diff options
authorFrederic Weisbecker <fweisbec@gmail.com>2012-12-18 17:32:19 +0100
committerFrederic Weisbecker <fweisbec@gmail.com>2013-03-21 15:38:33 +0100
commita831881be220358a1d28c5d95d69449fb6d623ca (patch)
parenta8d7ad52a7befbde896276d05c75c90fed48b5bf (diff)
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>
4 files changed, 94 insertions, 0 deletions
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4609e81..231698f 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -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
diff --git a/include/linux/tick.h b/include/linux/tick.h
index 553272e..44bfa8a 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -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 */
+extern int tick_nohz_extended_cpu(int cpu);
+static inline int tick_nohz_extended_cpu(int cpu) { return 0; }
extern void menu_hrtimer_cancel(void);
# else
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 24510d8..5a87c03 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.
+ bool "Full dynticks system"
+ 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.
bool "High Resolution Timer Support"
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a19a399..79c275f 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)
+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;
+#define have_nohz_extended_mask (0)
* NOHZ - aka dynamic tick functionality