ext4slower doesn't work and the error message is not very helpful

[afiskon]

Hi,

I would like to use ext4slower. I execute it like this:

sudo /usr/share/bcc/tools/ext4slower 1

... and it tells me:

ERROR: no ext4_file_operations in /proc/kallsyms. Exiting.

It's not very helpful since I can't understand from this error message whether I should rebuild a kernel with some additional flags or do something else. Could you please add some hint text that would explain how to make ext4slower work?

The environment is Arch Linux x64, uname -a:

Linux e733 4.13.3-1-ARCH #1 SMP PREEMPT Thu Sep 21 20:33:16 CEST 2017 x86_64 GNU/Linux

[goldshtn]

/cc @brendangregg

[samuelnair]

The tool is checking to see if that symbol exists in the kernel, as a means to determine if you have the ext4 compiled or loaded so as to ensure that those kprobes can attach successfully to the necessary functions.

The ext4_file_operations is a struct and should show up as a symbol of type 'd'. In all likelihood you need to compile the kernel with CONFIG_KALLSYMS_ALL option enabled to ensure that symbol table contains more than just text type symbols.

[afiskon]

Indeed, CONFIG_KALLSYMS_ALL is disabled:

$ zcat /proc/config.gz | grep CONFIG_KALLSYMS_ALL
# CONFIG_KALLSYMS_ALL is not set

I believe it worth adding a corresponding hint to the script:

diff --git a/home/eax/temp/ext4slower.orig b/usr/share/bcc/tools/ext4slower
index 3b5e7ae..09e8572 100755
--- a/home/eax/temp/ext4slower.orig
+++ b/usr/share/bcc/tools/ext4slower
@@ -262,6 +262,7 @@ with open(kallsyms) as syms:
             break
     if ops == '':
         print("ERROR: no ext4_file_operations in /proc/kallsyms. Exiting.")
+        print("HINT: the kernel should be built with CONFIG_KALLSYMS_ALL.")
         exit()
     bpf_text = bpf_text.replace('EXT4_FILE_OPERATIONS', ops)
 if min_ms == 0:

[yonghong-song]

Could you submit a formal patch for this? I think this is indeed helpful.
cc @brendangregg

[afiskon]

Done.

[afiskon]

For the record - also I've contacted Tobias Powalowski, the maintainer of the linux package for Arch Linux, and asked him if it's possible to enable CONFIG_KALLSYMS_ALL in this package.

[afiskon]

For the record - updated PR is here #1408