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

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

Em Thu, Apr 21, 2016 at 12:17:07PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Thu, Apr 21, 2016 at 12:48:58PM +0200, Peter Zijlstra escreveu:
> > On Wed, Apr 20, 2016 at 07:47:30PM -0300, Arnaldo Carvalho de Melo wrote:
> > > +++ b/kernel/events/callchain.c
> 
> > > @@ -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,

> > And this alloc _will_ fail if you put in a decent sized value..

> > Should we put in a dmesg WARN if this alloc fails and
> > perf_event_max_stack is 'large' ?
> 
> Unsure, it already returns -ENOMEM, see, some lines above, i.e. it
> better have error handling up this, ho-hum, call chain, I'm checking...
  
So, it is, using the tools, since it fits this bill:

[root@jouet acme]# perf probe get_callchain_buffers
Added new event:
  probe:get_callchain_buffers (on get_callchain_buffers)

You can now use it in all perf tools, such as:

	perf record -e probe:get_callchain_buffers -aR sleep 1

[root@jouet acme]# perf trace -e perf_event_open --event probe:get_callchain_buffers/call-graph=fp/ -- perf record -g usleep 1
[ perf record: Woken up 1 times to write data ]
    26.740 ( 0.008 ms): perf/1264 perf_event_open(attr_uptr: 0x1e6b618, pid: 1265, group_fd: -1, flags: FD_CLOEXEC) ...
    26.740 (         ): probe:get_callchain_buffers:(ffffffff811a9480))
                                       get_callchain_buffers+0xfe200001 ([kernel.kallsyms])
                                       SYSC_perf_event_open+0xfe2003bc ([kernel.kallsyms])
                                       sys_perf_event_open+0xfe200009 ([kernel.kallsyms])
                                       do_syscall_64+0xfe200062 ([kernel.kallsyms])
                                       return_from_SYSCALL_64+0xfe200000 ([kernel.kallsyms])
                                       syscall+0xffff0179d3c22019 (/usr/lib64/libc-2.22.so)
                                       cmd_record+0xffffffffff8005fc (/home/acme/bin/perf)
                                       run_builtin+0xffffffffff800061 (/home/acme/bin/perf)
                                       main+0xffffffffff800672 (/home/acme/bin/perf)
                                       __libc_start_main+0xffff0179d3c220f0 (/usr/lib64/libc-2.22.so)
    26.749 ( 0.016 ms): perf/1264  ... [continued]: perf_event_open()) = 4
<SNIP the perf_event_open syscalls for the other 3 CPUs in this system)

So, if we put some too big value there and the allocation fails, people asking
for callchains will get that -ENOMEM back in their face.

I'll do this test, i.e. put some huge value there and ask for callchains...

- Arnaldo