Subject: Re: [PATCH v2 06/25] timekeeping: Reorder struct timekeeper

From: Anna-Maria Behnsen <[email-masked]>
Date: Mon, 14 Oct 2024 11:30:22 +0200

John Stultz <[email-masked]> writes:

> On Wed, Oct 9, 2024 at 1:29 AM Anna-Maria Behnsen
> <[email-masked]> wrote:
>>
>> From: Thomas Gleixner <[email-masked]>
>>
>> From: Thomas Gleixner <[email-masked]>
>>
>> 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 <[email-masked]>
>> Signed-off-by: Anna-Maria Behnsen <[email-masked]>
>
> Acked-by: John Stultz <[email-masked]>
>
>> ---
>>  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 902c20ef495a..430e40549136 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 jiffy 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.
>
> Would it make sense to add divider comments or something in the struct
> to make this more visible? I fret in the context of a patch, a + line
> adding a new structure element that breaks the ordered alignment might
> not be obvious.

This is an argument! I'll add simple comments with /* Cachline X: */

Thanks,

	Anna-Maria