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

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: David Ahern <dsahern@gmail.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Brendan Gregg <brendan.d.gregg@gmail.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Alexei Starovoitov <ast@kernel.org>,
	He Kuang <hekuang@huawei.com>, Jiri Olsa <jolsa@redhat.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Milian Wolff <milian.wolff@kdab.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Stephane Eranian <eranian@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vince Weaver <vincent.weaver@maine.edu>,
	Wang Nan <wangnan0@huawei.com>, Zefan Li <lizefan@huawei.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH/RFC v2] perf core: Allow setting up max frame stack depth via sysctl
Date: Mon, 25 Apr 2016 13:14:25 -0300	[thread overview]
Message-ID: <20160425161425.GA25218@kernel.org> (raw)
In-Reply-To: <20160422221806.GA62857@ast-mbp.thefacebook.com>

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?

- Arnaldo