[PATCH] x86-64 TIF flags for debug regs and io bitmap in ctxsw
Hello,
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.
This patch covers the x86-64 modifications. A patch for i386 follows.
Changelog:
- add TIF_DEBUG to track when debug registers are active
- add TIF_IO_BITMAP to track when I/O bitmap is used
- modify __switch_to() to use the new TIF flags
<signed-off-by>: eranian@hpl.hp.com
Signed-off-by: Andi Kleen <ak@suse.de>
diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c
index bb6745d..6e05276 100644
--- a/arch/x86_64/kernel/process.c
+++ b/arch/x86_64/kernel/process.c
@@ -350,6 +350,7 @@
kfree(t->io_bitmap_ptr);
t->io_bitmap_ptr = NULL;
+ clear_thread_flag(TIF_IO_BITMAP);
/*
* Careful, clear this in the TSS too:
*/
@@ -369,6 +370,7 @@
if (t->flags & _TIF_IA32)
current_thread_info()->status |= TS_COMPAT;
}
+ t->flags &= ~_TIF_DEBUG;
tsk->thread.debugreg0 = 0;
tsk->thread.debugreg1 = 0;
@@ -461,7 +463,7 @@
asm("mov %%es,%0" : "=m" (p->thread.es));
asm("mov %%ds,%0" : "=m" (p->thread.ds));
- if (unlikely(me->thread.io_bitmap_ptr != NULL)) {
+ if (unlikely(test_tsk_thread_flag(me, TIF_IO_BITMAP))) {
p->thread.io_bitmap_ptr = kmalloc(IO_BITMAP_BYTES, GFP_KERNEL);
if (!p->thread.io_bitmap_ptr) {
p->thread.io_bitmap_max = 0;
@@ -469,6 +471,7 @@
}
memcpy(p->thread.io_bitmap_ptr, me->thread.io_bitmap_ptr,
IO_BITMAP_BYTES);
+ set_tsk_thread_flag(p, TIF_IO_BITMAP);
}
/*
@@ -498,6 +501,40 @@
*/
#define loaddebug(thread,r) set_debugreg(thread->debugreg ## r, r)
+static inline void __switch_to_xtra(struct task_struct *prev_p,
+ struct task_struct *next_p,
+ struct tss_struct *tss)
+{
+ struct thread_struct *prev, *next;
+
+ prev = &prev_p->thread,
+ next = &next_p->thread;
+
+ if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
+ loaddebug(next, 0);
+ loaddebug(next, 1);
+ loaddebug(next, 2);
+ loaddebug(next, 3);
+ /* no 4 and 5 */
+ loaddebug(next, 6);
+ loaddebug(next, 7);
+ }
+
+ if (test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {
+ /*
+ * Copy the relevant range of the IO bitmap.
+ * Normally this is 128 bytes or less:
+ */
+ memcpy(tss->io_bitmap, next->io_bitmap_ptr,
+ max(prev->io_bitmap_max, next->io_bitmap_max));
+ } else if (test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)) {
+ /*
+ * Clear any possible leftover bits:
+ */
+ memset(tss->io_bitmap, 0xff, prev->io_bitmap_max);
+ }
+}
+
/*
* switch_to(x,y) should switch tasks from x to y.
*
@@ -586,37 +623,11 @@
task_stack_page(next_p) + THREAD_SIZE - PDA_STACKOFFSET);
/*
- * Now maybe reload the debug registers
+ * Now maybe reload the debug registers and handle I/O bitmaps
*/
- if (unlikely(next->debugreg7)) {
- loaddebug(next, 0);
- loaddebug(next, 1);
- loaddebug(next, 2);
- loaddebug(next, 3);
- /* no 4 and 5 */
- loaddebug(next, 6);
- loaddebug(next, 7);
- }
-
-
- /*
- * Handle the IO bitmap
- */
- if (unlikely(prev->io_bitmap_ptr || next->io_bitmap_ptr)) {
- if (next->io_bitmap_ptr)
- /*
- * Copy the relevant range of the IO bitmap.
- * Normally this is 128 bytes or less:
- */
- memcpy(tss->io_bitmap, next->io_bitmap_ptr,
- max(prev->io_bitmap_max, next->io_bitmap_max));
- else {
- /*
- * Clear any possible leftover bits:
- */
- memset(tss->io_bitmap, 0xff, prev->io_bitmap_max);
- }
- }
+ if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW))
+ || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP))
+ __switch_to_xtra(prev_p, next_p, tss);
return prev_p;
}