Messages in this thread | | | Date | Thu, 6 Mar 2014 14:59:46 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC] [PATCH] Pre-emption control for userspace |
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On Thu, Mar 06, 2014 at 11:13:33PM +1100, Kevin Easton wrote: > On Tue, Mar 04, 2014 at 04:51:15PM -0800, Andi Kleen wrote: > > Anything else? > > If it was possible to make the time remaining in the current timeslice > available to userspace through the vdso, the thread could do something like:
Assuming we can do per-cpu values in the VDSO; this would mean hitting that cacheline on every context switch and wakeup. That's a complete non-starter performance wise.
> > if (sys_timeleft() < CRITICAL_SECTION_SIZE) > yield(); > lock(); > > to avoid running out of timeslice in the middle of the critical section.
Can still happen, the effective slice of a single runnable task is infinite, the moment another task gets woken this gets reduced to a finite amount, we then keep reducing the slice until there are about 8 runnable tasks (assuming you've not poked at any sysctls).
Also; depending on the state of the just woken task; it might land left of you in the tree, making it immediately eligible to run, completely obviating whatever number you just read.
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