[Linux-ia64] Re: [BUG] nanosleep() granularity bumps up in 2.5.64 (was: [PATCH]

[Linux-ia64] Re: [BUG] nanosleep() granularity bumps up in 2.5.64 (was: [PATCH]

[Eric Piel]

David Mosberger wrote:
> Are you running ntp?
Yes (I hadn't noticed it) but it was not connected to any server and
disabling it doesn't change the results.
 

> 
> On 2.5:
> 
>         $ time sleep 16
>         real    0m16.002s
>         user    0m0.001s
>         sys     0m0.002s
> 
>         $ time sleep 16.02
>         real    0m25.189s
>         user    0m0.000s
>         sys     0m0.001s
> 
> So clearly something very strange is going on.  My suspicion is that
> the bug was introduced back when x86 switched from 100Hz to 1000Hz
> ticks, but that's just a guess.  Eric, would you be
> able/willing/interested to look into this?
Sure, I aim at porting the high resolution timers but any annoying bug
related to the time can be interesting to remove.

Coincidently Vita has just reported a bug on the lkml which, after a
closer look, seems to be the same:
>   When playing with select() timeout values I found that granularity
> of nanosleep() in 2.5.64 kernel bumps to 256 msec. Trying to get finer
> granularity it ends up sleeping to the next multiple of 256 msec

From what I understand their is a bug in the timers that causes a big
granularity. The case of Vita is a very good example. Also, after 16s it
seems the granuality (slowly?!) jumps from 1/64th s to 16s! :
sleep requested	 time obtained
14.000000000     14.006201744
15.000000000     15.006647110
16.000000000     16.007089615
17.000000000     18.742679596
18.000000000     32.014190674
19.000000000     32.014190674
20.000000000     32.014190674

I think lines like that from patch-2.5.64 are very suspicious to be
related to the bug:
+	base->timer_jiffies = INITIAL_JIFFIES;
+	base->tv1.index = INITIAL_JIFFIES & TVR_MASK;
+	base->tv2.index = (INITIAL_JIFFIES >> TVR_BITS) & TVN_MASK;
+	base->tv3.index = (INITIAL_JIFFIES >> (TVR_BITS+TVN_BITS)) & TVN_MASK;
+	base->tv4.index = (INITIAL_JIFFIES >> (TVR_BITS+2*TVN_BITS)) &
TVN_MASK;
+	base->tv5.index = (INITIAL_JIFFIES >> (TVR_BITS+3*TVN_BITS)) &
TVN_MASK;

Any idea/sugestion/patch is welcomed. Whatever, I will try to fix this
as soon as I'm back from my week end :-)

	Eric

Re: [Linux-ia64] Re: [BUG] nanosleep() granularity bumps up in 2.5.64 (was: [PATCH] settimeofday() n

[Matthew Wilcox]

On Fri, Mar 14, 2003 at 03:34:36PM +0100, Eric Piel wrote:
> I think lines like that from patch-2.5.64 are very suspicious to be
> related to the bug:
> +	base->timer_jiffies = INITIAL_JIFFIES;
> +	base->tv1.index = INITIAL_JIFFIES & TVR_MASK;
> +	base->tv2.index = (INITIAL_JIFFIES >> TVR_BITS) & TVN_MASK;
> +	base->tv3.index = (INITIAL_JIFFIES >> (TVR_BITS+TVN_BITS)) & TVN_MASK;
> +	base->tv4.index = (INITIAL_JIFFIES >> (TVR_BITS+2*TVN_BITS)) &
> TVN_MASK;
> +	base->tv5.index = (INITIAL_JIFFIES >> (TVR_BITS+3*TVN_BITS)) &
> TVN_MASK;

No, I don't think so.  Those lines are for starting `jiffies' at a very
high number so we spot jiffie-wrap bugs early on.

-- 
"It's not Hollywood.  War is real, war is primarily not about defeat or
victory, it is about death.  I've seen thousands and thousands of dead bodies.
Do you think I want to have an academic debate on this subject?" -- Robert Fisk

Re: [Linux-ia64] Re: [BUG] nanosleep() granularity bumps up in 2.5.64 (was: [PATCH]

[David Mosberger]

>>>>> On Fri, 14 Mar 2003 15:34:36 +0100, Eric Piel <Eric.Piel@Bull.Net> said:

  Eric> Sure, I aim at porting the high resolution timers but any
  Eric> annoying bug related to the time can be interesting to remove.

Great!  It would be great to have someone who can focus on that.

  Eric> Any idea/sugestion/patch is welcomed. Whatever, I will try to
  Eric> fix this as soon as I'm back from my week end :-)

Does the x86 show the same behavior?  That would be a useful start to
narrow down the problem.

	--david

Re: [Linux-ia64] Re: [BUG] nanosleep() granularity bumps up in 2.5.64 (was: [PATCH] settimeofday() n

[Vitezslav Samel]

On Fri, Mar 14, 2003 at 02:48:59PM +0000, Matthew Wilcox wrote:
> On Fri, Mar 14, 2003 at 03:34:36PM +0100, Eric Piel wrote:
> > I think lines like that from patch-2.5.64 are very suspicious to be
> > related to the bug:
> > +	base->timer_jiffies = INITIAL_JIFFIES;
> > +	base->tv1.index = INITIAL_JIFFIES & TVR_MASK;
> > +	base->tv2.index = (INITIAL_JIFFIES >> TVR_BITS) & TVN_MASK;
> > +	base->tv3.index = (INITIAL_JIFFIES >> (TVR_BITS+TVN_BITS)) & TVN_MASK;
> > +	base->tv4.index = (INITIAL_JIFFIES >> (TVR_BITS+2*TVN_BITS)) &
> > TVN_MASK;
> > +	base->tv5.index = (INITIAL_JIFFIES >> (TVR_BITS+3*TVN_BITS)) &
> > TVN_MASK;
> 
> No, I don't think so.  Those lines are for starting `jiffies' at a very
> high number so we spot jiffie-wrap bugs early on.

  The nanosleep() bug narrowed down to 2.5.63-bk2. That's version, the "initial
jiffies" patch went in. And yes, it's on i686 machine.

	Cheers,
		Vita

[Linux-ia64] Re: [BUG] nanosleep() granularity bumps up in 2.5.64

[Tim Schmielau]

On Mon, 17 Mar 2003, Vitezslav Samel wrote:

> On Fri, Mar 14, 2003 at 02:48:59PM +0000, Matthew Wilcox wrote:
> > On Fri, Mar 14, 2003 at 03:34:36PM +0100, Eric Piel wrote:
> > > I think lines like that from patch-2.5.64 are very suspicious to be
> > > related to the bug:
> > > +	base->timer_jiffies = INITIAL_JIFFIES;
> > > +	base->tv1.index = INITIAL_JIFFIES & TVR_MASK;
> > > +	base->tv2.index = (INITIAL_JIFFIES >> TVR_BITS) & TVN_MASK;
> > > +	base->tv3.index = (INITIAL_JIFFIES >> (TVR_BITS+TVN_BITS)) & TVN_MASK;
> > > +	base->tv4.index = (INITIAL_JIFFIES >> (TVR_BITS+2*TVN_BITS)) &
> > > TVN_MASK;
> > > +	base->tv5.index = (INITIAL_JIFFIES >> (TVR_BITS+3*TVN_BITS)) &
> > > TVN_MASK;
> >
> > No, I don't think so.  Those lines are for starting `jiffies' at a very
> > high number so we spot jiffie-wrap bugs early on.
>
>   The nanosleep() bug narrowed down to 2.5.63-bk2. That's version, the "initial
> jiffies" patch went in. And yes, it's on i686 machine.

You can easily check whether it's connected with this change by setting
INITIAL_JIFFIES to zero. This should exactly recover the previous
situation.
I.e., something like the following (untested, hand-crafted) patch:

--- linux-2.5.64/include/linux/time.h
+++ linux-2.5.64/include/linux/time.h
@@ -28,7 +28,7 @@
  * Have the 32 bit jiffies value wrap 5 minutes after boot
  * so jiffies wrap bugs show up earlier.
  */
- #define INITIAL_JIFFIES ((unsigned int) (-300*HZ))
+ #define INITIAL_JIFFIES 0

 /*
  * Change timeval to jiffies, trying to avoid the


Tim