perf top: Allow asking for the maximum allowed sample rate

Add the handy '-F max' shortcut, just introduced to 'perf record', to
reading and using the kernel.perf_event_max_sample_rate value as the
user supplied sampling frequency:

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-hz04f296zccknnb5at06a6q0@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-top.txt

@@ -55,7 +55,9 @@ Default is to monitor all CPUS.
 
 -F <freq>::
 --freq=<freq>::
-	Profile at this frequency.
+	Profile at this frequency. Use 'max' to use the currently maximum
+	allowed frequency, i.e. the value in the kernel.perf_event_max_sample_rate
+	sysctl.
 
 -i::
 --inherit::

tools/perf/builtin-top.c

@@ -1307,7 +1307,9 @@ int cmd_top(int argc, const char **argv)
 	OPT_STRING(0, "sym-annotate", &top.sym_filter, "symbol name",
 		    "symbol to annotate"),
 	OPT_BOOLEAN('z', "zero", &top.zero, "zero history across updates"),
-	OPT_UINTEGER('F', "freq", &opts->user_freq, "profile at this frequency"),
+	OPT_CALLBACK('F', "freq", &top.record_opts, "freq or 'max'",
+		     "profile at this frequency",
+		      record__parse_freq),
 	OPT_INTEGER('E', "entries", &top.print_entries,
 		    "display this many functions"),
 	OPT_BOOLEAN('U', "hide_user_symbols", &top.hide_user_symbols,