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

From: Arjan van de Ven
Date: Fri Feb 20 2015 - 12:45:47 EST

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

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

the option exists today in sysfs and kernel parameter...

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)

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