Re: [PATCH 2/2] i386 TIF flags for debug regs and io bitmap in ctxsw

[Chuck Ebbert <76306.1226@compuserve.com>]

In-Reply-To: <20060704072939.GC5902@frankl.hpl.hp.com>

On Tue, 4 Jul 2006 00:29:39 -0700, Stephane Eranian wrote:

> Following my discussion with Andi. Here is a patch that introduces
> two new TIF flags to simplify the context switch code in __switch_to().
> The idea is to minimize the number of cache lines accessed in the common
> case, i.e., when neither the debug registers nor the I/O bitmap are used.

I get a 5-10% speedup in task switch times with this patch.
Some very minor comments:


> <signed-off-by>: eranian@hpl.hp.com

Should be: Signed-off-by: Stephane Eranian <eranian@hpl.hp.com>


> +	if (test_tsk_thread_flag(next_p, TIF_IO_BITMAP) == 0) {

preferred:

	if (!test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {


> @@ -674,18 +692,9 @@ struct task_struct fastcall * __switch_t
>  	/*
>  	 * Now maybe reload the debug registers
>  	 */

 	/*
 	 * Now maybe reload the debug registers and/or IO bitmap
 	 */


And this should be added to the patch:

--- 2.6.17-nb.orig/arch/i386/kernel/process.c
+++ 2.6.17-nb/arch/i386/kernel/process.c
@@ -360,13 +360,12 @@ EXPORT_SYMBOL(kernel_thread);
  */
 void exit_thread(void)
 {
-	struct task_struct *tsk = current;
-	struct thread_struct *t = &tsk->thread;
-
 	/* The process may have allocated an io port bitmap... nuke it. */
-	if (unlikely(NULL != t->io_bitmap_ptr)) {
+	if (unlikely(test_thread_flag(TIF_IO_BITMAP))) {
 		int cpu = get_cpu();
 		struct tss_struct *tss = &per_cpu(init_tss, cpu);
+		struct task_struct *tsk = current;
+		struct thread_struct *t = &tsk->thread;
 
 		kfree(t->io_bitmap_ptr);
 		t->io_bitmap_ptr = NULL;
-- 
Chuck
 "You can't read a newspaper if you can't read."  --George W. Bush