Re: perf: remount of tracefs required

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Aug 05, 2015 at 04:17:42PM +0200, Milian Wolff escreveu:
> On Tuesday 04 August 2015 12:27:46 Arnaldo Carvalho de Melo wrote:
> >   [acme@zoo linux]$ trace usleep 1
> >   Error:	No permissions to read
> >   /sys/kernel/debug/tracing/events/raw_syscalls/sys_(enter|exit)
> >   Hint:	Try 'sudo mount -o remount,mode=755 /sys/kernel/debug/tracing'
 
> This message I do not get, not even with my perf compiled from the v4.2-rc5 
> tag. And it is exactly what I was asking for.
 
> <snip>
 
> > I guess what fixes what you report is this cset:

> >   commit ab85785aa13c36440a91a8e9f7616357de411a1f

> >     tools lib api debugfs: Check for tracefs when reporting errors

> > [acme@zoo linux]$ git describe ab85785aa13c36440a91a8e9f7616357de411a1f
> > v4.2-rc1-17-gab85785aa13c

> > So, can you try running the perf tools, built from the latest 4.2-rc
> > codebase to see if it behaves as you expect?
 
> This is not included in 4.2-rc5 as far as I can see:
 
> $ git describe
> v4.2-rc5-2-ga64d3e5
> $ git show ab85785aa13c36440a91a8e9f7616357de411a1f
> fatal: bad object ab85785aa13c36440a91a8e9f7616357de411a1f
> $ git log --grep debugfs --oneline --author=Arnaldo
> f816b3c tools lib fs debugfs: Check if debugfs is mounted when handling ENOENT
> ...

Oops, it is just in tip/perf/core, i.e. scheduled for 4.3 :-\ Wonder how
difficult would be to cherry-pick that into perf/urgent.

I.e. can you try doing something like:

  git cherry-pick ab85785aa13c36440a91a8e9f7616357de411a1f

With a checked out v4.2-rc5, or better, torvalds/master, i.e. the top of
Linus's git tree? It is possible that one or more patches just before
this one, also in perf/core, may be needed.

If it is easy, perhaps we can push it for 4.2 final.

Ah, you may want to take a look at:

https://git.kernel.org/cgit/linux/kernel/git/acme/linux.git/log/?h=perf/core

To get from this:

  [root@zoo ~]# trace -e newfstatat,unlinkat rm /tmp/foo
     0.489 ( 0.011 ms): rm/8428 newfstatat(dfd: CWD, filename: 0x9153f8, statbuf: 0x915368, flag: 256 ) = 0
     0.508 ( 0.010 ms): rm/8428 unlinkat(dfd: CWD, pathname: 0x9140c0) = 0
  [root@zoo ~]#

After doing this:

  [root@zoo ~]# perf probe 'vfs_getname=getname_flags:72 pathname=filename:string'
  Added new event:
    probe:vfs_getname    (on getname_flags:72 with pathname=filename:string)

  You can now use it in all perf tools, such as:

	perf record -e probe:vfs_getname -aR sleep 1

To this:

  [root@zoo ~]# trace -e newfstatat,unlinkat rm /tmp/foo
  rm: cannot remove ‘/tmp/foo’: No such file or directory
     0.503 ( 0.007 ms): rm/8480 newfstatat(dfd: CWD, filename: /tmp/foo, statbuf: 0x2370368, flag: 256) = -1 ENOENT No such file or directory
     0.514 ( 0.004 ms): rm/8480 unlinkat(dfd: CWD, pathname: /tmp/foo) = -1 ENOENT No such file or directory
  [root@zoo ~]#

Auto inserting that probe remains to be done, but for now can be done just once
per boot.

- Arnaldo