Jan Kiszka wrote:
Sebastian Smolorz wrote:
Gilles Chanteperdrix wrote:
On Wed, Apr 2, 2008 at 5:58 PM, Sebastian Smolorz
<[EMAIL PROTECTED]> wrote:
Jan Kiszka wrote:
 > Sebastian Smolorz wrote:
 >> Jan Kiszka wrote:
 >>> Cornelius Köpp wrote:
 >>>> I talked with Sebastian Smolorz about this and he builds his own
>>>> independent kernel-config to check. He got the same drifting-effect
 >>>> with Xenomai 2.4.2 and Xenomai 2.4.3 running latency over several
 >>>> hours. His kernel-config ist attached as
 >>>> 'config-2.6.24-xenomai-2.4.3__ssm'.
 >>>>
>>>> Our kernel-configs are both based on a config used with Xenomai 2.3.4
 >>>> and Linux 2.6.20.15 without any drifting effects.
>>> 2.3.x did not incorporate the new TSC-to-ns conversion. Maybe it is >>> not a PIC vs. APIC thing, but rather a rounding problem of larger TSC >>> values (that naturally show up when the system runs for a longer time).
 >> This hint seems to point into the right direction. I tried out a
>> modified pod_32.h (xnarch_tsc_to_ns() commented out) so that the old >> implementation in include/asm-generic/bits/pod.h was used. The drifting
 >> bug disappeared. So there seems so be a buggy x86-specific
 >> implementation of this routine.
 >
 > Hmm, maybe even a conceptional issue: the multiply-shift-based
> xnarch_tsc_to_ns is not as precise as the still multiply-divide-based > xnarch_ns_to_tsc. So when converting from tsc over ns back to tsc, we
 > may loose some bits, maybe too many bits...
 >
 > It looks like this bites us in the kernel latency tests (-t2 should
> suffer as well). Those recalculate their timeouts each round based on > absolute nanoseconds. In contrast, the periodic user mode task of -t0
 > uses a periodic timer that is forwarded via a tsc-based interval.
 >
 > You (or Cornelius) could try to analyse the calculation path of the
> involved timeouts, specifically to understand why the scheduled timeout > of the underlying task timer (which is tsc-based) tend to diverge from
 > the calculated one (ns-based).

 So here comes the explanation. The error is inside the function
 rthal_llmulshft(). It returns wrong values which are too small - the
 higher the given TSC value the bigger the error. The function
 rtdm_clock_read_monotonic() calls rthal_llmulshft(). As
 rtdm_clock_read_monotonic() is called every time the latency kernel
thread runs [1] the values reported by latency become smaller over time.

 In contrast, the latency task in user space only uses the conversion
 from TSC to ns only once when calling rt_timer_inquire [2].
timer_info.date is too small, timer_info.tsc is right. So all calculated
  deltas in [3] are shifted to a smaller value. This value is constant
 during the runtime of lateny in user space because no more conversion
 from TSC to ns occurs.

latency does conversions from tsc to ns, but it converts time
differences, so the error is small relative to the results.

Of course. I wasn't precise with my last statement. It should be: No more conversions from *absolute* TSC values to ns occur.


This patch may do the trick: it uses the inverted tsc-to-ns function instead of the frequency-based one. Be warned, it is totally untested inside Xenomai, I just ran it in a user space test program. But it may give an idea.

Your patch needed two minor corrections (ns instead of ts in functions xnarch_ns_to_tsc()) in order to compile. A short run (30 minutes) of latency -t1 seems to prove your bug-fix: There seems to be no drift.

If I got your patch correctly, it doesn't make xnarch_tsc_to_ns more precise but introduces a new function xnarch_ns_to_tsc() which is also less precise than the generic xnarch_ns_to_tsc(), right? So isn't there still the danger of getting wrong values when calling xnarch_tsc_to_ns() not in combination with xnarch_ns_to_tsc()?

--
Sebastian
---
 include/asm-x86/bits/init_32.h |    3 ++-
 include/asm-x86/bits/init_64.h |    3 ++-
 include/asm-x86/bits/pod_32.h  |    7 +++++++
 include/asm-x86/bits/pod_64.h  |    7 +++++++
 4 files changed, 18 insertions(+), 2 deletions(-)

Index: b/include/asm-x86/bits/init_32.h
===================================================================
--- a/include/asm-x86/bits/init_32.h
+++ b/include/asm-x86/bits/init_32.h
@@ -73,7 +73,7 @@ int xnarch_calibrate_sched(void)
 
 static inline int xnarch_init(void)
 {
-       extern unsigned xnarch_tsc_scale, xnarch_tsc_shift;
+       extern unsigned xnarch_tsc_scale, xnarch_tsc_shift, xnarch_tsc_divide;
        int err;
 
        err = rthal_init();
@@ -89,6 +89,7 @@ static inline int xnarch_init(void)
 
        xnarch_init_llmulshft(1000000000, RTHAL_CPU_FREQ,
                              &xnarch_tsc_scale, &xnarch_tsc_shift);
+       xnarch_tsc_divide = 1 << xnarch_tsc_shift;
 
        err = xnarch_calibrate_sched();
 
Index: b/include/asm-x86/bits/init_64.h
===================================================================
--- a/include/asm-x86/bits/init_64.h
+++ b/include/asm-x86/bits/init_64.h
@@ -70,7 +70,7 @@ int xnarch_calibrate_sched(void)
 
 static inline int xnarch_init(void)
 {
-       extern unsigned xnarch_tsc_scale, xnarch_tsc_shift;
+       extern unsigned xnarch_tsc_scale, xnarch_tsc_shift, xnarch_tsc_divide;
        int err;
 
        err = rthal_init();
@@ -86,6 +86,7 @@ static inline int xnarch_init(void)
 
        xnarch_init_llmulshft(1000000000, RTHAL_CPU_FREQ,
                              &xnarch_tsc_scale, &xnarch_tsc_shift);
+       xnarch_tsc_divide = 1 << xnarch_tsc_shift;
 
        err = xnarch_calibrate_sched();
 
Index: b/include/asm-x86/bits/pod_32.h
===================================================================
--- a/include/asm-x86/bits/pod_32.h
+++ b/include/asm-x86/bits/pod_32.h
@@ -25,6 +25,7 @@
 
 unsigned xnarch_tsc_scale;
 unsigned xnarch_tsc_shift;
+unsigned xnarch_tsc_divide;
 
 long long xnarch_tsc_to_ns(long long ts)
 {
@@ -32,6 +33,12 @@ long long xnarch_tsc_to_ns(long long ts)
 }
 #define XNARCH_TSC_TO_NS
 
+long long xnarch_ns_to_tsc(long long ns)
+{
+       return xnarch_llimd(ns, xnarch_tsc_divide, xnarch_tsc_scale);
+}
+#define XNARCH_NS_TO_TSC
+
 #include <asm-generic/xenomai/bits/pod.h>
 #include <asm/xenomai/switch.h>
 
Index: b/include/asm-x86/bits/pod_64.h
===================================================================
--- a/include/asm-x86/bits/pod_64.h
+++ b/include/asm-x86/bits/pod_64.h
@@ -24,6 +24,7 @@
 
 unsigned xnarch_tsc_scale;
 unsigned xnarch_tsc_shift;
+unsigned xnarch_tsc_divide;
 
 long long xnarch_tsc_to_ns(long long ts)
 {
@@ -31,6 +32,12 @@ long long xnarch_tsc_to_ns(long long ts)
 }
 #define XNARCH_TSC_TO_NS
 
+long long xnarch_ns_to_tsc(long long ns)
+{
+       return xnarch_llimd(ns, xnarch_tsc_divide, xnarch_tsc_scale);
+}
+#define XNARCH_NS_TO_TSC
+
 #include <asm-generic/xenomai/bits/pod.h>
 #include <asm/xenomai/switch.h>
 
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