Re: [PATCH/RFC v2] perf core: Allow setting up max frame stack depth via sysctl

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Mon, Apr 25, 2016 at 01:06:48PM -0700, Alexei Starovoitov escreveu:
> On Mon, Apr 25, 2016 at 04:22:29PM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Mon, Apr 25, 2016 at 01:27:06PM -0300, Arnaldo Carvalho de Melo escreveu:
> > > Em Mon, Apr 25, 2016 at 01:14:25PM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > Em Fri, Apr 22, 2016 at 03:18:08PM -0700, Alexei Starovoitov escreveu:
> > > > > right... and looking into it further, realized that the patch is broken,
> > > > > since get_callchain_entry() is doing:
> > > > >   return &entries->cpu_entries[cpu][*rctx];
> > > > > whereas it should be dynamic offset based on sysctl_perf_event_max_stack*8
> > > > > So definitely needs another respin.
> > 
> > > struct perf_callchain_entry {
> > >         __u64     nr;
> > >         __u64     ip[0]; /* /proc/sys/kernel/perf_event_max_stack */
> > > };
> >  
> > > But perf_callchain_entry->ip is not a pointer... Got it ;-\
> > 
> > This is what I am building now, a patch on top of the previous, that
> > will be combined and sent as v3, if I don't find any more funnies:
> > 
> > diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> > index 6fe77349fa9d..40657892a7e0 100644
> > --- a/kernel/events/callchain.c
> > +++ b/kernel/events/callchain.c
> > @@ -20,11 +20,10 @@ struct callchain_cpus_entries {
> >  
> >  int sysctl_perf_event_max_stack __read_mostly = PERF_MAX_STACK_DEPTH;
> >  
> > -static size_t perf_callchain_entry__sizeof(void)
> > -{
> > -	return sizeof(struct perf_callchain_entry) +
> > -	       sizeof(__u64) * sysctl_perf_event_max_stack;
> > -}
> > +#define __perf_callchain_entry_size(entries) \
> > +	(sizeof(struct perf_callchain_entry) + sizeof(__u64) * entries)
> > +
> > +static size_t perf_callchain_entry_size __read_mostly = __perf_callchain_entry_size(PERF_MAX_STACK_DEPTH);
> >  
> >  static DEFINE_PER_CPU(int, callchain_recursion[PERF_NR_CONTEXTS]);
> >  static atomic_t nr_callchain_events;
> > @@ -81,7 +80,7 @@ static int alloc_callchain_buffers(void)
> >  	if (!entries)
> >  		return -ENOMEM;
> >  
> > -	size = perf_callchain_entry__sizeof() * PERF_NR_CONTEXTS;
> > +	size = perf_callchain_entry_size * PERF_NR_CONTEXTS;
> >  
> >  	for_each_possible_cpu(cpu) {
> >  		entries->cpu_entries[cpu] = kmalloc_node(size, GFP_KERNEL,
> > @@ -155,7 +154,8 @@ static struct perf_callchain_entry *get_callchain_entry(int *rctx)
> >  
> >  	cpu = smp_processor_id();
> >  
> > -	return &entries->cpu_entries[cpu][*rctx];
> > +	return (((void *)&entries->cpu_entries[cpu][0]) +
> > +		(*rctx * perf_callchain_entry_size));
> 
> I think the following would be easier to read:
> 
> +	return (void *)entries->cpu_entries[cpu] +
> +		*rctx * perf_callchain_entry_size;

Well, I thought that multiline expressions required parentheses, to make
them easier to read for someone, maybe Ingo? ;-)

Whatever, both generate the same result, really want me to change this?

> ?
> if didn't mixed up the ordering...

If you are not sure, then its not clearer, huh? ;-P
 
> and probably we could do the math on the spot instead of introducing
> perf_callchain_entry_size global variable?

I was trying to avoid the calc for each alloc, just doing it when we
change that number via the sysctl, probably not that big a deal, do you
really think we should do the math per-alloc instead of
per-sysctl-changing?

- Arnaldo