Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

From: Josh Triplett
Date: Sat Feb 21 2015 - 22:58:32 EST

On Sat, Feb 21, 2015 at 07:51:34AM -0800, Arjan van de Ven wrote:
> >>
> >>there's a few others as well that I'm chasing down...
> >>.. but the flip side, prior to running ring 3 code, why NOT do fast expedites?
> >
> >It would be good to have before-and-after measurements of actual
> >boot time. Are these numbers available?
> To show the boot time, I'm using the timestamp of the "Write protecting" line,
> that's pretty much the last thing we print prior to ring 3 execution.

That's a little sad; we ought to be write-protecting kernel read-only
data as *early* as possible.

> A kernel with default RCU behavior (inside KVM, only virtual devices) looks like this:
> [ 0.038724] Write protecting the kernel read-only data: 10240k
> a kernel with expedited RCU (using the command line option, so that I don't have
> to recompile between measurements and thus am completely oranges-to-oranges)
> [ 0.031768] Write protecting the kernel read-only data: 10240k
> which, in percentage, is an 18% improvement.

Nice improvement, but that suggests that we're spending far too much
time waiting on RCU grace periods at boot time.

- Josh Triplett
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