[Xen-devel] [PATCH] adjust non-default sized console ring allocation

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

Using xmalloc() for objects that are guaranteed to be at least as
large as a page is wasteful, as it will always result in more (here:
double the amount) being allocated.

The other adjustments are more cosmetic:
- Updating conring and conring_size can be done so NMI/MCE generated
  messages don't use the new (larger) size with the old (smaller)
  buffer.
- The size printed can be in KiB (for the value to be easier to grasp)
  since it is always a multiple of the default of 16KiB.

One remaining problem I see is that the allocation happens after all
CPUs got brought up: With that process (plus the prior ACPI table
parsing) being what generates (on large systems) by far the largest
amount of messages, one of the primary uses of that option - getting at
all boot messages - seems to be out of reach at present. Prior to
discovering this boot option exists at all (it didn't in 3.3) I was
about to implement a solution where the ring would sit at the end of
.bss, and thus be sizeable at very early boot time; maybe that's still
worth considering as an alternative to the current approach?

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- 2009-08-18.orig/xen/drivers/char/console.c  2009-08-24 17:51:39.000000000 
+0200
+++ 2009-08-18/xen/drivers/char/console.c       2009-08-26 16:47:56.000000000 
+0200
@@ -608,7 +608,7 @@ void __init console_init_postirq(void)
     if ( opt_conring_size < conring_size )
         return;
     
-    ring = xmalloc_bytes(opt_conring_size);
+    ring = alloc_xenheap_pages(get_order_from_bytes(opt_conring_size), 0);
     if ( ring == NULL )
     {
         printk("Unable to allocate console ring of %u bytes.\n",
@@ -619,11 +619,12 @@ void __init console_init_postirq(void)
     spin_lock_irq(&console_lock);
     for ( i = conringc ; i != conringp; i++ )
         ring[i & (opt_conring_size - 1)] = conring[i & (conring_size - 1)];
-    conring_size = opt_conring_size;
     conring = ring;
+    wmb();
+    conring_size = opt_conring_size;
     spin_unlock_irq(&console_lock);
 
-    printk("Allocated console ring of %u bytes.\n", opt_conring_size);
+    printk("Allocated console ring of %u KiB.\n", opt_conring_size >> 10);
 }
 
 void __init console_endboot(void)




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