nohz: Basic full dynticks interface
For extreme usecases such as Real Time or HPC, having the ability to shutdown the tick when a single task runs on a CPU is a desired feature: * Reducing the amount of interrupts improves throughput for CPU-bound tasks. The CPU is less distracted from its real job, from an execution time and from the cache point of views. * This also improve latency response as we have less critical sections. Start with introducing a very simple interface to define full dynticks CPU: use a boot time option defined cpumask through the "nohz_extended=" kernel parameter. CPUs that are part of this range will have their tick shutdown whenever possible: provided they run a single task and they don't do kernel activity that require the periodic tick. These details will be later documented in Documentation/* An online CPU must be kept outside this range to handle the timekeeping. Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Chris Metcalf <cmetcalf@tilera.com> Cc: Christoph Lameter <cl@linux.com> Cc: Geoff Levand <geoff@infradead.org> Cc: Gilad Ben Yossef <gilad@benyossef.com> Cc: Hakan Akkan <hakanakkan@gmail.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Kevin Hilman <khilman@linaro.org> Cc: Li Zhong <zhong@linux.vnet.ibm.com> Cc: Namhyung Kim <namhyung.kim@lge.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Thomas Gleixner <tglx@linutronix.de>
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@ -1913,6 +1913,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
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Valid arguments: on, off
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Default: on
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nohz_extended= [KNL,BOOT]
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In kernels built with CONFIG_NO_HZ_EXTENDED=y, set
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the specified list of CPUs whose tick will be stopped
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whenever possible. You need to keep at least one online
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CPU outside the range to maintain the timekeeping.
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noiotrap [SH] Disables trapped I/O port accesses.
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noirqdebug [X86-32] Disables the code which attempts to detect and
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@ -157,6 +157,13 @@ static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
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static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
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# endif /* !NO_HZ */
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#ifdef CONFIG_NO_HZ_EXTENDED
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extern int tick_nohz_extended_cpu(int cpu);
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#else
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static inline int tick_nohz_extended_cpu(int cpu) { return 0; }
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#endif
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# ifdef CONFIG_CPU_IDLE_GOV_MENU
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extern void menu_hrtimer_cancel(void);
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# else
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@ -79,6 +79,25 @@ config NO_HZ
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only trigger on an as-needed basis both when the system is
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busy and when the system is idle.
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config NO_HZ_EXTENDED
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bool "Full dynticks system"
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depends on NO_HZ && RCU_USER_QS && VIRT_CPU_ACCOUNTING_GEN && RCU_NOCB_CPU && SMP
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select CONTEXT_TRACKING_FORCE
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help
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Adaptively try to shutdown the tick whenever possible, even when
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the CPU is running tasks. Typically this requires running a single
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task on the CPU. Chances for running tickless are maximized when
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the task mostly runs in userspace and has few kernel activity.
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You need to fill up the nohz_extended boot parameter with the
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desired range of dynticks CPUs.
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This is implemented at the expense of some overhead in user <-> kernel
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transitions: syscalls, exceptions and interrupts. Even when it's
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dynamically off.
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Say N.
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config HIGH_RES_TIMERS
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bool "High Resolution Timer Support"
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depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
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@ -142,6 +142,68 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
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profile_tick(CPU_PROFILING);
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}
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#ifdef CONFIG_NO_HZ_EXTENDED
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static cpumask_var_t nohz_extended_mask;
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bool have_nohz_extended_mask;
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int tick_nohz_extended_cpu(int cpu)
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{
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if (!have_nohz_extended_mask)
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return 0;
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return cpumask_test_cpu(cpu, nohz_extended_mask);
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}
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/* Parse the boot-time nohz CPU list from the kernel parameters. */
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static int __init tick_nohz_extended_setup(char *str)
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{
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alloc_bootmem_cpumask_var(&nohz_extended_mask);
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if (cpulist_parse(str, nohz_extended_mask) < 0)
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pr_warning("NOHZ: Incorrect nohz_extended cpumask\n");
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else
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have_nohz_extended_mask = true;
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return 1;
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}
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__setup("nohz_extended=", tick_nohz_extended_setup);
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static int __init init_tick_nohz_extended(void)
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{
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cpumask_var_t online_nohz;
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int cpu;
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if (!have_nohz_extended_mask)
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return 0;
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if (!zalloc_cpumask_var(&online_nohz, GFP_KERNEL)) {
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pr_warning("NO_HZ: Not enough memory to check extended nohz mask\n");
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return -ENOMEM;
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}
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/*
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* CPUs can probably not be concurrently offlined on initcall time.
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* But we are paranoid, aren't we?
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*/
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get_online_cpus();
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/* Ensure we keep a CPU outside the dynticks range for timekeeping */
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cpumask_and(online_nohz, cpu_online_mask, nohz_extended_mask);
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if (cpumask_equal(online_nohz, cpu_online_mask)) {
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cpu = cpumask_any(cpu_online_mask);
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pr_warning("NO_HZ: Must keep at least one online CPU "
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"out of nohz_extended range\n");
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pr_warning("NO_HZ: Clearing %d from nohz_extended range\n", cpu);
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cpumask_clear_cpu(cpu, nohz_extended_mask);
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}
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put_online_cpus();
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free_cpumask_var(online_nohz);
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return 0;
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}
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core_initcall(init_tick_nohz_extended);
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#else
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#define have_nohz_extended_mask (0)
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#endif
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/*
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* NOHZ - aka dynamic tick functionality
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*/
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