED_CPUFREQ_MIGRATION (1U << 1)
>
> #ifdef CONFIG_CPU_FREQ
> +struct cpufreq_policy;
> +
> struct update_util_data {
> void (*func)(struct update_util_data *data, u64 time, unsigned int
> flags); }; @@ -20,6 +22,7 @@ void cpufreq_add_update_util_hook(int cp
> void (*func)(struct update_util_data *data, u64
> time,
> unsigned int flags));
> void cpufreq_remove_update_util_hook(int cpu);
> +bool cpufreq_this_cpu_can_update(struct cpufreq_policy *policy);
>
> static inline unsigned long map_util_freq(unsigned long util,
> unsigned long freq, unsigned long cap)
> Index: linux-pm/kernel/sched/cpufreq_schedutil.c
> ==============================================================
> =====
> --- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
> +++ linux-pm/kernel/sched/cpufreq_schedutil.c
> @@ -82,12 +82,10 @@ static bool sugov_should_update_freq(str
> * by the hardware, as calculating the frequency is pointless if
> * we cannot in fact act on it.
> *
> - * For the slow switching platforms, the kthread is always scheduled on
> - * the right set of CPUs and any CPU can find the next frequency and
> - * schedule the kthread.
> + * This is needed on the slow switching platforms too to prevent CPUs
> + * going offline from leaving stale IRQ work items behind.
> */
> - if (sg_policy->policy->fast_switch_enabled &&
> - !cpufreq_this_cpu_can_update(sg_policy->policy))
> + if (!cpufreq_this_cpu_can_update(sg_policy->policy))
> return false;
>
> if (unlikely(sg_policy->limits_changed)) {
>
>

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