nohz: Only enable context tracking on full dynticks CPUs

The context tracking subsystem has the ability to selectively
enable the tracking on any defined subset of CPU. This means that
we can define a CPU range that doesn't run the context tracking
and another range that does.

Now what we want in practice is to enable the tracking on full
dynticks CPUs only. In order to perform this, we just need to pass
our full dynticks CPU range selection from the full dynticks
subsystem to the context tracking.

This way we can spare the overhead of RCU user extended quiescent
state and vtime maintainance on the CPUs that are outside the
full dynticks range. Just keep in mind the raw context tracking
itself is still necessary everywhere.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Kevin Hilman <khilman@linaro.org>

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index d883ff0dd8fc72fb6ef3938bd7adcbe54956ff8b..1ae37c708c63033a37a5efc593e379e7593a7a41 100644 (file)
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -34,6 +34,8 @@ static inline bool context_tracking_active(void)
        return __this_cpu_read(context_tracking.active);
 }
 
+extern void context_tracking_cpu_set(int cpu);
+
 extern void user_enter(void);
 extern void user_exit(void);
 

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 7b095de356c5775f22da4cc168293c62c96f6059..72bcb2570d3e0db89bddf7f8e2aa770751c49eec 100644 (file)
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -26,6 +26,11 @@ DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
 #endif
 };
 
+void context_tracking_cpu_set(int cpu)
+{
+       per_cpu(context_tracking.active, cpu) = true;
+}
+
 /**
  * user_enter - Inform the context tracking that the CPU is going to
  *              enter userspace mode.

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 9563c744dad2cd5823624e127e1c266dc45d3cf5..91a2528b5f44c8600da227ad05c054d5226c2996 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -23,6 +23,7 @@
 #include <linux/irq_work.h>
 #include <linux/posix-timers.h>
 #include <linux/perf_event.h>
+#include <linux/context_tracking.h>
 
 #include <asm/irq_regs.h>
 
@@ -344,11 +345,16 @@ static int tick_nohz_init_all(void)
 
 void __init tick_nohz_init(void)
 {
+       int cpu;
+
        if (!have_nohz_full_mask) {
                if (tick_nohz_init_all() < 0)
                        return;
        }
 
+       for_each_cpu(cpu, nohz_full_mask)
+               context_tracking_cpu_set(cpu);
+
        cpu_notifier(tick_nohz_cpu_down_callback, 0);
        cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), nohz_full_mask);
        pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf);