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

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Date:	Thu, 27 Sep 2012 17:23:08 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Michael Neuling <mikey@...ling.org>
Cc:	K Prasad <prasad@...ux.vnet.ibm.com>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: powerpc/perf: hw breakpoints return ENOSPC

2012/9/25 Michael Neuling <mikey@...ling.org>:
> Michael Neuling <mikey@...ling.org> wrote:
>
>> Frederic Weisbecker <fweisbec@...il.com> wrote:
>>
>> > On Thu, Aug 16, 2012 at 02:23:54PM +1000, Michael Neuling wrote:
>> > > Hi,
>> > >
>> > > I've been trying to get hardware breakpoints with perf to work on POWER7
>> > > but I'm getting the following:
>> > >
>> > >   % perf record -e mem:0x10000000 true
>> > >
>> > >     Error: sys_perf_event_open() syscall returned with 28 (No space left on device).  /bin/dmesg may provide additional information.
>> > >
>> > >     Fatal: No CONFIG_PERF_EVENTS=y kernel support configured?
>> > >
>> > >   true: Terminated
>> > >
>> > > (FWIW adding -a and it works fine)
>> > >
>> > > Debugging it seems that __reserve_bp_slot() is returning ENOSPC because
>> > > it thinks there are no free breakpoint slots on this CPU.
>> > >
>> > > I have a 2 CPUs, so perf userspace is doing two perf_event_open syscalls
>> > > to add a counter to each CPU [1].  The first syscall succeeds but the
>> > > second is failing.
>> > >
>> > > 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.
>> > >
>> > > The following patch fixes this by checking the associated CPU for each
>> > > breakpoint in task_bp_pinned.  I'm not familiar with this code, so it's
>> > > provided as a reference to the above issue.
>> > >
>> > > Mikey
>> > >
>> > > 1. not sure why it doesn't just do one syscall and specify all CPUs, but
>> > > that's another issue.  Using two syscalls should work.
>> >
>> > This patch seems to make sense. I'll try it and run some tests.
>> > Can I have your Signed-off-by ?
>
> Frederic,
>
> Did you ever get to testing or integrating this patch?
>
> Mikey

Sorry, I forgot this in my mailbox. I'm going to look at this in the
next few days.
Feel free to harass me by email or IRC if I don't give news on this soon.
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