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>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4609e81dbc37fc2dbfa005ff607890df3a8bbc6b..231698feaddcdf669e5969ec5efd0a3e74bf2130 100644 (file)
--- 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 553272e6af554844fbea920f0c3b08707c52e13a..44bfa8aa439f266c2df0947bd3c74b673a0d4bbb 100644 (file)
--- 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 */
 
+#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

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 24510d84efd760b287f532727dfc20f0d8e57405..5a87c03e45adbe4068dd3dcd673cbf185e232b95 100644 (file)
--- 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 a19a39952c1b24d3dd0ab0e9bd354fa9c5a7cb46..79c275f08b7db41099f477bc22ccb690b40ea4d5 100644 (file)
--- 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
  */