timers/nohz: Convert tick dependency mask to atomic_t

The tick dependency mask was intially unsigned long because this is the
type on which clear_bit() operates on and fetch_or() accepts it.

But now that we have atomic_fetch_or(), we can instead use
atomic_andnot() to clear the bit. This consolidates the type of our
tick dependency mask, reduce its size on structures and benefit from
possible architecture optimizations on atomic_t operations.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1458830281-4255-3-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 33 insertions, 34 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 60bba7e032dc..52c4847b05e2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -720,7 +720,7 @@ struct signal_struct {
         struct task_cputime cputime_expires;
 
 #ifdef CONFIG_NO_HZ_FULL
-        unsigned long tick_dep_mask;
+        atomic_t tick_dep_mask;
 #endif
 
         struct list_head cpu_timers[3];
@@ -1549,7 +1549,7 @@ struct task_struct {
 #endif
 
 #ifdef CONFIG_NO_HZ_FULL
-        unsigned long tick_dep_mask;
+        atomic_t tick_dep_mask;
 #endif
         unsigned long nvcsw, nivcsw; /* context switch counts */
         u64 start_time;         /* monotonic time in nsec */
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 084b79f5917e..58e3310c9b21 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -157,52 +157,50 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
 cpumask_var_t tick_nohz_full_mask;
 cpumask_var_t housekeeping_mask;
 bool tick_nohz_full_running;
-static unsigned long tick_dep_mask;
+static atomic_t tick_dep_mask;
 
-static void trace_tick_dependency(unsigned long dep)
+static bool check_tick_dependency(atomic_t *dep)
 {
-        if (dep & TICK_DEP_MASK_POSIX_TIMER) {
+        int val = atomic_read(dep);
+
+        if (val & TICK_DEP_MASK_POSIX_TIMER) {
                 trace_tick_stop(0, TICK_DEP_MASK_POSIX_TIMER);
-                return;
+                return true;
         }
 
-        if (dep & TICK_DEP_MASK_PERF_EVENTS) {
+        if (val & TICK_DEP_MASK_PERF_EVENTS) {
                 trace_tick_stop(0, TICK_DEP_MASK_PERF_EVENTS);
-                return;
+                return true;
         }
 
-        if (dep & TICK_DEP_MASK_SCHED) {
+        if (val & TICK_DEP_MASK_SCHED) {
                 trace_tick_stop(0, TICK_DEP_MASK_SCHED);
-                return;
+                return true;
         }
 
-        if (dep & TICK_DEP_MASK_CLOCK_UNSTABLE)
+        if (val & TICK_DEP_MASK_CLOCK_UNSTABLE) {
                 trace_tick_stop(0, TICK_DEP_MASK_CLOCK_UNSTABLE);
+                return true;
+        }
+
+        return false;
 }
 
 static bool can_stop_full_tick(struct tick_sched *ts)
 {
         WARN_ON_ONCE(!irqs_disabled());
 
-        if (tick_dep_mask) {
-                trace_tick_dependency(tick_dep_mask);
+        if (check_tick_dependency(&tick_dep_mask))
                 return false;
-        }
 
-        if (ts->tick_dep_mask) {
-                trace_tick_dependency(ts->tick_dep_mask);
+        if (check_tick_dependency(&ts->tick_dep_mask))
                 return false;
-        }
 
-        if (current->tick_dep_mask) {
-                trace_tick_dependency(current->tick_dep_mask);
+        if (check_tick_dependency(&current->tick_dep_mask))
                 return false;
-        }
 
-        if (current->signal->tick_dep_mask) {
-                trace_tick_dependency(current->signal->tick_dep_mask);
+        if (check_tick_dependency(&current->signal->tick_dep_mask))
                 return false;
-        }
 
         return true;
 }
@@ -259,12 +257,12 @@ static void tick_nohz_full_kick_all(void)
         preempt_enable();
 }
 
-static void tick_nohz_dep_set_all(unsigned long *dep,
+static void tick_nohz_dep_set_all(atomic_t *dep,
                                   enum tick_dep_bits bit)
 {
-        unsigned long prev;
+        int prev;
 
-        prev = fetch_or(dep, BIT_MASK(bit));
+        prev = atomic_fetch_or(dep, BIT(bit));
         if (!prev)
                 tick_nohz_full_kick_all();
 }
@@ -280,7 +278,7 @@ void tick_nohz_dep_set(enum tick_dep_bits bit)
 
 void tick_nohz_dep_clear(enum tick_dep_bits bit)
 {
-        clear_bit(bit, &tick_dep_mask);
+        atomic_andnot(BIT(bit), &tick_dep_mask);
 }
 
 /*
@@ -289,12 +287,12 @@ void tick_nohz_dep_clear(enum tick_dep_bits bit)
  */
 void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit)
 {
-        unsigned long prev;
+        int prev;
         struct tick_sched *ts;
 
         ts = per_cpu_ptr(&tick_cpu_sched, cpu);
 
-        prev = fetch_or(&ts->tick_dep_mask, BIT_MASK(bit));
+        prev = atomic_fetch_or(&ts->tick_dep_mask, BIT(bit));
         if (!prev) {
                 preempt_disable();
                 /* Perf needs local kick that is NMI safe */
@@ -313,7 +311,7 @@ void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
 {
         struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);
 
-        clear_bit(bit, &ts->tick_dep_mask);
+        atomic_andnot(BIT(bit), &ts->tick_dep_mask);
 }
 
 /*
@@ -331,7 +329,7 @@ void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit)
 
 void tick_nohz_dep_clear_task(struct task_struct *tsk, enum tick_dep_bits bit)
 {
-        clear_bit(bit, &tsk->tick_dep_mask);
+        atomic_andnot(BIT(bit), &tsk->tick_dep_mask);
 }
 
 /*
@@ -345,7 +343,7 @@ void tick_nohz_dep_set_signal(struct signal_struct *sig, enum tick_dep_bits bit)
 
 void tick_nohz_dep_clear_signal(struct signal_struct *sig, enum tick_dep_bits bit)
 {
-        clear_bit(bit, &sig->tick_dep_mask);
+        atomic_andnot(BIT(bit), &sig->tick_dep_mask);
 }
 
 /*
@@ -366,7 +364,8 @@ void __tick_nohz_task_switch(void)
         ts = this_cpu_ptr(&tick_cpu_sched);
 
         if (ts->tick_stopped) {
-                if (current->tick_dep_mask || current->signal->tick_dep_mask)
+                if (atomic_read(&current->tick_dep_mask) ||
+                    atomic_read(&current->signal->tick_dep_mask))
                         tick_nohz_full_kick();
         }
 out:
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index eb4e32566a83..bf38226e5c17 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -60,7 +60,7 @@ struct tick_sched {
         u64                             next_timer;
         ktime_t                         idle_expires;
         int                             do_timer_last;
-        unsigned long                   tick_dep_mask;
+        atomic_t                        tick_dep_mask;
 };
 
 extern struct tick_sched *tick_get_tick_sched(int cpu);