timers, sched/clock: Optimize cache line usage

Currently sched_clock(), a very hot code path, is not optimized
to minimise its cache profile. In particular:

  1. cd is not ____cacheline_aligned,

  2. struct clock_data does not distinguish between hotpath and
     coldpath data, reducing locality of reference in the hotpath,

  3. Some hotpath data is missing from struct clock_data and is marked
     __read_mostly (which more or less guarantees it will not share a
     cache line with cd).

This patch corrects these problems by extracting all hotpath
data into a separate structure and using ____cacheline_aligned
to ensure the hotpath uses a single (64 byte) cache line.

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: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1427397806-20889-3-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 1751e956add9f165547e13710a6dbd26f0b97723..872e0685d1fbbd2428c56ba205034679738a5b4f 100644 (file)
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -18,28 +18,59 @@
 #include <linux/seqlock.h>
 #include <linux/bitops.h>
 
-struct clock_data {
-       ktime_t wrap_kt;
+/**
+ * struct clock_read_data - data required to read from sched_clock
+ *
+ * @epoch_ns:          sched_clock value at last update
+ * @epoch_cyc:         Clock cycle value at last update
+ * @sched_clock_mask:   Bitmask for two's complement subtraction of non 64bit
+ *                     clocks
+ * @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
+ * with the seqcount used to synchronize access, comfortably fits into
+ * a 64 byte cache line.
+ */
+struct clock_read_data {
        u64 epoch_ns;
        u64 epoch_cyc;
-       seqcount_t seq;
-       unsigned long rate;
+       u64 sched_clock_mask;
+       u64 (*read_sched_clock)(void);
        u32 mult;
        u32 shift;
        bool suspended;
 };
 
+/**
+ * struct clock_data - all data needed for sched_clock (including
+ *                     registration of a new clock source)
+ *
+ * @seq:               Sequence counter for protecting updates.
+ * @read_data:         Data required to read from sched_clock.
+ * @wrap_kt:           Duration for which clock can run before wrapping
+ * @rate:              Tick rate of the registered clock
+ * @actual_read_sched_clock: Registered clock read function
+ *
+ * The ordering of this structure has been chosen to optimize cache
+ * performance. In particular seq and read_data (combined) should fit
+ * into a single 64 byte cache line.
+ */
+struct clock_data {
+       seqcount_t seq;
+       struct clock_read_data read_data;
+       ktime_t wrap_kt;
+       unsigned long rate;
+};
+
 static struct hrtimer sched_clock_timer;
 static int irqtime = -1;
 
 core_param(irqtime, irqtime, int, 0400);
 
-static struct clock_data cd = {
-       .mult   = NSEC_PER_SEC / HZ,
-};
-
-static u64 __read_mostly sched_clock_mask;
-
 static u64 notrace jiffy_sched_clock_read(void)
 {
        /*
@@ -49,7 +80,10 @@ static u64 notrace jiffy_sched_clock_read(void)
        return (u64)(jiffies - INITIAL_JIFFIES);
 }
 
-static u64 __read_mostly (*read_sched_clock)(void) = jiffy_sched_clock_read;
+static struct clock_data cd ____cacheline_aligned = {
+       .read_data = { .mult = NSEC_PER_SEC / HZ,
+                      .read_sched_clock = jiffy_sched_clock_read, },
+};
 
 static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
 {
@@ -60,15 +94,16 @@ unsigned long long notrace sched_clock(void)
 {
        u64 cyc, res;
        unsigned long seq;
+       struct clock_read_data *rd = &cd.read_data;
 
        do {
                seq = raw_read_seqcount_begin(&cd.seq);
 
-               res = cd.epoch_ns;
-               if (!cd.suspended) {
-                       cyc = read_sched_clock();
-                       cyc = (cyc - cd.epoch_cyc) & sched_clock_mask;
-                       res += cyc_to_ns(cyc, cd.mult, cd.shift);
+               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);
                }
        } while (read_seqcount_retry(&cd.seq, seq));
 
@@ -83,16 +118,17 @@ static void notrace update_sched_clock(void)
        unsigned long flags;
        u64 cyc;
        u64 ns;
+       struct clock_read_data *rd = &cd.read_data;
 
-       cyc = read_sched_clock();
-       ns = cd.epoch_ns +
-               cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
-                         cd.mult, cd.shift);
+       cyc = rd->read_sched_clock();
+       ns = rd->epoch_ns +
+            cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
+                      rd->mult, rd->shift);
 
        raw_local_irq_save(flags);
        raw_write_seqcount_begin(&cd.seq);
-       cd.epoch_ns = ns;
-       cd.epoch_cyc = cyc;
+       rd->epoch_ns = ns;
+       rd->epoch_cyc = cyc;
        raw_write_seqcount_end(&cd.seq);
        raw_local_irq_restore(flags);
 }
@@ -111,6 +147,7 @@ void __init sched_clock_register(u64 (*read)(void), int bits,
        u32 new_mult, new_shift;
        unsigned long r;
        char r_unit;
+       struct clock_read_data *rd = &cd.read_data;
 
        if (cd.rate > rate)
                return;
@@ -129,17 +166,18 @@ 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 = read_sched_clock();
-       ns = cd.epoch_ns + cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,
-                         cd.mult, cd.shift);
+       cyc = rd->read_sched_clock();
+       ns = rd->epoch_ns +
+            cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,
+                      rd->mult, rd->shift);
 
        raw_write_seqcount_begin(&cd.seq);
-       read_sched_clock = read;
-       sched_clock_mask = new_mask;
-       cd.mult = new_mult;
-       cd.shift = new_shift;
-       cd.epoch_cyc = new_epoch;
-       cd.epoch_ns = ns;
+       rd->read_sched_clock = read;
+       rd->sched_clock_mask = new_mask;
+       rd->mult = new_mult;
+       rd->shift = new_shift;
+       rd->epoch_cyc = new_epoch;
+       rd->epoch_ns = ns;
        raw_write_seqcount_end(&cd.seq);
 
        r = rate;
@@ -171,7 +209,7 @@ void __init sched_clock_postinit(void)
         * If no sched_clock function has been provided at that point,
         * make it the final one one.
         */
-       if (read_sched_clock == jiffy_sched_clock_read)
+       if (cd.read_data.read_sched_clock == jiffy_sched_clock_read)
                sched_clock_register(jiffy_sched_clock_read, BITS_PER_LONG, HZ);
 
        update_sched_clock();
@@ -187,17 +225,21 @@ void __init sched_clock_postinit(void)
 
 static int sched_clock_suspend(void)
 {
+       struct clock_read_data *rd = &cd.read_data;
+
        update_sched_clock();
        hrtimer_cancel(&sched_clock_timer);
-       cd.suspended = true;
+       rd->suspended = true;
        return 0;
 }
 
 static void sched_clock_resume(void)
 {
-       cd.epoch_cyc = read_sched_clock();
+       struct clock_read_data *rd = &cd.read_data;
+
+       rd->epoch_cyc = rd->read_sched_clock();
        hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
-       cd.suspended = false;
+       rd->suspended = false;
 }
 
 static struct syscore_ops sched_clock_ops = {