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

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Date:	Thu, 16 Aug 2012 21:17:29 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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

Peter,

> > 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...

Maybe I should wait for Frederic to respond but I'm not sure I
understand what you're saying.

I can see how using ptrace hw breakpoints and perf hw breakpoints at the
same time could be a problem, but I'm not sure how this would stop it.

Are you saying that we need to keep at least 1 slot free at all times,
so that we can use it for ptrace?

Is "perf record -e mem:0x10000000 true" ever going to be able to work on
POWER7 with only one hw breakpoint resource per CPU?  

Thanks,
Mikey
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