On 30.07.2012 09:25, Alexander Motin wrote:
On 30.07.2012 07:33, Bruce Evans wrote:
On Sun, 29 Jul 2012, Alexander Motin wrote:
On 29.07.2012 15:26, Bruce Evans wrote:
On Sun, 29 Jul 2012, Alexander Motin wrote:
On 29.07.2012 11:37, Bruce Evans wrote:
...
binuptime() is more accurate than uncalibrated scaling. Is accuracy
required?
Accuracy is not required at all. +-20% is not a problem.
If not, the CPU ticker might work, and is faster than HPET,
and and is not under user control for perverse settings. It normally
reduces to readtsc() with no serializing instruction even in proposed
changes. This is good enough for process times (not very good) and
depends on the CPU not changing. Its calibration is very accurate
(similar to timecounters) modulo bugs, but not always up to date.
Problem with ticker that it may stop during idle periods, and idle is
exactly what happens here. Unlike timecounter usage here we don't need
CPU synchronicity, but we need it working during deep sleeps.
The ticker is the same as the timecounter in many cases of
interest. If
the TSC stops then it cannot be used for timecounting unless
timecounting
is reinitialized. Timecounting should be reinitialized after deep
sleeps,
but you say you need it to work during deep sleeps.
Timecounter already has detection logic to disable TSC in cases where
it is unreliable. I don't want to replicate it here. I need not
precise and not synchronized by reliable and fast time source.
Yes, this logic gives exactly what you don't want (an inefficient
timecounter), by preventing use of the TSC for the timecounter, although
the TSC is perfectly usable for the ticker and here.
Can you teach me how to use ticker that is not ticking? If TSC was
considered unusable for timecounter for reasons unrelated to SMP, how
can I use it as ticker.
I wouldn't trust timecounters for some time after waking up after a
deep sleep. If their clock stopped then the times read might only be
very out of date. If their clock didn't stop, then they might have
wrapped or otherwise overflowed and the times read would be garbage.
Is there any locking or ordering to prevent them being used before they
are reinitialized?
I am not sure what reinitialization are you talking about. IIRC, there
is no any waking up code for TSC. None other time counters have
problems with C-states.
It is the timecounter code that needs reinitializing. If the TSC stops,
or wraps mod 2**32, then its counts become garbage for the purpose of
timecounting. Maybe it is not used for timecounting in either of these
cases. But these cases shouldn't prevent its use for timecounting.
The 2**32 number is because timecounters only use 32 bits of hardware
counters (for efficiency). So even if the hardware has some magic to
not stop the TSC while sleeping (maybe it fakes not stopping it be
reloading on wakeup), it is still unusable by timecounters after sleeping
for a second or 2 so that it wraps. The software needs similar faking
to reload the timecounter on wakeup. This makes use of timecounters in
sleep/wakeup code fragile.
At this moment I am not talking about S-states sleeping for hours. I am
talking about C-states for milliseconds. It means that TSC may stop and
start 10K times each second or even more. Attempt to save and restore
its state will consume so much resources, that probably make it useless.
What's about wrap after 2 seconds, I would be happy to make CPU sleep
for so long, but now 100ms is all I can hope even on idle system.
At boot time there is a dummy timecounter that returns bogo-times.
Apparently sleeping doesn't occur before the timecounter is switched to
a real one. The dummy timecounter isn't switched back to after boot
time. But it probably should be, since the hardware timecounter may
have stopped or wrapped. Sleeping could just set a flag to indicate
this state, but then you would have to provide a fake time anyway on
finding the flag set. Boot time just points to the dummy timecounter
so as not to check this flag in all early timecounter "hardware" calls.
And how dummy timecounter that counts something, but not time, can help
me to measure sleep time?
Nevermind, let it be compromise solution -- ticker for C1 state where
performance is the most important and where TSC works and ACPI timer for
others.
--
Alexander Motin
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