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Message-Id: <1363879460-21595-2-git-send-email-fweisbec@gmail.com>
Date: Thu, 21 Mar 2013 16:24:18 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Chris Metcalf <cmetcalf@...era.com>,
Christoph Lameter <cl@...ux.com>,
Geoff Levand <geoff@...radead.org>,
Gilad Ben Yossef <gilad@...yossef.com>,
Hakan Akkan <hakanakkan@...il.com>,
Ingo Molnar <mingo@...nel.org>,
Kevin Hilman <khilman@...aro.org>,
Li Zhong <zhong@...ux.vnet.ibm.com>,
Namhyung Kim <namhyung.kim@....com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Paul Gortmaker <paul.gortmaker@...driver.com>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 1/3] 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@...ux.vnet.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Chris Metcalf <cmetcalf@...era.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Geoff Levand <geoff@...radead.org>
Cc: Gilad Ben Yossef <gilad@...yossef.com>
Cc: Hakan Akkan <hakanakkan@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Kevin Hilman <khilman@...aro.org>
Cc: Li Zhong <zhong@...ux.vnet.ibm.com>
Cc: Namhyung Kim <namhyung.kim@....com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@...driver.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
---
Documentation/kernel-parameters.txt | 6 +++
include/linux/tick.h | 7 ++++
kernel/time/Kconfig | 19 +++++++++++
kernel/time/tick-sched.c | 62 +++++++++++++++++++++++++++++++++++
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 */
+#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 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.
+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 a19a399..79c275f0 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
*/
--
1.7.5.4
--
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