From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 936D33A63EB for ; Tue, 24 Feb 2026 16:36:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771951003; cv=none; b=bqD71504E+0C9rPO/aKiJr4JByPQ7LIkUtzhVmUHg7glusDxq7f/bBk0KW5sNoqdQnEEDWQGJ5mj+DTETHGt6kqbGsaXxVslQgvNsvV2Y+EoFvz2TOjJBl9jV1urc09LTgdj9ilpTBHzHzk8XrTXiNbhBNIzk7TPLIAwRRk1cVM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771951003; c=relaxed/simple; bh=kNINeP5ljjuNHJTHfRjrgrsBfkEf98j5jk496403GS0=; h=Date:Message-ID:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=a/zAHCqhHfRSP8H2R9niIkcvIcI1x9bggnlvnGBocm1J653BMwS5IdmSqD69gIsQqUkN72QwAbyZy5e63p5FVhL1ZCn/1+q6ARMqgi8pK6+uvKPtdhG/jo9WzCYuQxu1qv3RQAtwW2/QXdeU+q569tiw/VmuvT1NeTsg+SK27sw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=cKSQRMqu; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="cKSQRMqu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A3196C116D0; Tue, 24 Feb 2026 16:36:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771951003; bh=kNINeP5ljjuNHJTHfRjrgrsBfkEf98j5jk496403GS0=; h=Date:From:To:Cc:Subject:References:From; b=cKSQRMquyuqvNq/EzSbbV9vsSIT+VAFwvjoIsxa3rpNivF/FfL7qojlpjo1cbIqTJ WfP9RAZ2kISUXS1TC8Ta5Z3jkyL/59iYCL4CrTaQ9jo42FUT6zCGa+zZ5oar3mko/4 FICvYbU5U3KIdUoxhneVL4+Et2QpMvK2Nht6lsg7cZlu729ZkW6n7L8bPwQHL/16zQ eebhriQvOQu8zot/A8vR+/0EbA/KWAM6OOZIrQEvDHYG8kafK8DGGQqna2ot3dgY68 mru4V3NgAnJofS4LZWunJa3BFHSf8dfrFl6jCxQCdVsJPcFte0viVcBGYEN4D/bLAb cB4zlLngdfSuQ== Date: Tue, 24 Feb 2026 17:36:40 +0100 Message-ID: <20260224163429.944763521@kernel.org> User-Agent: quilt/0.68 From: Thomas Gleixner To: LKML Cc: Anna-Maria Behnsen , John Stultz , Stephen Boyd , Daniel Lezcano , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , x86@kernel.org, Peter Zijlstra , Frederic Weisbecker , Eric Dumazet Subject: [patch 18/48] timekeeping: Provide infrastructure for coupled clockevents References: <20260224163022.795809588@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Some architectures have clockevent devices which are coupled to the system clocksource by implementing a less than or equal comparator which compares the programmed absolute expiry time against the underlying time counter. Well known examples are TSC/TSC deadline timer and the S390 TOD clocksource/comparator. While the concept is nice it has some downsides: 1) The clockevents core code is strictly based on relative expiry times as that's the most common case for clockevent device hardware. That requires to convert the absolute expiry time provided by the caller (hrtimers, NOHZ code) to a relative expiry time by reading and substracting the current time. The clockevent::set_next_event() callback must then read the counter again to convert the relative expiry back into a absolute one. 2) The conversion factors from nanoseconds to counter clock cycles are set up when the clockevent is registered. When NTP applies corrections then the clockevent conversion factors can deviate from the clocksource conversion substantially which either results in timers firing late or in the worst case early. The early expiry then needs to do a reprogam with a short delta. In most cases this is papered over by the fact that the read in the set_next_event() callback happens after the read which is used to calculate the delta. So the tendency is that timers expire mostly late. All of this can be avoided by providing support for these devices in the core code: 1) The timekeeping core keeps track of the last update to the clocksource by storing the base nanoseconds and the corresponding clocksource counter value. That's used to keep the conversion math for reading the time within 64-bit in the common case. This information can be used to avoid both reads of the underlying clocksource in the clockevents reprogramming path: delta = expiry - base_ns; cycles = base_cycles + ((delta * clockevent::mult) >> clockevent::shift); The resulting cycles value can be directly used to program the comparator. 2) As #1 does not longer provide the "compensation" through the second read the deviation of the clocksource and clockevent conversions caused by NTP become more prominent. This can be cured by letting the timekeeping core compute and store the reverse conversion factors when the clocksource cycles to nanoseconds factors are modified by NTP: CS::MULT (1 << NS_TO_CYC_SHIFT) --------------- = ---------------------- (1 << CS:SHIFT) NS_TO_CYC_MULT Ergo: NS_TO_CYC_MULT = (1 << (CS::SHIFT + NS_TO_CYC_SHIFT)) / CS::MULT The NS_TO_CYC_SHIFT value is calculated when the clocksource is installed so that it aims for a one hour maximum sleep time. Signed-off-by: Thomas Gleixner --- include/linux/clocksource.h | 1 include/linux/timekeeper_internal.h | 8 ++ kernel/time/Kconfig | 3 kernel/time/timekeeping.c | 110 ++++++++++++++++++++++++++++++++++++ kernel/time/timekeeping.h | 2 5 files changed, 124 insertions(+) --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -150,6 +150,7 @@ struct clocksource { #define CLOCK_SOURCE_RESELECT 0x100 #define CLOCK_SOURCE_VERIFY_PERCPU 0x200 #define CLOCK_SOURCE_CAN_INLINE_READ 0x400 +#define CLOCK_SOURCE_HAS_COUPLED_CLOCK_EVENT 0x800 /* simplify initialization of mask field */ #define CLOCKSOURCE_MASK(bits) GENMASK_ULL((bits) - 1, 0) --- a/include/linux/timekeeper_internal.h +++ b/include/linux/timekeeper_internal.h @@ -72,6 +72,10 @@ struct tk_read_base { * @id: The timekeeper ID * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds + * @cs_id: The ID of the current clocksource + * @cs_ns_to_cyc_mult: Multiplicator for nanoseconds to cycles conversion + * @cs_ns_to_cyc_shift: Shift value for nanoseconds to cycles conversion + * @cs_ns_to_cyc_maxns: Maximum nanoseconds to cyles conversion range * @clock_was_set_seq: The sequence number of clock was set events * @cs_was_changed_seq: The sequence number of clocksource change events * @clock_valid: Indicator for valid clock @@ -159,6 +163,10 @@ struct timekeeper { u64 raw_sec; /* Cachline 3 and 4 (timekeeping internal variables): */ + enum clocksource_ids cs_id; + u32 cs_ns_to_cyc_mult; + u32 cs_ns_to_cyc_shift; + u64 cs_ns_to_cyc_maxns; unsigned int clock_was_set_seq; u8 cs_was_changed_seq; u8 clock_valid; --- a/kernel/time/Kconfig +++ b/kernel/time/Kconfig @@ -47,6 +47,9 @@ config GENERIC_CLOCKEVENTS_BROADCAST_IDL config GENERIC_CLOCKEVENTS_MIN_ADJUST bool +config GENERIC_CLOCKEVENTS_COUPLED + bool + # Generic update of CMOS clock config GENERIC_CMOS_UPDATE bool --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -391,6 +391,20 @@ static void tk_setup_internals(struct ti tk->tkr_raw.mult = clock->mult; tk->ntp_err_mult = 0; tk->skip_second_overflow = 0; + + tk->cs_id = clock->id; + + /* Coupled clockevent data */ + if (IS_ENABLED(CONFIG_GENERIC_CLOCKEVENTS_COUPLED) && + clock->flags & CLOCK_SOURCE_HAS_COUPLED_CLOCK_EVENT) { + /* + * Aim for an one hour maximum delta and use KHz to handle + * clocksources with a frequency above 4GHz correctly as + * the frequency argument of clocks_calc_mult_shift() is u32. + */ + clocks_calc_mult_shift(&tk->cs_ns_to_cyc_mult, &tk->cs_ns_to_cyc_shift, + NSEC_PER_MSEC, clock->freq_khz, 3600 * 1000); + } } /* Timekeeper helper functions. */ @@ -720,6 +734,36 @@ static inline void tk_update_ktime_data( tk->tkr_raw.base = ns_to_ktime(tk->raw_sec * NSEC_PER_SEC); } +static inline void tk_update_ns_to_cyc(struct timekeeper *tks, struct timekeeper *tkc) +{ + struct tk_read_base *tkrs = &tks->tkr_mono; + struct tk_read_base *tkrc = &tkc->tkr_mono; + unsigned int shift; + + if (!IS_ENABLED(CONFIG_GENERIC_CLOCKEVENTS_COUPLED) || + !(tkrs->clock->flags & CLOCK_SOURCE_HAS_COUPLED_CLOCK_EVENT)) + return; + + if (tkrs->mult == tkrc->mult && tkrs->shift == tkrc->shift) + return; + /* + * The conversion math is simple: + * + * CS::MULT (1 << NS_TO_CYC_SHIFT) + * --------------- = ---------------------- + * (1 << CS:SHIFT) NS_TO_CYC_MULT + * + * Ergo: + * + * NS_TO_CYC_MULT = (1 << (CS::SHIFT + NS_TO_CYC_SHIFT)) / CS::MULT + * + * NS_TO_CYC_SHIFT has been set up in tk_setup_internals() + */ + shift = tkrs->shift + tks->cs_ns_to_cyc_shift; + tks->cs_ns_to_cyc_mult = (u32)div_u64(1ULL << shift, tkrs->mult); + tks->cs_ns_to_cyc_maxns = div_u64(tkrs->clock->mask, tks->cs_ns_to_cyc_mult); +} + /* * Restore the shadow timekeeper from the real timekeeper. */ @@ -754,6 +798,7 @@ static void timekeeping_update_from_shad tk->tkr_mono.base_real = tk->tkr_mono.base + tk->offs_real; if (tk->id == TIMEKEEPER_CORE) { + tk_update_ns_to_cyc(tk, &tkd->timekeeper); update_vsyscall(tk); update_pvclock_gtod(tk, action & TK_CLOCK_WAS_SET); @@ -808,6 +853,71 @@ static void timekeeping_forward_now(stru tk_update_coarse_nsecs(tk); } +/* + * ktime_expiry_to_cycles - Convert a expiry time to clocksource cycles + * @id: Clocksource ID which is required for validity + * @expires_ns: Absolute CLOCK_MONOTONIC expiry time (nsecs) to be converted + * @cycles: Pointer to storage for corresponding absolute cycles value + * + * Convert a CLOCK_MONOTONIC based absolute expiry time to a cycles value + * based on the correlated clocksource of the clockevent device by using + * the base nanoseconds and cycles values of the last timekeeper update and + * converting the delta between @expires_ns and base nanoseconds to cycles. + * + * This only works for clockevent devices which are using a less than or + * equal comparator against the clocksource. + * + * Utilizing this avoids two clocksource reads for such devices, the + * ktime_get() in clockevents_program_event() to calculate the delta expiry + * value and the readout in the device::set_next_event() callback to + * convert the delta back to a absolute comparator value. + * + * Returns: True if @id matches the current clocksource ID, false otherwise + */ +bool ktime_expiry_to_cycles(enum clocksource_ids id, ktime_t expires_ns, u64 *cycles) +{ + struct timekeeper *tk = &tk_core.timekeeper; + struct tk_read_base *tkrm = &tk->tkr_mono; + ktime_t base_ns, delta_ns, max_ns; + u64 base_cycles, delta_cycles; + unsigned int seq; + u32 mult, shift; + + /* + * Racy check to avoid the seqcount overhead when ID does not match. If + * the relevant clocksource is installed concurrently, then this will + * just delay the switch over to this mechanism until the next event is + * programmed. If the ID is not matching the clock events code will use + * the regular relative set_next_event() callback as before. + */ + if (data_race(tk->cs_id) != id) + return false; + + do { + seq = read_seqcount_begin(&tk_core.seq); + + if (tk->cs_id != id) + return false; + + base_cycles = tkrm->cycle_last; + base_ns = tkrm->base + (tkrm->xtime_nsec >> tkrm->shift); + + mult = tk->cs_ns_to_cyc_mult; + shift = tk->cs_ns_to_cyc_shift; + max_ns = tk->cs_ns_to_cyc_maxns; + + } while (read_seqcount_retry(&tk_core.seq, seq)); + + /* Prevent negative deltas and multiplication overflows */ + delta_ns = min(expires_ns - base_ns, max_ns); + delta_ns = max(delta_ns, 0); + + /* Convert to cycles */ + delta_cycles = ((u64)delta_ns * mult) >> shift; + *cycles = base_cycles + delta_cycles; + return true; +} + /** * ktime_get_real_ts64 - Returns the time of day in a timespec64. * @ts: pointer to the timespec to be set --- a/kernel/time/timekeeping.h +++ b/kernel/time/timekeeping.h @@ -9,6 +9,8 @@ extern ktime_t ktime_get_update_offsets_ ktime_t *offs_boot, ktime_t *offs_tai); +bool ktime_expiry_to_cycles(enum clocksource_ids id, ktime_t expires_ns, u64 *cycles); + extern int timekeeping_valid_for_hres(void); extern u64 timekeeping_max_deferment(void); extern void timekeeping_warp_clock(void);