linux-kernel - Re: powerpc/perf: hw breakpoints return ENOSPC

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Date:	Thu, 16 Aug 2012 09:40:12 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Michael Neuling <mikey@...ling.org>
Cc:	K Prasad <prasad@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: powerpc/perf: hw breakpoints return ENOSPC

On Thu, 2012-08-16 at 14:23 +1000, Michael Neuling wrote:
> 
> 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=1), 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...
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