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>

2 files changed, 81 insertions, 0 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 24510d84efd7..5a87c03e45ad 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -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
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a19a39952c1b..79c275f08b7d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -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
  */