Re: [PATCH 37/37] s390: Prevent from cputime leaks

[Martin Schwidefsky]

On Wed, 25 Jan 2017 16:25:20 +0100
Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

> On Mon, Jan 23, 2017 at 10:44:56AM +0100, Martin Schwidefsky wrote:
> > On Sun, 22 Jan 2017 19:20:13 +0100
> > Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> >   
> > > The s390 clock has a higher granularity than nanoseconds. 1 nanosec
> > > equals 4.096 in s390 cputime_t. Therefore we leak a remainder while
> > > flushing the cputime through cputime_to_nsecs().
> > > 
> > > For more precision, make sure we keep that remainder on cputime
> > > accumulators for later accounting.
> > > 
> > > Reported-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> > > Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Paul Mackerras <paulus@xxxxxxxxx>
> > > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > > Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> > > Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> > > Cc: Tony Luck <tony.luck@xxxxxxxxx>
> > > Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Stanislaw Gruszka <sgruszka@xxxxxxxxxx>
> > > Cc: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > > Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>  
> > 
> > NAK. Good intention but the patch is just broken. with 36 of the 37
> > patches applied all looks good but the last one completely breaks the
> > accounting for s390. This is from an idle system:
> > 
> > top - 10:39:33 up 0 min,  1 user,  load average: 0,00, 0,00, 0,00
> > Tasks: 106 total,   1 running, 105 sleeping,   0 stopped,   0 zombie
> > %Cpu0  :  8,9 us, 21,6 sy,  0,0 ni,  0,0 id,  0,0 wa, 10,8 hi,  4,3 si, 54,4 st
> > %Cpu1  :  0,0 us, 23,5 sy,  0,0 ni,  0,0 id,  0,0 wa, 19,0 hi, 13,1 si, 44,3 st
> > %Cpu2  :  0,0 us, 30,3 sy,  0,0 ni,  0,0 id,  0,0 wa, 14,7 hi, 14,8 si, 40,2 st
> > KiB Mem :  1009304 total,   818808 free,    57284 used,   133212 buff/cache
> > KiB Swap:  1048556 total,  1048556 free,        0 used.   917356 avail Mem  
> 
> Oh ok. I must have done something wrong.
> 
> > 
> > There is another issue that affects precision, there is no s390 specific
> > version of cputime_to_nsecs. The generic version uses cputime_to_usecs
> > and mulitplies by 1000 to get nano-seconds. That already looses precision.  
> 
> That's right. And that's the point of this patch. I'm not sure we can have a
> more precise version of cputime_to_nsecs() if 1 nsec == 4.096 cputime_t

I am thinking about a version that does the calculation (cputime * 1000) >> 12
in several steps, like this

  nsecs = (((((cputime * 5) >> 3) * 5) >> 3) * 5) >> 3

To multiply with 5 is a shift by 2 and an add. In total that would gives us
6 shifts and 3 adds for the conversion.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.