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

From: Peter Zijlstra
Date: Fri Feb 20 2015 - 12:44:21 EST

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

Because as soon as the option exists, people will use it for other
things too.

- The proposed interface is very much exposed to everybody who wants
it; this again is wide open to (ab)use.

Once it exists people will start to use, and before you know it we'll
always have that fast counter incremented and we're in IPI hell. Most
likely because someone was lazy and it seemed like a quick fix for
some stupid code.

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.
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