[PATCH 06/24] timekeeping: Reorder struct timekeeper

Anna-Maria Behnsen
2024-09-11T13:30:05Z

From: Thomas Gleixner <tglx@linutronix.de>

struct timekeeper is ordered suboptimal vs. cachelines. The layout,
including the preceding seqcount (see struct tk_core in timekeeper.c) is:

cacheline 0: seqcount, tkr_mono
cacheline 1: tkr_raw, xtime_sec
cacheline 2: ktime_sec ... tai_offset, internal variables
cacheline 3: next_leap_ktime, raw_sec, internal variables
cacheline 4: internal variables

So any access to via ktime_get*() except for access to CLOCK_MONOTONIC_RAW
will use either cachelines 0 + 1 or cachelines 0 + 2. Access to
CLOCK_MONOTONIC_RAW uses cachelines 0 + 1 + 3.

Reorder the members so that the result is more efficient:

cacheline 0: seqcount, tkr_mono
cacheline 1: xtime_sec, ktime_sec ... tai_offset
cacheline 2: tkr_raw, raw_sec
cacheline 3: internal variables
cacheline 4: internal variables

That means ktime_get*() will access cacheline 0 + 1 and CLOCK_MONOTONIC_RAW
access will use cachelines 0 + 2.

Update kernel-doc and fix formatting issues while at it. Also fix a typo
in struct tk_read_base kernel-doc.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
---
include/linux/timekeeper_internal.h | 102 +++++++++++++++++++++---------------
1 file changed, 61 insertions(+), 41 deletions(-)

diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index 84ff2844df2a..5805c5f61df4 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -26,7 +26,7 @@
* occupies a single 64byte cache line.
*
* The struct is separate from struct timekeeper as it is also used
- * for a fast NMI safe accessors.
+ * for the fast NMI safe accessors.
*
* @base_real is for the fast NMI safe accessor to allow reading clock
* realtime from any context.
@@ -44,33 +44,41 @@ struct tk_read_base {

/**
* struct timekeeper - Structure holding internal timekeeping values.
- * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
- * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
- * @xtime_sec: Current CLOCK_REALTIME time in seconds
- * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
- * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
- * @offs_real: Offset clock monotonic -> clock realtime
- * @offs_boot: Offset clock monotonic -> clock boottime
- * @offs_tai: Offset clock monotonic -> clock tai
- * @tai_offset: The current UTC to TAI offset in seconds
- * @clock_was_set_seq: The sequence number of clock was set events
- * @cs_was_changed_seq: The sequence number of clocksource change events
- * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
- * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
- * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
- * @cycle_interval: Number of clock cycles in one NTP interval
- * @xtime_interval: Number of clock shifted nano seconds in one NTP
- * interval.
- * @xtime_remainder: Shifted nano seconds left over when rounding
- * @cycle_interval
- * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
- * @ntp_error: Difference between accumulated time and NTP time in ntp
- * shifted nano seconds.
- * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
- * ntp shifted nano seconds.
- * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
- * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
- * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
+ * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
+ * @xtime_sec: Current CLOCK_REALTIME time in seconds
+ * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
+ * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
+ * @offs_real: Offset clock monotonic -> clock realtime
+ * @offs_boot: Offset clock monotonic -> clock boottime
+ * @offs_tai: Offset clock monotonic -> clock tai
+ * @tai_offset: The current UTC to TAI offset in seconds
+ * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
+ * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
+ * @clock_was_set_seq: The sequence number of clock was set events
+ * @cs_was_changed_seq: The sequence number of clocksource change events
+ * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
+ * @cycle_interval: Number of clock cycles in one NTP interval
+ * @xtime_interval: Number of clock shifted nano seconds in one NTP
+ * interval.
+ * @xtime_remainder: Shifted nano seconds left over when rounding
+ * @cycle_interval
+ * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
+ * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
+ * @ntp_tick: The ntp_tick_length() value currently being
+ * used. This cached copy ensures we consistently
+ * apply the tick length for an entire tick, as
+ * ntp_tick_length may change mid-tick, and we don't
+ * want to apply that new value to the tick in
+ * progress.
+ * @ntp_error: Difference between accumulated time and NTP time in ntp
+ * shifted nano seconds.
+ * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
+ * ntp shifted nano seconds.
+ * @ntp_err_mult: Multiplication factor for scaled math conversion
+ * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
+ * @last_warning: Warning ratelimiter (DEBUG_TIMEKEEPING)
+ * @underflow_seen: Underflow warning flag (DEBUG_TIMEKEEPING)
+ * @overflow_seen: Overflow warning flag (DEBUG_TIMEKEEPING)
*
* Note: For timespec(64) based interfaces wall_to_monotonic is what
* we need to add to xtime (or xtime corrected for sub jiffie times)
@@ -88,10 +96,25 @@ struct tk_read_base {
*
* @monotonic_to_boottime is a timespec64 representation of @offs_boot to
* accelerate the VDSO update for CLOCK_BOOTTIME.
+ *
+ * The cacheline ordering of the structure is optimized for in kernel usage
+ * of the ktime_get() and ktime_get_ts64() family of time accessors. Struct
+ * timekeeper is prepended in the core timekeeeping code with a sequence
+ * count, which results in the following cacheline layout:
+ *
+ * 0: seqcount, tkr_mono
+ * 1: xtime_sec ... tai_offset
+ * 2: tkr_raw, raw_sec
+ * 3,4: Internal variables
+ *
+ * Cacheline 0,1 contain the data which is used for accessing
+ * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
+ * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
+ * variables which are only accessed during timekeeper updates once per
+ * tick.
*/
struct timekeeper {
struct tk_read_base tkr_mono;
- struct tk_read_base tkr_raw;
u64 xtime_sec;
unsigned long ktime_sec;
struct timespec64 wall_to_monotonic;
@@ -99,31 +122,28 @@ struct timekeeper {
ktime_t offs_boot;
ktime_t offs_tai;
s32 tai_offset;
+
+ struct tk_read_base tkr_raw;
+ u64 raw_sec;
+
+ /* The following members are for timekeeping internal use */
unsigned int clock_was_set_seq;
u8 cs_was_changed_seq;
- ktime_t next_leap_ktime;
- u64 raw_sec;
+
struct timespec64 monotonic_to_boot;

- /* The following members are for timekeeping internal use */
u64 cycle_interval;
u64 xtime_interval;
s64 xtime_remainder;
u64 raw_interval;
- /* The ntp_tick_length() value currently being used.
- * This cached copy ensures we consistently apply the tick
- * length for an entire tick, as ntp_tick_length may change
- * mid-tick, and we don't want to apply that new value to
- * the tick in progress.
- */
+
+ ktime_t next_leap_ktime;
u64 ntp_tick;
- /* Difference between accumulated time and NTP time in ntp
- * shifted nano seconds. */
s64 ntp_error;
u32 ntp_error_shift;
u32 ntp_err_mult;
- /* Flag used to avoid updating NTP twice with same second */
u32 skip_second_overflow;
+
#ifdef CONFIG_DEBUG_TIMEKEEPING
long last_warning;
/*

--
2.39.2