Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755280Ab0KFAkH (ORCPT ); Fri, 5 Nov 2010 20:40:07 -0400 Received: from e3.ny.us.ibm.com ([32.97.182.143]:41644 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755273Ab0KFAkE (ORCPT ); Fri, 5 Nov 2010 20:40:04 -0400 From: John Stultz To: LKML Cc: John Stultz , Thomas Gleixner , Ingo Molnar , Martin Schwidefsky , Clark Williams , Andi Kleen Subject: [PATCH] Greatly improve TSC calibration using a delayed workqueue Date: Fri, 5 Nov 2010 17:39:45 -0700 Message-Id: <1289003985-29060-1-git-send-email-johnstul@us.ibm.com> X-Mailer: git-send-email 1.7.3.2.146.gca209 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4471 Lines: 140 Boot to boot the TSC calibration may vary by quite a large amount. While normal variance of 50-100ppm can easily be seen, the quick calibration code only requires 500ppm accuracy, which is the limit of what NTP can correct for. This can cause problems for systems being used as NTP servers, as every time they reboot it can take hours for them to calculate the new drift error caused by the calibration. The classic trade-off here is calibration accuracy vs slow boot times, as during the calibration nothing else can run. This patch uses a delayed workqueue to calibrate the TSC over the period of a second. This allows very accurate calibration (in my tests only varying by 1khz or 0.4ppm boot to boot). Additionally this refined calibration step does not block the boot process, and only delays the TSC clocksoure registration by a few seconds in early boot. Credit to Andi Kleen who suggested using a timer quite awhile back, but I dismissed it thinking the timer calibration would be done after the clocksource was registered (which would break things). Forgive me for my short-sightedness. This patch has worked very well in my testing, but TSC hardware is quite varied so it would probably be good to get some extended testing, possibly pushing inclusion out to 2.6.39. Signed-off-by: John Stultz CC: Thomas Gleixner CC: Ingo Molnar CC: Martin Schwidefsky CC: Clark Williams CC: Andi Kleen --- arch/x86/kernel/tsc.c | 69 ++++++++++++++++++++++++++++++++++++++++++++++-- 1 files changed, 66 insertions(+), 3 deletions(-) diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 0c40d8b..e88b71e 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -885,7 +885,69 @@ __cpuinit int unsynchronized_tsc(void) return tsc_unstable; } -static void __init init_tsc_clocksource(void) + +static void calibrate_tsc_work(struct work_struct *work); +static DECLARE_DELAYED_WORK(tsc_irqwork, calibrate_tsc_work); + +static void calibrate_tsc_work(struct work_struct *work) +{ + static u64 tsc_start = -1, ref_start; + static int hpet; + u64 tsc_stop, ref_stop, delta; + unsigned long freq; + + /* Don't bother refining TSC on unstable systems */ + if (check_tsc_unstable()) + goto out; + + /* + * Since the timer is started early in boot, we may be + * delayed the first time we expire. So set the timer + * again once we know timers are working. + */ + if (tsc_start == -1) { + /* + * Only set hpet once, to avoid mixing hardware + * if the hpet becomes enabled later. + */ + hpet = is_hpet_enabled(); + schedule_delayed_work(&tsc_irqwork, HZ); + tsc_start = tsc_read_refs(&ref_start, hpet); + return; + } + + tsc_stop = tsc_read_refs(&ref_stop, hpet); + + /* hpet or pmtimer available ? */ + if (!hpet && !ref_start && !ref_stop) + goto out; + + /* Check, whether the sampling was disturbed by an SMI */ + if (tsc_start == ULLONG_MAX || tsc_stop == ULLONG_MAX) + goto out; + + delta = tsc_stop - tsc_start; + delta *= 1000000LL; + if (hpet) + freq = calc_hpet_ref(delta, ref_start, ref_stop); + else + freq = calc_pmtimer_ref(delta, ref_start, ref_stop); + + /* Make sure we're within 1% */ + if (abs(tsc_khz - freq) > tsc_khz/100) + goto out; + + tsc_khz = freq; + printk(KERN_INFO "Refined TSC clocksource calibration: " + "%lu.%03lu MHz.\n", (unsigned long)tsc_khz / 1000, + (unsigned long)tsc_khz % 1000); + +out: + clocksource_register_khz(&clocksource_tsc, tsc_khz); +} + + +static int __init init_tsc_clocksource(void) { if (tsc_clocksource_reliable) clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY; @@ -894,8 +956,10 @@ static void __init init_tsc_clocksource(void) clocksource_tsc.rating = 0; clocksource_tsc.flags &= ~CLOCK_SOURCE_IS_CONTINUOUS; } - clocksource_register_khz(&clocksource_tsc, tsc_khz); + schedule_delayed_work(&tsc_irqwork, 0); + return 0; } +core_initcall(init_tsc_clocksource); void __init tsc_init(void) { @@ -949,6 +1013,5 @@ void __init tsc_init(void) mark_tsc_unstable("TSCs unsynchronized"); check_system_tsc_reliable(); - init_tsc_clocksource(); } -- 1.7.3.2.146.gca209 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/