linux-kernel - Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread wakeups

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Date:	Wed, 02 Jul 2014 13:29:38 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread
 wakeups

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On 07/02/2014 01:26 PM, Peter Zijlstra wrote:
> On Wed, Jul 02, 2014 at 10:08:38AM -0700, Paul E. McKenney wrote:
>> As were others, not that long ago.  Today is the first hint that
>> I got that you feel otherwise.  But it does look like the softirq
>> approach to callback processing needs to stick around for awhile
>> longer.  Nice to hear that softirq is now "sane and normal"
>> again, I guess.  ;-)
> 
> Nah, softirqs are still totally annoying :-)
> 
> So I've lost detail again, but it seems to me that on all CPUs that
> are actually getting ticks, waking tasks to process the RCU state
> is entirely over doing it. Might as well keep processing their RCU
> state from the tick as was previously done.

For CPUs that are not getting ticks (eg. because they are idle),
is it worth waking up anything on that CPU, or would it make more
sense to simply process their RCU callbacks on a different CPU,
if there aren't too many pending?

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