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

[Arnaldo Carvalho de Melo]

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:
> > On Fri, Apr 22, 2016 at 04:05:31PM -0600, David Ahern wrote:
> > > On 4/22/16 2:52 PM, Arnaldo Carvalho de Melo wrote:
> > > >+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;
> > > >+}
> > > >+
> 
> > > To Alexei's comment, a max_stack of 0 still has a non-zero alloc size so
> > > that should be ok.
> 
> > > >  static DEFINE_PER_CPU(int, callchain_recursion[PERF_NR_CONTEXTS]);
> > > >  static atomic_t nr_callchain_events;
> > > >  static DEFINE_MUTEX(callchain_mutex);
> > > >@@ -73,7 +81,7 @@ static int alloc_callchain_buffers(void)
> > > >  	if (!entries)
> > > >  		return -ENOMEM;
> 
> > > >-	size = sizeof(struct perf_callchain_entry) * PERF_NR_CONTEXTS;
> > > >+	size = perf_callchain_entry__sizeof() * PERF_NR_CONTEXTS;
> 
> > > >  	for_each_possible_cpu(cpu) {
> > > >  		entries->cpu_entries[cpu] = kmalloc_node(size, GFP_KERNEL,
>  
> > 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.
> 
> Huh? Can you elaborate a bit more?
> 
> Are you saying this is a bug introduced by this patch or something
> pre-existing?

When we alloc we alloc:

/*
 * Number of contexts where an event can trigger:
 *      task, softirq, hardirq, nmi.
 */
#define PERF_NR_CONTEXTS        4

        callchain_cpus_entries[0 .. (nr_cpu_ids - 1)][in_nmi() (3)]
        callchain_cpus_entries[0 .. (nr_cpu_ids - 1)][in_irq() (2)]
        callchain_cpus_entries[0 .. (nr_cpu_ids - 1)][in_sortirq() (1)]
        callchain_cpus_entries[0 .. (nr_cpu_ids - 1)][(0)]

And all of these have this type:

struct perf_callchain_entry {
        __u64     nr;
        __u64     ip[0]; /* /proc/sys/kernel/perf_event_max_stack */
};

So, it will return a struct_callchain_entry pointer to a 8-byte sized
chunk, with just perf_callchain_entry->nr, and will not try to touch
perf_callchain_entry->ip[] since sysctl_perf_event_max_stack is zero.

But perf_callchain_entry->ip is not a pointer... Got it ;-\

Touché, respinning...

Brendan, this probably affected you results...

- Arnaldo