From: Michael Neuling <mikey@neuling.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: K Prasad <prasad@linux.vnet.ibm.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org,
Ingo Molnar <mingo@kernel.org>
Subject: Re: powerpc/perf: hw breakpoints return ENOSPC
Date: Fri, 17 Aug 2012 09:34:07 +1000 [thread overview]
Message-ID: <8649.1345160047@neuling.org> (raw)
In-Reply-To: <1345117498.29668.23.camel@twins>
> > > > 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.
>
> ptrace uses perf for hwbp support so we're stuck with all kinds of
> stupid ptrace constraints.. or somesuch.
OK
> > Are you saying that we need to keep at least 1 slot free at all times,
> > so that we can use it for ptrace?
>
> No, I'm saying perf-hwbp is weird because of ptrace, maybe the ptrace
> weirdness shouldn't live in perf-hwpb but in the ptrace-perf glue
> however..
OK.
> > Is "perf record -e mem:0x10000000 true" ever going to be able to work on
> > POWER7 with only one hw breakpoint resource per CPU?
>
> I think it should work... but I'm fairly sure it currently doesn't
> because of how things are done. 'perf record -ie mem:0x100... true'
> might just work.
Adding -i doesn't help.
Mikey
next prev parent reply other threads:[~2012-08-16 23:34 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-16 4:23 powerpc/perf: hw breakpoints return ENOSPC Michael Neuling
2012-08-16 7:40 ` Peter Zijlstra
2012-08-16 11:17 ` Michael Neuling
2012-08-16 11:44 ` Peter Zijlstra
2012-08-16 14:02 ` Michael Ellerman
2012-08-16 14:15 ` Peter Zijlstra
2012-08-17 1:20 ` Michael Ellerman
2012-08-16 23:34 ` Michael Neuling [this message]
2012-08-17 16:15 ` Frederic Weisbecker
2012-08-17 21:58 ` Michael Neuling
2012-09-25 7:10 ` Michael Neuling
2012-09-27 15:23 ` Frederic Weisbecker
2012-09-27 1:02 ` Jovi Zhang
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