fix strange stack calculation for secondary cpus

in arch/i386/kernel/smpboot.c:
stack_start.esp = (void *) (1024 + PAGE_SIZE + (char *)idle);

This causes problems when I switch to 4k stacks? What is supposed to be going on here? Why point esp into the middle of the stack? If you wanted to do that, why not just use PAGE_SIZE>>2?

In any case, I think THREAD_SIZE needs to be here instead of PAGE_SIZE.

Dave Hansen
[email protected]

@@ -806,7 +806,7 @@
 
 	/* So we see what's up   */
 	printk("Booting processor %d/%d eip %lx\n", cpu, apicid, start_eip);
-	stack_start.esp = (void *) (1024 + PAGE_SIZE + (char *)idle->thread_info);
+	stack_start.esp = (void *) (THREAD_SIZE + (char *)idle->thread_info);
 
 	/*
 	 * This grunge runs the startup process for

Yes, it is weird: I wondered the same when we did our bigstack patch for debugging 2.4 stack overflows.

The conclusion I came to was, it was trying to start the stack somewhere that wouldn't clash with where it's set up for the trampoline at the top of the stack area, see in particular initialize_secondary(): was choosing somewhere arbitrarily far below that.

To avoid mysterious magic numbers, I chose instead to start it immediately below that area i.e. set the top esp here to the bottom esp there. That worked fine for 2.4, I don't see why the same shouldn't work for 2.5.

Whereas with your patch, you might be overwriting that area. So below I've munged your patch into what we found worked back then.

To be honest, I can't quite remember my way around that stuff now, and my words above may make little sense!

Hugh

@@ -806,7 +806,7 @@
 
 	/* So we see what's up   */
 	printk("Booting processor %d/%d eip %lx\n", cpu, apicid, start_eip);
-	stack_start.esp = (void *) (1024 + PAGE_SIZE + (char *)idle->thread_info);
+	stack_start.esp = (void *) idle->thread.esp;
 
 	/*
 	 * This grunge runs the startup process for

It should. I just want to be able to use arbitrary stack sizes.

Agreed. I was really just trying to eliminate the magic number without much real knowledge about what was going on.

Would you like to send your patch on to Alan or Linus?

Dave Hansen
[email protected]