On 2016/6/17 5:47, Arnaldo Carvalho de Melo wrote:
Em Wed, Jun 15, 2016 at 02:23:34AM +0000, Wang Nan escreveu:
Before this patch, when using overwritable ring buffer on an old
kernel, error message is misleading:
# ~/perf record -m 1 -e raw_syscalls:*/overwrite/ -a
Error:
The raw_syscalls:sys_enter event is not supported.
This patch output clear error message to tell user his/her kernel
is too old:
# ~/perf record -m 1 -e raw_syscalls:*/overwrite/ -a
Reading from overwrite event is not supported by this kernel
Error:
The raw_syscalls:sys_enter event is not supported.
So I went to see if exposing that missing_features struct outside
evsel.c was strictly needed and found that we already have fallbacking
for this feature (attr.write_backwards) i.e. if we set it and
sys_perf_event_open() fails, we will check if we are asking the kernel
for some attr. field that it doesn't supports, set that missing_features
and try again.
But the way this was done for attr.write_backwards was buggy, as we need
to check features in the inverse order of their introduction to the
kernel, so that a newer tool checks first the newest perf_event_attr
fields, detecting that the older kernel doesn't have support for them.
The patch that introduced write_backwards support ([1]) in perf_evsel__open()
did this checking after all the other older attributes, wrongly.
[1]: b90dc17a5d14 ("perf evsel: Add overwrite attribute and check
write_backward")
Also we shouldn't even try to call sys_perf_event_open if
perf_missing_features.write_backward is true and evsel->overwrite is
also true, the old code would check this only after successfully opening
the fd, do it before the open loop.
Please take a look at the following patch, see if it is sufficient for
handling older kernels, probably we need to emit a message to the user,
but that has to be done at the builtin- level, i.e. at the tool, i.e.
perf_evsel_open__strerror() should have what it takes to figure out this
extra error and provide/ a proper string, lemme add this to the patch...
done, please check:
write_backwards_fallback.patch:
[SNIP]
@@ -1496,7 +1493,10 @@ try_fallback:
* Must probe features in the order they were added to the
* perf_event_attr interface.
*/
I read this comment but misunderstand. I thought 'order' means newest last.
Will try your patch. Thank you.
- if (!perf_missing_features.clockid_wrong && evsel->attr.use_clockid) {
+ if (!perf_missing_features.write_backward &&
evsel->attr.write_backward) {
+ perf_missing_features.write_backward = true;
+ goto fallback_missing_features;
+ } else if (!perf_missing_features.clockid_wrong &&
evsel->attr.use_clockid) {
perf_missing_features.clockid_wrong = true;
goto fallback_missing_features;
} else if (!perf_missing_features.clockid && evsel->attr.use_clockid) {
@@ -1521,10 +1521,6 @@ try_fallback:
PERF_SAMPLE_BRANCH_NO_FLAGS))) {
perf_missing_features.lbr_flags = true;
goto fallback_missing_features;
- } else if (!perf_missing_features.write_backward &&
- evsel->attr.write_backward) {
- perf_missing_features.write_backward = true;
- goto fallback_missing_features;
}
out_close:
@@ -2409,6 +2405,8 @@ int perf_evsel__open_strerror(struct perf_evsel *evsel,
struct target *target,
"We found oprofile daemon running, please stop it and try again.");
break;
case EINVAL:
+ if (evsel->overwrite && perf_missing_features.write_backward)
+ return scnprintf(msg, size, "Reading from overwrite event is
not supported by this kernel.");
if (perf_missing_features.clockid)
return scnprintf(msg, size, "clockid feature not
supported.");
if (perf_missing_features.clockid_wrong)
