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SubjectRe: gettimeofday order of magnitude slower with pmtimer, which is default
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Con Kolivas <kernel@kolivas.org> 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.

Actually, pmtmr not seems very fast, rather it's slow like usual I/O port.

Simple userland test

rdtsc(l1, h1);
val = inl(0x808); /* 0x808 is PMTMR address */
asm volatile("lfence");
rdtsc(l2, h2);
is about 1us.

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@mail.parknet.co.jp>


Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
---
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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