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

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Date:	Mon, 25 Apr 2016 16:22:29 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	David Ahern <dsahern@...il.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Alexei Starovoitov <ast@...nel.org>,
	He Kuang <hekuang@...wei.com>, Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Milian Wolff <milian.wolff@...b.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Wang Nan <wangnan0@...wei.com>, Zefan Li <lizefan@...wei.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC v2] perf core: Allow setting up max frame stack depth
 via sysctl

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));
 }
 
 static void
@@ -236,10 +236,12 @@ int perf_event_max_stack_handler(struct ctl_table *table, int write,
 		return ret;
 
 	mutex_lock(&callchain_mutex);
-	if (atomic_read(&nr_callchain_events))
+	if (atomic_read(&nr_callchain_events)) {
 		ret = -EBUSY;
-	else
+	} else {
 		sysctl_perf_event_max_stack = new_value;
+		perf_callchain_entry_size   = __perf_callchain_entry_size(new_value);
+	}
 
 	mutex_unlock(&callchain_mutex);