Re: [RFC/HACK] x86: Fast return to kernel

[Linus Torvalds]

On Sun, May 4, 2014 at 12:59 PM, H. Peter Anvin <h.peter.anvin@xxxxxxxxx> 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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