Subject: Re: powerpc/perf: hw breakpoints return ENOSPC

From: Michael Neuling <[email-masked]>
Date: Sat, 18 Aug 2012 07:58:15 +1000

Frederic Weisbecker <[email-masked]> 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 ?

Of course...

Signed-off-by: Michael Neuling <[email-masked]>

> 
> Thanks.
> 
> > 
> > diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> > index bb38c4d..e092daa 100644
> > --- a/kernel/events/hw_breakpoint.c
> > +++ b/kernel/events/hw_breakpoint.c
> > @@ -111,14 +111,16 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
> >   * Count the number of breakpoints of the same type and same task.
> >   * The given event must be not on the list.
> >   */
> > -static int task_bp_pinned(struct perf_event *bp, enum bp_type_idx type)
> > +static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
> >  {
> >  	struct task_struct *tsk = bp->hw.bp_target;
> >  	struct perf_event *iter;
> >  	int count = 0;
> >  
> >  	list_for_each_entry(iter, &bp_task_head, hw.bp_list) {
> > -		if (iter->hw.bp_target == tsk && find_slot_idx(iter) == type)
> > +		if (iter->hw.bp_target == tsk &&
> > +		    find_slot_idx(iter) == type &&
> > +		    cpu == iter->cpu)
> >  			count += hw_breakpoint_weight(iter);
> >  	}
> >  
> > @@ -141,7 +143,7 @@ fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
> >  		if (!tsk)
> >  			slots->pinned += max_task_bp_pinned(cpu, type);
> >  		else
> > -			slots->pinned += task_bp_pinned(bp, type);
> > +			slots->pinned += task_bp_pinned(cpu, bp, type);
> >  		slots->flexible = per_cpu(nr_bp_flexible[type], cpu);
> >  
> >  		return;
> > @@ -154,7 +156,7 @@ fetch_bp_busy_slots(struct bp_busy_slots *slots, struct perf_event *bp,
> >  		if (!tsk)
> >  			nr += max_task_bp_pinned(cpu, type);
> >  		else
> > -			nr += task_bp_pinned(bp, type);
> > +			nr += task_bp_pinned(cpu, bp, type);
> >  
> >  		if (nr > slots->pinned)
> >  			slots->pinned = nr;
> > @@ -188,7 +190,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu, bool enable,
> >  	int old_idx = 0;
> >  	int idx = 0;
> >  
> > -	old_count = task_bp_pinned(bp, type);
> > +	old_count = task_bp_pinned(cpu, bp, type);
> >  	old_idx = old_count - 1;
> >  	idx = old_idx + weight;
> >  
>