linux-kernel - Re: [PATCH tip/core/rcu] Make RCU_FAST_NO

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Date:	Thu, 5 Jul 2012 20:46:48 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu] Make RCU_FAST_NO_HZ respect nohz= boot
 parameter

On Thu, Jul 05, 2012 at 05:29:44PM -0700, Paul E. McKenney wrote:
> On Thu, Jul 05, 2012 at 04:02:08PM -0700, Josh Triplett wrote:
> > On Thu, Jul 05, 2012 at 03:37:31PM -0700, Paul E. McKenney wrote:
> > > If the nohz= boot parameter disables nohz, then RCU_FAST_NO_HZ needs to
> > > also disable itself.  This commit therefore checks for tick_nohz_enabled
> > > being zero, disabling rcu_prepare_for_idle() if so.  This patch assumes
> > > that tick_nohz_enabled can change at runtime: If this is not the case,
> > > then a simpler approach suffices.
> > 
> > Allowing nohz to change at runtime seems like an entirely unnecessary
> > bit of added complexity.  (So does having a boot parameter for it, but
> > that one at least seems easier to handle.)
> 
> I will let representatives from the various distros expound to you on
> their one-binary-only strategy for kernel builds.  ;-)

I'm aware.  However, the subset of people wanting to turn off nohz seems
sufficiently small at this point that I'd *hope* distro kernels could
just always have it turned on. :)

In any case, as I said, the ability to change it at runtime seems like
the primary bit of complexity; the ability to change it at boot time
seems straightforward to handle.

> > What would the patch look like if you can assume nohz will never change
> > at runtime?  And does anyone have a use case for changing nohz at
> > runtime, rather than at boot time?
> 
> It would be a little bit simpler, but would break in very odd and
> difficult-to-debug ways if anyone ever did allow it to change at runtime,
> for example, to accommodate systems subject to varying workloads.

Granted, but it doesn't seem worth preemtively making RCU more
complicated to accomodate a use case that nobody has said they have yet.
:)

- Josh Triplett
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