Messages in this thread | | | Date | Thu, 11 Apr 2013 10:36:35 +0200 | From | Stanislaw Gruszka <> | Subject | Re: [RFC 4/4] cputime: remove scaling |
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On Wed, Apr 10, 2013 at 02:02:28PM +0200, Ingo Molnar wrote: > > * Stanislaw Gruszka <sgruszka@redhat.com> wrote: > > > Scaling cputime cause problems, bunch of them was fixed, but still is possible > > to hit multiplication overflow issue, which make {u,s}time values incorrect. > > This problem has no good solution in kernel. > > Wasn't 128-bit math a solution to the overflow problems? 128-bit math isn't nice, > but at least for multiplication it's defensible.
128 bit division is needed unfortunately. Though on 99.9% of cases, it will go through 64 bit fast path.
> > This patch remove scaling code and export raw values of {u,t}ime . Procps > > programs can use newly introduced sum_exec_runtime to find out precisely > > calculated process cpu time and scale utime, stime values accordingly. > > > > Unfortunately times(2) syscall has no such option. > > > > This change affect kernels compiled without CONFIG_VIRT_CPU_ACCOUNTING_*. > > So, the concern here is that 'top hiding' code can now hide again. It's also that > we are not really solving the problem, we are pushing it to user-space - which in > the best case gets updated to solve the problem in some similar fashion - and in > the worst case does not get updated or does it in a buggy way. > > So while user-space has it a bit easier because it can do floating point math, is > there really no workable solution to the current kernel side integer overflow bug?
I do not see any. Basically all we have make problem less reproducible or just defer it. The best solution, except full 128 bit math I found is something like this (dropping precision if values are big and overflow will happen):
u64 _scale_time(u64 rtime, u64 total, u64 time) { const int zero_bits = clzll(time) + clzll(rtime); u64 scaled;
if (zero_bits < 64) { /* Drop precision */ const int drop_bits = 64 - zero_bits;
time >>= drop_bits; rtime >>= drop_bits; total >>= 2*drop_bits;
if (total == 0) return time; }
scaled = (time * rtime) / total;
return scaled; }
It defer problem to quite long period. My testing script detect failure at:
FAIL! rtime: 1954463459156 <- 22621 days (one thread , CONFIG_HZ=1000) total: 1771603722423 stime: 354320744484 kernel: 391351504748 <- kernel value python: 390892691830 <- correct value
For one thread this is fine, but for 512 threads inaccuracy will happen after only 40 days (due to dropping too many of "total" variable bits).
> I really prefer robust kernel side accounting/instrumentation.
We have CONFIG_IRQ_TIME_ACCOUNTING and CONFIG_VIRT_CPU_ACCOUNTING_GEN. Perhaps we can change to use one of those options by default. I wonder if the additional performance cost related with them is really something that we should care about. Are there any measurement that show those will make performance worse ?
Stanislaw
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