Re: gettimeofday order of magnitude slower with pmtimer, which isdefault
From: john stultz
Date: Tue Mar 21 2006 - 14:20:29 EST
On Tue, 2006-03-21 at 10:26 +0900, OGAWA Hirofumi wrote:
> Con Kolivas <kernel@xxxxxxxxxxx> writes:
>
> > On Tuesday 21 March 2006 01:50, Andreas Mohr wrote:
> >
> >> I think it's crazy to do a safe tripled readout (with *very* expensive
> >> I/O!) of the PM timer unconditionally on *all* systems when only a
> >> (albeit not that small) subset of systems is affected by buggy (un-latched)
> >> PM timers.
> >> I want to improve things there; I can see three ways to do it:
> >> a) maintain a blacklist (or probably better a whitelist) of systems that
> >> are (not) affected
> >> b) detect long-time timer accuracy, then switch to fast readout if timer
> >> is verified to be accurate (no white/blacklist needed this way)
> >> c) give up on PM timer completely
> >>
> >> Any comments on which way and how this could/should be done?
> >
> > The pm timer is very fast when the timer is latched and that strange loop uses
> > hardly any cpu time. The same can't be said about the unlatched timer case
> > where absurd amounts of cpu seem the norm. You have a catch 22 situation if
> > you depend on the accuracy of the pm timer only to end up wasting time trying
> > to actually use that timer.
[snip]
> And the following is test of gettimeofday(). Probably, we need a patch. Umm....
>
> 2.6.16 (pmtmr)
> Simple gettimeofday: 3.6532 microseconds
> Simple gettimeofday: 3.6502 microseconds
> Simple gettimeofday: 3.6522 microseconds
> Simple gettimeofday: 3.6486 microseconds
> Simple gettimeofday: 3.6539 microseconds
>
> 2.6.16+patch (pmtmr)
> Simple gettimeofday: 1.4582 microseconds
> Simple gettimeofday: 1.4593 microseconds
> Simple gettimeofday: 1.4671 microseconds
> Simple gettimeofday: 1.4650 microseconds
>
> 2.6.16 (tsc)
> Simple gettimeofday: 0.4037 microseconds
> Simple gettimeofday: 0.4037 microseconds
> Simple gettimeofday: 0.4040 microseconds
> Simple gettimeofday: 0.4037 microseconds
> Simple gettimeofday: 0.4038 microseconds
> --
> OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
>
>
> Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
In my TOD rework I've dropped the triple read, figuring if a problem
arose we could blacklist the specific box. This patch covers that, so it
looks like a good idea to me.
I've not tested it myself, but if you feel good about it, please send it
to Andrew.
thanks
-john
Acked-by: John Stultz <johnstul@xxxxxxxxxx>
> ---
>
> arch/i386/kernel/timers/timer_pm.c | 74 ++++++++++++++++++++++++++++++-------
> 1 file changed, 60 insertions(+), 14 deletions(-)
>
> diff -puN arch/i386/kernel/timers/timer_pm.c~pm-kill-workaround arch/i386/kernel/timers/timer_pm.c
> --- linux-2.6/arch/i386/kernel/timers/timer_pm.c~pm-kill-workaround 2006-03-21 04:20:27.000000000 +0900
> +++ linux-2.6-hirofumi/arch/i386/kernel/timers/timer_pm.c 2006-03-21 04:31:48.000000000 +0900
> @@ -15,6 +15,7 @@
> #include <linux/module.h>
> #include <linux/device.h>
> #include <linux/init.h>
> +#include <linux/pci.h>
> #include <asm/types.h>
> #include <asm/timer.h>
> #include <asm/smp.h>
> @@ -35,6 +36,7 @@
> * in arch/i386/acpi/boot.c */
> u32 pmtmr_ioport = 0;
>
> +static int pmtmr_need_workaround __read_mostly = 1;
>
> /* value of the Power timer at last timer interrupt */
> static u32 offset_tick;
> @@ -45,24 +47,68 @@ static seqlock_t monotonic_lock = SEQLOC
>
> #define ACPI_PM_MASK 0xFFFFFF /* limit it to 24 bits */
>
> +#ifdef CONFIG_PCI
> +/*
> + * PIIX4 Errata:
> + *
> + * The power management timer may return improper result when read.
> + * Although the timer value settles properly after incrementing,
> + * while incrementing there is a 3 ns window every 69.8 ns where the
> + * timer value is indeterminate (a 4.2% chance that the data will be
> + * incorrect when read). As a result, the ACPI free running count up
> + * timer specification is violated due to erroneous reads.
> + */
> +static int __init pmtmr_bug_check(void)
> +{
> + struct pci_dev *dev;
> + int pmtmr_has_bug = 0;
> + u8 rev;
> +
> + dev = pci_get_device(PCI_VENDOR_ID_INTEL,
> + PCI_DEVICE_ID_INTEL_82371AB_3, NULL);
> + if (dev) {
> + pci_read_config_byte(dev, PCI_REVISION_ID, &rev);
> + /* the bug has been fixed in PIIX4M */
> + if (rev < 3)
> + pmtmr_has_bug = 1;
> + pci_dev_put(dev);
> + }
> +
> + if (pmtmr_has_bug)
> + printk(KERN_INFO
> + "*** Found PM-Timer Bug on this chip. For workarounds a bug, this timer\n"
> + "*** source is slow. Use other timer source (clock=).\n");
> + else
> + pmtmr_need_workaround = 0;
> +
> + return 0;
> +}
> +device_initcall(pmtmr_bug_check);
> +#endif
> +
> /*helper function to safely read acpi pm timesource*/
> static inline u32 read_pmtmr(void)
> {
> - u32 v1=0,v2=0,v3=0;
> - /* It has been reported that because of various broken
> - * chipsets (ICH4, PIIX4 and PIIX4E) where the ACPI PM time
> - * source is not latched, so you must read it multiple
> - * times to insure a safe value is read.
> - */
> - do {
> - v1 = inl(pmtmr_ioport);
> - v2 = inl(pmtmr_ioport);
> - v3 = inl(pmtmr_ioport);
> - } while ((v1 > v2 && v1 < v3) || (v2 > v3 && v2 < v1)
> - || (v3 > v1 && v3 < v2));
> + if (unlikely(pmtmr_need_workaround)) {
> + u32 v1, v2, v3;
> +
> + /* It has been reported that because of various broken
> + * chipsets (ICH4, PIIX4 and PIIX4E) where the ACPI PM time
> + * source is not latched, so you must read it multiple
> + * times to insure a safe value is read.
> + */
> + do {
> + v1 = inl(pmtmr_ioport);
> + v2 = inl(pmtmr_ioport);
> + v3 = inl(pmtmr_ioport);
> + } while ((v1 > v2 && v1 < v3) || (v2 > v3 && v2 < v1)
> + || (v3 > v1 && v3 < v2));
> +
> + /* mask the output to 24 bits */
> + return v2 & ACPI_PM_MASK;
> + }
>
> - /* mask the output to 24 bits */
> - return v2 & ACPI_PM_MASK;
> + return inl(pmtmr_ioport) & ACPI_PM_MASK;
> }
>
>
> _
> -
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