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Subject: Re: powerpc/perf: hw breakpoints return ENOSPC
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Michael Neuling <mikey@neuling.org>
Date: Thu, 16 Aug 2012 09:40:12 +0200
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Cc: Ingo Molnar <mingo@kernel.org>, Frederic Weisbecker <fweisbec@gmail.com>,
 K Prasad <prasad@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org,
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On Thu, 2012-08-16 at 14:23 +1000, Michael Neuling wrote:
>=20
> On this second syscall, fetch_bp_busy_slots() sets slots.pinned to be 1,
> despite there being no breakpoint on this CPU.  This is because the call
> the task_bp_pinned, checks all CPUs, rather than just the current CPU.
> POWER7 only has one hardware breakpoint per CPU (ie. HBP_NUM=3D1), so we
> return ENOSPC.

I think this comes from the ptrace legacy, we register a breakpoint on
all cpus because when we migrate a task it cannot fail to migrate the
breakpoint.

Its one of the things I hate most about the hwbp stuff as it relates to
perf.

Frederic knows more...