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Date: Fri, 8 Aug 2014 13:27:16 -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 18:27:14 +0200
Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Aug 08, 2014 at 10:58:58AM -0400, Steven Rostedt wrote:
>
> > > > No, they are also used by optimized kprobes. This is why optimized
> > > > kprobes depend on !CONFIG_PREEMPT. [ added Masami to the discussion ].
> > >
> > > How do those work? Is that one where the INT3 relocates the instruction
> > > stream into an alternative 'text' and that JMPs back into the original
> > > stream at the end?
> >
> > No, it's where we replace the 'int3' with a jump to a trampoline that
> > simulates an INT3. Speeds things up quite a bit.
>
> OK, so the trivial 'fix' for that is to patch the probe site like:
>
> preempt_disable(); INC GS:%__preempt_count
> call trampoline; CALL 0xDEADBEEF
> preempt_enable(); DEC GS:%__preempt_count
> JNZ 1f
> CALL ___preempt_schedule
> 1f:
>
> At which point the preempt_disable/enable() are the read side primitives
> and call_rcu_sched/synchronize_sched are sufficient to release it.
>
> With the per-cpu preempt count stuff we have on x86 that is 4
> instructions for the preempt_*() stuff -- they're 'big' instructions
> though, since 3 have memops and 2 have a segment prefix.
>
>
Well, this looks like it may make kprobes a bit more complex, and even
slow down slightly the optimized probe.
Also note that if we add call_rcu_tasks(), then perf function tracing
can be called directly instead of being added to the trampoline that
disables and enables preemption before calling it.
-- Steve
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