Re: Final: Add 32 bit VDSO time function support

[Stefani Seibold <stefani@seibold.net>]

Am Mittwoch, den 26.02.2014, 16:55 -0800 schrieb Andy Lutomirski:
> Um.  This code doesn't work.  I'll send a patch.  I can't speak
> towards how well it compiles in different configurations.
> 
> I can't speak towards how well it compiles in different
> configurations.  Also, vdso_fallback_gettime needs .cfi annotations, I
> think.  I could probably dredge the required incantations from
> somewhere, but someone else may know how to do it.
> 
> Once I patch it to work, your 32-bit code is considerably faster than
> the 64-bit case.  It's enough faster that I suspect a bug.  Dumping
> the in-memory shows some rather suspicious nops before the rdtsc
> instruction.  I suspect that you've forgotten to run the 32-bit vdso
> through the alternatives code.  The is a nasty bug: it will appear to
> work, but you'll see non-monotonic times on some SMP systems.
> 

I didn't know this. My basic test case is a KVM which defaults to 1 cpu.
Thanks for discovering the issue.

> In my configuration, with your patches, I get (64-bit):
> 
> CLOCK_REALTIME:
> 100000000 loops in 2.07105s = 20.71 nsec / loop
> 100000000 loops in 2.06874s = 20.69 nsec / loop
> 100000000 loops in 2.29415s = 22.94 nsec / loop
> CLOCK_MONOTONIC:
> 100000000 loops in 2.06526s = 20.65 nsec / loop
> 100000000 loops in 2.10134s = 21.01 nsec / loop
> 100000000 loops in 2.10615s = 21.06 nsec / loop
> CLOCK_REALTIME_COARSE:
> 100000000 loops in 0.37440s = 3.74 nsec / loop
> [  503.011756] perf samples too long (2550 > 2500), lowering
> kernel.perf_event_max_sample_rate to 50000
> 100000000 loops in 0.37399s = 3.74 nsec / loop
> 100000000 loops in 0.38445s = 3.84 nsec / loop
> CLOCK_MONOTONIC_COARSE:
> 100000000 loops in 0.40238s = 4.02 nsec / loop
> 100000000 loops in 0.40939s = 4.09 nsec / loop
> 100000000 loops in 0.41152s = 4.12 nsec / loop
> 
> Without the patches, I get:
> 
> CLOCK_REALTIME:
> 100000000 loops in 2.07348s = 20.73 nsec / loop
> 100000000 loops in 2.07346s = 20.73 nsec / loop
> 100000000 loops in 2.06922s = 20.69 nsec / loop
> CLOCK_MONOTONIC:
> 100000000 loops in 1.98955s = 19.90 nsec / loop
> 100000000 loops in 1.98895s = 19.89 nsec / loop
> 100000000 loops in 1.98881s = 19.89 nsec / loop
> CLOCK_REALTIME_COARSE:
> 100000000 loops in 0.37462s = 3.75 nsec / loop
> 100000000 loops in 0.37460s = 3.75 nsec / loop
> 100000000 loops in 0.37428s = 3.74 nsec / loop
> CLOCK_MONOTONIC_COARSE:
> 100000000 loops in 0.40081s = 4.01 nsec / loop
> 100000000 loops in 0.39834s = 3.98 nsec / loop
> [   36.706696] perf samples too long (2565 > 2500), lowering
> kernel.perf_event_max_sample_rate to 50000
> 100000000 loops in 0.39949s = 3.99 nsec / loop
> 
> This looks like a wash, except for CLOCK_MONOTONIC, which got a bit
> slower.  I'll send a followup patch once the bugs are fixed that
> improves the timings to:
> 
> CLOCK_REALTIME:
> 100000000 loops in 2.08621s = 20.86 nsec / loop
> 100000000 loops in 2.07122s = 20.71 nsec / loop
> 100000000 loops in 2.07089s = 20.71 nsec / loop
> CLOCK_MONOTONIC:
> 100000000 loops in 2.06831s = 20.68 nsec / loop
> 100000000 loops in 2.06862s = 20.69 nsec / loop
> 100000000 loops in 2.06195s = 20.62 nsec / loop
> CLOCK_REALTIME_COARSE:
> 100000000 loops in 0.37274s = 3.73 nsec / loop
> 100000000 loops in 0.37247s = 3.72 nsec / loop
> 100000000 loops in 0.37234s = 3.72 nsec / loop
> CLOCK_MONOTONIC_COARSE:
> 100000000 loops in 0.39944s = 3.99 nsec / loop
> 100000000 loops in 0.39940s = 3.99 nsec / loop
> 100000000 loops in 0.40054s = 4.01 nsec / loop
> 
> I'm not quite sure that causes the remaining loss.
> 
> Test code is here:
> 
> https://gitorious.org/linux-test-utils/linux-clock-tests