* Rik van Riel <[email protected]> wrote:
> On 05/01/2015 12:03 PM, Andy Lutomirski wrote:
>
> > The last time I asked, the impression I got was that we needed two things:
> >
> > 1. We can't pluck things from the RCU list without knowing whether the
> > CPU is in an RCU read-side critical section, and we can't know that
> > unless we have regular grade periods or we know that the CPU is idle.
> > To make the CPU detectably idle, we need to set a bit somewhere.
>
> More than that. We also need a way for another CPU to identify the
> callbacks they could run for us, without confusing them with new
> callbacks queued after we transitioned back from USER to KERNEL
> context.
That's easy: a simple, constant-work IPI sent to nohz-full CPUs could
just dequeue these callbacks, if it sees that we are still in
user-mode.
It's super easy because such an IPI would essentially be equivalent to
a system call context from RCU's POV, if it uses this test:
if (user_mode(regs)) {
... pluck RCU callbacks ...
}
That way we can have lots of overhead removed from the syscall call
path ...
Thanks,
Ingo
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