On Sun, May 4, 2014 at 12:59 PM, H. Peter Anvin <h.peter.an...@intel.com> wrote:
>
> Maybe let userspace sit in a tight loop doing RDTSC, and look for data
> points too far apart to have been uninterrupted?
That won't work, since Andy's patch improves on the "interrupt
happened in kernel space", not on the user-space interrupt case.
But some variation on that with a kernel module that does something like
- take over one CPU and force tons of timer interrupts on that CPU
using the local APIC
- for (say) ten billion cycles, do something like this in that kernel module:
#define TEN_BILLION (10000000000)
unsigned long prev = 0, sum = 0, end = rdtsc() + TEN_BILLION;
for (;;) {
unsigned long tsc = rdtsc();
if (tsc > end)
break;
if (tsc < prev + 500) {
sum += tsc - prev;
}
prev = tsc;
}
and see how big a fraction of the 10 billion cycles you capture in
'sum'. The bigger the fraction, the less time the timer interrupts
stole from your CPU.
That "500" is just a random cut-off. Any interrupt will take more than
that many TSC cycles. So the above basically counts how much
uninterrupted time that thread gets.
Hmm?
Linus
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