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

[David Ahern <dsahern@gmail.com>]

On 4/22/16 2:52 PM, Arnaldo Carvalho de Melo wrote:
> Em Wed, Apr 20, 2016 at 04:04:12PM -0700, Alexei Starovoitov escreveu:
>> On Wed, Apr 20, 2016 at 07:47:30PM -0300, Arnaldo Carvalho de Melo wrote:
>
>> Nice. I like it. That's a great approach to hard problem.
>> Java guys will be happy too.
>> Please also adjust two places in kernel/bpf/stackmap.c
>
>>> +	{
>>> +		.procname	= "perf_event_max_stack",
>>> +		.data		= NULL, /* filled in by handler */
>>> +		.maxlen		= sizeof(sysctl_perf_event_max_stack),
>>> +		.mode		= 0644,
>>> +		.proc_handler	= perf_event_max_stack_handler,
>>> +		.extra1		= &zero,
>
>> zero seems to be the wrong minimum. I think it should be at least 2 to
>> to fit user/kernel tags ?  Probably needs to define max as well.
>
> So, if someone asks for zero, it will not copy anything, but then, this
> would be what the user had asked for :-)
>
> Ditto for the max, if someone asks for too big a callchain, then when
> allocating it it will fail and no callchain will be produced, that or it
> will be able to allocate but will take too long copying that many
> addresses, and we would be prevented from doing so by some other
> protection, iirc there is perf_cpu_time_max_percent, and then buffer
> space will run out.
>
> So I think that leaving it as is is enough, no?
>
> Can I keep your Acked-by? David, can I keep yours?

Yes


> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index 343c22f5e867..6fe77349fa9d 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c
> @@ -18,6 +18,14 @@ struct callchain_cpus_entries {
>   	struct perf_callchain_entry	*cpu_entries[0];
>   };
>
> +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,