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From: john stultz on 13 Aug 2010 03:40
On Thu, 2010-08-12 at 22:17 -0500, Larry Finger wrote:
> On 08/12/2010 03:52 PM, john stultz wrote:
> >
> > Ugh. I'm surprised it picks *this* loop to optimize instead of the
> > similar one right above. I'm guessing its the local raw_nsecs value, but
> > whatever. Also surprised Jason's testing didn't hit this issue, but its
> > probably a gcc version thing.
> >
> > Regardless, I clearly need to give i386 more love in my testing.
> > My profuse apologies.
> >
> > As suggested by Linus, here's the do_div explicit version. It builds ok
> > on i386 & x86_64, but I have not yet tested it.
> >
> > Larry, Jason: Could you verify it works for you (and avoids the original
> > issue)?
>
> This one builds for me with both compilers. It appears to run OK. As to the
> original issue - I don't think I ever saw the problem. I'll leave that question
> for Jason.

Thanks for the testing!

I also managed to trigger the link issue with a 64bit gcc-4.3 cross
compiling to 32bit. However both 32bit and 64bit gcc-4.4 didn't trigger
the link issue, so it looks like its fixed in gcc.

Regardless, after my own testing, the change looks good to me. Raw time
is accumulating properly relative to monotonic time.

Assuming Jason has no complaints it should be able to be pushed in.

Thanks again for the quick reporting and verification.
-john


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From: Mikael Pettersson on 13 Aug 2010 04:50
Larry Finger writes:
> On 08/12/2010 03:26 PM, Linus Torvalds wrote:
> > On Thu, Aug 12, 2010 at 1:18 PM, Larry Finger <Larry.Finger(a)lwfinger.net> wrote:
> >>
> >> With the above commit, building an i386 version of the kernel results in the
> >> following from the build:
> >>
> >> kernel/built-in.o: In function `logarithmic_accumulation':
> >> /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to
> >> `__umoddi3'
> >> /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to
> >> `__udivdi3'
> >> make: *** [.tmp_vmlinux1] Error 1
> >>
> >> Reverting the patch allows the system to build correctly.
> >
> > Damn. It's your compiler turning a while-loop into a divide. Which
> > likely isn't even an optimization, but whatever.
> >
> > John: I think that while-loop needs to be something like
> >
> > if (raw_nsecs >= NSEC_PER_SEC) {
> > u64 raw_secs = raw_nsecs;
> > raw_nsecs = do_div(raw_secs, NSEC_PER_SEC);
> > raw_time.tv_sec += taw_secs;
> > }
> > raw_time.tc_nsec = raw_nsecs;
> >
> > which is sad and overly complicated, but the simple thing seems to get
> > messed up by the compiler.
> >
> > Untested. Maybe I got the complex do_div() semantics wrong. Somebody
> > needs to check.
>
> I'll try it. The system with the problem has gcc (SUSE Linux) 4.3.2
> [gcc-4_3-branch revision 141291]. That is on a 64-bit system cross compiling
> with ARCH=i386.
>
> The real 32-bit system with gcc (SUSE Linux) 4.5.0 20100604 [gcc-4_5-branch
> revision 160292] builds OK. It just wasn't finished when I wrote the first message.

The loops-turns-into-div is a known bug in early gcc-4.3 releases,
possibly only with -Os. It's fixed in 4.4 and 4.3.latest. I can
probably dig up the gcc PR number if needed.

/Mikael
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From: Jason Wessel on 13 Aug 2010 08:00
On 08/13/2010 02:30 AM, john stultz wrote:
> On Thu, 2010-08-12 at 22:17 -0500, Larry Finger wrote:
>
>> On 08/12/2010 03:52 PM, john stultz wrote:
>>
>>> Ugh. I'm surprised it picks *this* loop to optimize instead of the
>>> similar one right above. I'm guessing its the local raw_nsecs value, but
>>> whatever. Also surprised Jason's testing didn't hit this issue, but its
>>> probably a gcc version thing.
>>>
>>> Regardless, I clearly need to give i386 more love in my testing.
>>> My profuse apologies.
>>>
>>> As suggested by Linus, here's the do_div explicit version. It builds ok
>>> on i386 & x86_64, but I have not yet tested it.
>>>
>>> Larry, Jason: Could you verify it works for you (and avoids the original
>>> issue)?
>>>
>> This one builds for me with both compilers. It appears to run OK. As to the
>> original issue - I don't think I ever saw the problem. I'll leave that question
>> for Jason.
>>
>
> Thanks for the testing!
>
> I also managed to trigger the link issue with a 64bit gcc-4.3 cross
> compiling to 32bit. However both 32bit and 64bit gcc-4.4 didn't trigger
> the link issue, so it looks like its fixed in gcc.
>
> Regardless, after my own testing, the change looks good to me. Raw time
> is accumulating properly relative to monotonic time.
>
> Assuming Jason has no complaints it should be able to be pushed in.
>
>

No complaints here. The edge case remains solved on the low MHZ 32 bit
system.

The reason I never saw any problem in all my test configurations was
that gcc 4.4 is the oldest compiler I have in any of the configurations
I am using. I could have been more specific in my original mail on the
subject but it was tested with a 32bit cross compiler as well as a 64bit
cross compiler.

Many thanks to all who helped draw this odd ball case to closure.

Jason.
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