On 05/04/2014 02:31 PM, Linus Torvalds wrote:
> 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.
>
I was thinking about your proposal, not Andy's.
> 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.
Yes, same idea, but in a kernel module.
-hpa
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