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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Feb 20, 2015 at 09:45:39AM -0800, Arjan van de Ven wrote:
> On 2/20/2015 9:43 AM, Peter Zijlstra wrote:
> >On Fri, Feb 20, 2015 at 09:32:39AM -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?
> >
> >So my objections are twofold:
> >
> >  - I object to fast expedites in principle; they spray IPIs across the
> >    system, so ideally we'd not have them at all, therefore also not at
> >    boot.
> >
> >    Because as soon as the option exists, people will use it for other
> >    things too.
> 
> the option exists today in sysfs and kernel parameter...

Yeah, Paul and me have been having this argument for a while now ;-)

> >And esp. in bootup code you can special case a lot of stuff; there's
> >limited concurrency esp. because userspace it not there yet. So we might
> >not actually need those sync calls.
> 
> yeah I am going down that angle as well absolutely.
> but there are cases that may well be legit (or are 5 function calls deep into common code)

Good ;-)