linux-kernel - Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks

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Date:	Thu, 7 Aug 2014 10:45:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>, 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, tglx@...utronix.de, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period
 waiting for RCU-tasks

On Wed, Aug 06, 2014 at 03:45:18PM -0700, Paul E. McKenney wrote:
> > > > But I still very much hate the polling stuff...
> 
> The nice thing about the polling approach is minimal overhead in the
> common case where RCU-tasks is not in use.

No, quite the reverse, there is overhead when its not in use, as opposed
to no overhead at all.

I'm still not convinced we need this 'generic' rcu-task stuff and create
yet another kthread with polling semantics, we want to let the system
idle out when there's nothing to do, not keep waking it up.

So do we really need the call_rcu_task() thing and why isn't something
like synchronize_tasks() good enough?

So the thing is, the one proposed user is very rare (*) and for that
you're adding overhead outside of that user (a separate kthread) and
your adding overhead when its not used.

* I'm assuming that, since tracing is 'rare' and this is some tracing
thing.

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