Re: Bug: ll_rw_blk.c, elevator.c and displaying "default" IO Schedule r at boot-time (Cosmetic only)

[Srihari Vijayaraghavan]

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
From harisri@xxxxxxxxxxxxxxxx Mon Jan 31 23:10:25 2005
From: Srihari Vijayaraghavan <harisri@xxxxxxxxxxxxxxxx>
To: Jens Axboe <axboe@xxxxxxx>
Subject: Re: [PATCH] elevator: print default selection
Date: Mon, 31 Jan 2005 23:10:25 +1100
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At a cost of one extra variable, this one works great for all cases:

Hopefully the extra steps in elevator_init() won't hinder the performance, as 
elevator_init() seems to be the safest place to determine the default 
elevator, as all elevators are registered, when it is called.

Sorry if it is no good.

Thank you.
Hari.

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--- 2.6.11/drivers/block/elevator.c	2005-01-22 15:22:55.000000000 +1100
+++ test/drivers/block/elevator.c	2005-01-31 22:38:36.000000000 +1100
@@ -180,6 +180,8 @@
 
 __setup("elevator=", elevator_setup);
 
+static int default_msg = 0;
+
 int elevator_init(request_queue_t *q, char *name)
 {
 	struct elevator_type *e = NULL;
@@ -195,6 +197,12 @@
 	if (!e)
 		return -EINVAL;
 
+	if (!default_msg && !strcmp(e->elevator_name, chosen_elevator)) {
+		printk(KERN_INFO "using %s as default io scheduler\n",
+						chosen_elevator);
+		default_msg = 1;
+	}
+
 	eq = kmalloc(sizeof(struct elevator_queue), GFP_KERNEL);
 	if (!eq) {
 		elevator_put(e->elevator_type);
@@ -513,10 +521,7 @@
 	list_add_tail(&e->list, &elv_list);
 	spin_unlock_irq(&elv_list_lock);
 
-	printk(KERN_INFO "io scheduler %s registered", e->elevator_name);
-	if (!strcmp(e->elevator_name, chosen_elevator))
-		printk(" (default)");
-	printk("\n");
+	printk(KERN_INFO "io scheduler %s registered\n", e->elevator_name);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(elv_register);

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