linux-kernel - Re: [BELATED CORE TOPIC] context tracking / nohz / RCU state

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Date:	Tue, 11 Aug 2015 14:52:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	"ksummit-discuss@...ts.linuxfoundation.org" 
	<ksummit-discuss@...ts.linuxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [BELATED CORE TOPIC] context tracking / nohz / RCU state

On Tue, Aug 11, 2015 at 2:47 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Tue, Aug 11, 2015 at 12:07:54PM -0700, Andy Lutomirski wrote:
>> On Tue, Aug 11, 2015 at 11:33 AM, Paul E. McKenney
>> <paulmck@...ux.vnet.ibm.com> wrote:
>> > On Tue, Aug 11, 2015 at 10:49:36AM -0700, Andy Lutomirski wrote:
>> >> This is a bit late, but here goes anyway.
>> >>
>> >> Having played with the x86 context tracking hooks for awhile, I think
>> >> it would be nice if core code that needs to be aware of CPU context
>> >> (kernel, user, idle, guest, etc) could come up with single,
>> >> comprehensible, easily validated set of hooks that arch code is
>> >> supposed to call.
>> >>
>> >> Currently we have:
>> >>
>> >>  - RCU hooks, which come in a wide variety to notify about IRQs, NMIs, etc.
>> >
>> > Something about people yelling at me for waking up idle CPUs, thus
>> > degrading their battery lifetimes.  ;-)
>> >
>> >>  - Context tracking hooks.  Only used by some arches.  Calling these
>> >> calls the RCU hooks for you in most cases.  They have weird
>> >> interactions with interrupts and they're slow.
>> >
>> > Combining these would be good, but there are subtleties.  For example,
>> > some arches don't have context tracking, but RCU still needs to correctly
>> > identify idle CPUs without in any way interrupting or awakening that CPU.
>> > It would be good to make this faster, but it does have to work.
>>
>> Could we maybe have one set of old RCU-only (no context tracking)
>> callbacks and a completely separate set of callbacks for arches that
>> support full context tracking?  The implementation of the latter would
>> presumably call into RCU.
>
> It should be possible for RCU to use context tracking if it is available
> and to have RCU maintain its own state otherwise, if that is what you
> are getting at.  Assuming that the decision is global and made at either
> build or boot time, anyway.  Having some CPUs tracking context and others
> not sounds like an invitation for subtle bugs.

I think that, if this happens, the decision should be made at build
time, per arch, and not be configurable.  If x86_64 uses context
tracking, then I think x86_64 shouldn't need additional RCU callbacks,
assuming that context tracking is comprehensive enough for RCU's
purposes.

--Andy
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