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Commit a831881b authored by Frederic Weisbecker's avatar Frederic Weisbecker
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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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: default avatarFrederic 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>
parent a8d7ad52

Documentation/kernel-parameters.txt

+6 −0
Original line number Diff line number Diff line
@@ -1913,6 +1913,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted. 
			Valid arguments: on, off

			Default: on



	nohz_extended=  [KNL,BOOT]

			In kernels built with CONFIG_NO_HZ_EXTENDED=y, set

			the specified list of CPUs whose tick will be stopped

			whenever possible. You need to keep at least one online

			CPU outside the range to maintain the timekeeping.



	noiotrap	[SH] Disables trapped I/O port accesses.



	noirqdebug	[X86-32] Disables the code which attempts to detect and

include/linux/tick.h

+7 −0
Original line number Diff line number Diff line
@@ -157,6 +157,13 @@ static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; } 
static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }

# endif /* !NO_HZ */



#ifdef CONFIG_NO_HZ_EXTENDED

extern int tick_nohz_extended_cpu(int cpu);

#else

static inline int tick_nohz_extended_cpu(int cpu) { return 0; }

#endif





# ifdef CONFIG_CPU_IDLE_GOV_MENU

extern void menu_hrtimer_cancel(void);

# else

kernel/time/Kconfig

+19 −0
Original line number Diff line number Diff line
@@ -79,6 +79,25 @@ config NO_HZ 
	  only trigger on an as-needed basis both when the system is

	  busy and when the system is idle.



config NO_HZ_EXTENDED

       bool "Full dynticks system"

       depends on NO_HZ && RCU_USER_QS && VIRT_CPU_ACCOUNTING_GEN && RCU_NOCB_CPU && SMP

       select CONTEXT_TRACKING_FORCE

       help

         Adaptively try to shutdown the tick whenever possible, even when

	 the CPU is running tasks. Typically this requires running a single

	 task on the CPU. Chances for running tickless are maximized when

	 the task mostly runs in userspace and has few kernel activity.



	 You need to fill up the nohz_extended boot parameter with the

	 desired range of dynticks CPUs.



	 This is implemented at the expense of some overhead in user <-> kernel

	 transitions: syscalls, exceptions and interrupts. Even when it's

	 dynamically off.



	 Say N.



config HIGH_RES_TIMERS

	bool "High Resolution Timer Support"

	depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS

kernel/time/tick-sched.c

+62 −0
Original line number Diff line number Diff line
@@ -142,6 +142,68 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs) 
	profile_tick(CPU_PROFILING);

}



#ifdef CONFIG_NO_HZ_EXTENDED

static cpumask_var_t nohz_extended_mask;

bool have_nohz_extended_mask;



int tick_nohz_extended_cpu(int cpu)

{

	if (!have_nohz_extended_mask)

		return 0;



	return cpumask_test_cpu(cpu, nohz_extended_mask);

}



/* Parse the boot-time nohz CPU list from the kernel parameters. */

static int __init tick_nohz_extended_setup(char *str)

{

	alloc_bootmem_cpumask_var(&nohz_extended_mask);

	if (cpulist_parse(str, nohz_extended_mask) < 0)

		pr_warning("NOHZ: Incorrect nohz_extended cpumask\n");

	else

		have_nohz_extended_mask = true;

	return 1;

}

__setup("nohz_extended=", tick_nohz_extended_setup);



static int __init init_tick_nohz_extended(void)

{

	cpumask_var_t online_nohz;

	int cpu;



	if (!have_nohz_extended_mask)

		return 0;



	if (!zalloc_cpumask_var(&online_nohz, GFP_KERNEL)) {

		pr_warning("NO_HZ: Not enough memory to check extended nohz mask\n");

		return -ENOMEM;

	}



	/*

	 * CPUs can probably not be concurrently offlined on initcall time.

	 * But we are paranoid, aren't we?

	 */

	get_online_cpus();



	/* Ensure we keep a CPU outside the dynticks range for timekeeping */

	cpumask_and(online_nohz, cpu_online_mask, nohz_extended_mask);

	if (cpumask_equal(online_nohz, cpu_online_mask)) {

		cpu = cpumask_any(cpu_online_mask);

		pr_warning("NO_HZ: Must keep at least one online CPU "

			   "out of nohz_extended range\n");

		pr_warning("NO_HZ: Clearing %d from nohz_extended range\n", cpu);

		cpumask_clear_cpu(cpu, nohz_extended_mask);

	}

	put_online_cpus();

	free_cpumask_var(online_nohz);



	return 0;

}

core_initcall(init_tick_nohz_extended);

#else

#define have_nohz_extended_mask (0)

#endif



/*

 * NOHZ - aka dynamic tick functionality

 */