Roberts-Thomson, James wrote:
> I've been trying to investigate an IO performance issue on my machine, as
> part of this I've noticed what is (presumably only a cosmetic) issue with
> the messages displayed at kernel boot-time.

Agree, purely cosmetic.

> In the "good old days" (i.e. older 2.6.x kernel versions), one of the many
> messages displayed at kernel boot-time was "elevator: using XXX as default
> io scheduler", where XXX was one of the IO schedulers (cfq, anticipatory,
> deadline, etc) depending on kernel .config at compile time.
> 
> I noticed in 2.6.11, this message has vanished (although this may have
> happened in an earlier kernel), and I now get some messages "io scheduler
> XXX registered".  Unfortunately, the "default" scheduler is no longer
> tagged.
> 
> The Bug, however, is that code to tag the default clearly exists in
> elevator.c, thus:
> 
> In function "elv_register":
> 
>         printk(KERN_INFO "io scheduler %s registered", e->elevator_name);
>         if (!strcmp(e->elevator_name, chosen_elevator))
>                 printk(" (default)");
>         printk("\n");
> 
> Some investigation has shown that when this code is called at kernel boot
> time with no "elevator=xxx" kernel argument, chosen_elevator is undefined.
> The code that defines chosen_elevator (elevator_setup_default) is only
> called for the first time AFTER all the compiled in schedulers call
> "elv_register".
> 
> However, if "elevator=xxx" is passed as a kernel argument, the code in
> elv_register works.

Absolutely right. I am afraid, I introduced this bug (which is no worse than 
the bug I was trying to fix), while I was trying to get the elevator.c to 
print the default selection info correct, when a user uses "elevator=" boot 
parameter.

Please refer to the attachment, which contains an email, and a patch I sent to 
Jens the moment I realised the mistake (unfortunately I had no reply from 
Jens, perhaps he was/is very busy). I acknowledge that it is an ugly fix, for 
it seems silly to use an extra variable. I could not think of a better idea I 
am afraid.

I strongly believe that while elv_register() is the right place to print info 
about an elevator that is being registered, elevator_init() perhaps is the 
right place to print what is the chosen/default elevator. But the catch 22 is 
that elv_init() is called once for every block device, so I cannot see an 
easy solution to print just once.

I will try to think more about it, and if I could come up with a better idea, 
I shall inform you and LKML.

Thank you.
Hari