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

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Message-ID: <20140807172753.GG3588@twins.programming.kicks-ass.net>
Date:	Thu, 7 Aug 2014 19:27:53 +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 Thu, Aug 07, 2014 at 05:26:00PM +0200, Peter Zijlstra wrote:
> > > So do we really need the call_rcu_task() thing and why isn't something
> > > like synchronize_tasks() good enough?
> > 
> > Sounds like a question for Steven.
> > 
> > > 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.
> > 
> > If that really was the case, that would be bad.  However, in the latest
> > versions, that is no longer the case.
> > 
> > > * I'm assuming that, since tracing is 'rare' and this is some tracing
> > > thing.
> > 
> > Another good point for Steven.
> 
> Yes.. and he's back now, so please :-)

Right, Steve (and Paul) please explain _why_ this is an 'RCU' at all?
_Why_ do we have call_rcu_task(), and why is it entwined in the 'normal'
RCU stuff? We've got SRCU -- which btw started out simple, without
call_srcu() -- and that lives entirely independent. And SRCU is far more
an actual RCU than this thing is, its got read side primitives and
everything.

Also, I cannot think of any other use besides trampolines for this
thing, but that might be my limited imagination.



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