timers, sched/clock: Remove suspend from clock_read_data()

Currently cd.read_data.suspended is read by the hotpath function
sched_clock(). This variable need not be accessed on the
hotpath. In fact, once it is removed, we can remove the
conditional branches from sched_clock() and install a dummy
read_sched_clock function to suspend the clock.

The new master copy of the function pointer
(actual_read_sched_clock) is introduced and is used for all
reads of the clock hardware except those within sched_clock
itself.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1427397806-20889-4-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 25 insertions, 15 deletions

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 872e0685d1fb..52ea5d976393 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -28,10 +28,9 @@
  * @read_sched_clock:   Current clock source (or dummy source when suspended)
  * @mult:               Multipler for scaled math conversion
  * @shift:              Shift value for scaled math conversion
- * @suspended:          Flag to indicate if the clock is suspended (stopped)
  *
  * Care must be taken when updating this structure; it is read by
- * some very hot code paths. It occupies <=48 bytes and, when combined
+ * some very hot code paths. It occupies <=40 bytes and, when combined
  * with the seqcount used to synchronize access, comfortably fits into
  * a 64 byte cache line.
  */
@@ -42,7 +41,6 @@ struct clock_read_data {
         u64 (*read_sched_clock)(void);
         u32 mult;
         u32 shift;
-        bool suspended;
 };
 
 /**
@@ -64,6 +62,7 @@ struct clock_data {
         struct clock_read_data read_data;
         ktime_t wrap_kt;
         unsigned long rate;
+        u64 (*actual_read_sched_clock)(void);
 };
 
 static struct hrtimer sched_clock_timer;
@@ -83,6 +82,8 @@ static u64 notrace jiffy_sched_clock_read(void)
 static struct clock_data cd ____cacheline_aligned = {
         .read_data = { .mult = NSEC_PER_SEC / HZ,
                        .read_sched_clock = jiffy_sched_clock_read, },
+        .actual_read_sched_clock = jiffy_sched_clock_read,
+
 };
 
 static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
@@ -99,12 +100,9 @@ unsigned long long notrace sched_clock(void)
         do {
                 seq = raw_read_seqcount_begin(&cd.seq);
 
-                res = rd->epoch_ns;
-                if (!rd->suspended) {
-                        cyc = rd->read_sched_clock();
-                        cyc = (cyc - rd->epoch_cyc) & rd->sched_clock_mask;
-                        res += cyc_to_ns(cyc, rd->mult, rd->shift);
-                }
+                cyc = (rd->read_sched_clock() - rd->epoch_cyc) &
+                      rd->sched_clock_mask;
+                res = rd->epoch_ns + cyc_to_ns(cyc, rd->mult, rd->shift);
         } while (read_seqcount_retry(&cd.seq, seq));
 
         return res;
@@ -120,7 +118,7 @@ static void notrace update_sched_clock(void)
         u64 ns;
         struct clock_read_data *rd = &cd.read_data;
 
-        cyc = rd->read_sched_clock();
+        cyc = cd.actual_read_sched_clock();
         ns = rd->epoch_ns +
              cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
                        rd->mult, rd->shift);
@@ -166,10 +164,11 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
 
         /* update epoch for new counter and update epoch_ns from old counter*/
         new_epoch = read();
-        cyc = rd->read_sched_clock();
+        cyc = cd.actual_read_sched_clock();
         ns = rd->epoch_ns +
              cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
                        rd->mult, rd->shift);
+        cd.actual_read_sched_clock = read;
 
         raw_write_seqcount_begin(&cd.seq);
         rd->read_sched_clock = read;
@@ -209,7 +208,7 @@ void __init sched_clock_postinit(void)
          * If no sched_clock function has been provided at that point,
          * make it the final one one.
          */
-        if (cd.read_data.read_sched_clock == jiffy_sched_clock_read)
+        if (cd.actual_read_sched_clock == jiffy_sched_clock_read)
                 sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);
 
         update_sched_clock();
@@ -223,13 +222,24 @@ void __init sched_clock_postinit(void)
         hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
 }
 
+/*
+ * Clock read function for use when the clock is suspended.
+ *
+ * This function makes it appear to sched_clock() as if the clock
+ * stopped counting at its last update.
+ */
+static u64 notrace suspended_sched_clock_read(void)
+{
+        return cd.read_data.epoch_cyc;
+}
+
 static int sched_clock_suspend(void)
 {
         struct clock_read_data *rd = &cd.read_data;
 
         update_sched_clock();
         hrtimer_cancel(&sched_clock_timer);
-        rd->suspended = true;
+        rd->read_sched_clock = suspended_sched_clock_read;
         return 0;
 }
 
@@ -237,9 +247,9 @@ static void sched_clock_resume(void)
 {
         struct clock_read_data *rd = &cd.read_data;
 
-        rd->epoch_cyc = rd->read_sched_clock();
+        rd->epoch_cyc = cd.actual_read_sched_clock();
         hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
-        rd->suspended = false;
+        rd->read_sched_clock = cd.actual_read_sched_clock;
 }
 
 static struct syscore_ops sched_clock_ops = {