Re: [PATCH] Only process_die notifier in ia64_do_page_fault if KPROBES is configured.

[Christoph Lameter <clameter@engr.sgi.com>]

On Wed, 31 Aug 2005, Andi Kleen wrote:

> Also with the inline the test should be essentially a single test of 
> a global variable and jump. Hardly a big performance issue, no? 

There are multiple effects of this code.

- Additional cacheline in use in the page fault handler 
  increasing the cache foot print.

- There are registers in use for checking the global variable.

- The compilers will reserve registers for the code that is never 
  executed which may affect other elements of performance. From the 
  register perspective a function call may be better on ia64.

Certainly not a big effect (if we make sure the compiler knows that 
this test mostly fails and insure that the variable is in 
__mostly_read) but this is a frequently executed code path and the code
is there without purpose if CONFIG_KPROBES is off.

It wont get too bad unless lots of other people have similar ideas about 
fixing their race conditions using similar methods. But we will be setting 
a bad precedent if we allow this.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>