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: <20140808121055.52112b54@gandalf.local.home>
Date:	Fri, 8 Aug 2014 12:10:55 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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, masami.hiramatsu.pt@...achi.com
Subject: Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period
 waiting for RCU-tasks

On Fri, 8 Aug 2014 18:01:28 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

 
> Well, see the _BIG_ difference is that currently, when I do nop >
> current_tracer all that memory is instantly freed again.
> 
> With the proposed scheme, if I setup state, reconsider, destroy state,
> try again, and generally muck about I can tie up unspecified amounts of
> memory.
> 
> And being the bumbling idiot that I am, that's actually fairly typical
> of how I end up tracing. There's no neat and tidy, I trace something,
> look at the trace, script a little, muck about with the settings and
> goto 1.


It would actually be trivial to make that case never free the
trampoline associated to function tracing. As that is a static ops that
never gets freed, the trampoline it uses doesn't need to be freed
either.

But, if you were to do:

   # cd /sys/kernel/debug/tracing
   # mkdir instances/foo
   # cd instances/foo
1: # echo function > current_tracer
   # echo nop > current_tracer
   goto 1

Then, yeah that could do it.

> 
> 
> In any case, I think I now fully understand what you're trying to do,
> just not sure its all win.

Fair enough.

-- Steve
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