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

From: Stephane Eranian
Date: Tue Jul 04 2006 - 03:36:13 EST

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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 i386 modifications.

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@xxxxxxxxxx

diff -urNp linux-2.6.17.2.orig/arch/i386/kernel/ioport.c linux-2.6.17.2-tif/arch/i386/kernel/ioport.c
--- linux-2.6.17.2.orig/arch/i386/kernel/ioport.c 2006-06-17 18:49:35.000000000 -0700
+++ linux-2.6.17.2-tif/arch/i386/kernel/ioport.c 2006-07-03 09:28:07.000000000 -0700
@@ -79,6 +79,7 @@ asmlinkage long sys_ioperm(unsigned long

memset(bitmap, 0xff, IO_BITMAP_BYTES);
t->io_bitmap_ptr = bitmap;
+ set_thread_flag(TIF_IO_BITMAP);
}

/*
diff -urNp linux-2.6.17.2.orig/arch/i386/kernel/process.c linux-2.6.17.2-tif/arch/i386/kernel/process.c
--- linux-2.6.17.2.orig/arch/i386/kernel/process.c 2006-06-17 18:49:35.000000000 -0700
+++ linux-2.6.17.2-tif/arch/i386/kernel/process.c 2006-07-04 00:06:16.000000000 -0700
@@ -370,6 +370,7 @@ void exit_thread(void)

kfree(t->io_bitmap_ptr);
t->io_bitmap_ptr = NULL;
+ clear_thread_flag(TIF_IO_BITMAP);
/*
* Careful, clear this in the TSS too:
*/
@@ -388,6 +389,7 @@ void flush_thread(void)

memset(tsk->thread.debugreg, 0, sizeof(unsigned long)*8);
memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
+ clear_tsk_thread_flag(tsk, TIF_DEBUG);
/*
* Forget coprocessor state..
*/
@@ -432,7 +434,7 @@ int copy_thread(int nr, unsigned long cl
savesegment(gs,p->thread.gs);

tsk = current;
- if (unlikely(NULL != tsk->thread.io_bitmap_ptr)) {
+ if (unlikely(test_tsk_thread_flag(tsk, 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;
@@ -440,6 +442,7 @@ int copy_thread(int nr, unsigned long cl
}
memcpy(p->thread.io_bitmap_ptr, tsk->thread.io_bitmap_ptr,
IO_BITMAP_BYTES);
+ set_tsk_thread_flag(p, TIF_IO_BITMAP);
}

/*
@@ -534,10 +537,24 @@ int dump_task_regs(struct task_struct *t
return 1;
}

-static inline void
-handle_io_bitmap(struct thread_struct *next, struct tss_struct *tss)
+static inline void __switch_to_xtra(struct task_struct *next_p,
+ struct tss_struct *tss)
{
- if (!next->io_bitmap_ptr) {
+ struct thread_struct *next;
+
+ next = &next_p->thread;
+
+ if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
+ set_debugreg(next->debugreg[0], 0);
+ set_debugreg(next->debugreg[1], 1);
+ set_debugreg(next->debugreg[2], 2);
+ set_debugreg(next->debugreg[3], 3);
+ /* no 4 and 5 */
+ set_debugreg(next->debugreg[6], 6);
+ set_debugreg(next->debugreg[7], 7);
+ }
+
+ if (test_tsk_thread_flag(next_p, TIF_IO_BITMAP) == 0) {
/*
* Disable the bitmap via an invalid offset. We still cache
* the previous bitmap owner and the IO bitmap contents:
@@ -545,6 +562,7 @@ handle_io_bitmap(struct thread_struct *n
tss->io_bitmap_base = INVALID_IO_BITMAP_OFFSET;
return;
}
+
if (likely(next == tss->io_bitmap_owner)) {
/*
* Previous owner of the bitmap (hence the bitmap content)
@@ -674,18 +692,9 @@ struct task_struct fastcall * __switch_t
/*
* Now maybe reload the debug registers
*/
- if (unlikely(next->debugreg[7])) {
- set_debugreg(next->debugreg[0], 0);
- set_debugreg(next->debugreg[1], 1);
- set_debugreg(next->debugreg[2], 2);
- set_debugreg(next->debugreg[3], 3);
- /* no 4 and 5 */
- set_debugreg(next->debugreg[6], 6);
- set_debugreg(next->debugreg[7], 7);
- }
-
- if (unlikely(prev->io_bitmap_ptr || next->io_bitmap_ptr))
- handle_io_bitmap(next, tss);
+ if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW))
+ || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP))
+ __switch_to_xtra(next_p, tss);

disable_tsc(prev_p, next_p);

diff -urNp linux-2.6.17.2.orig/arch/i386/kernel/ptrace.c linux-2.6.17.2-tif/arch/i386/kernel/ptrace.c
--- linux-2.6.17.2.orig/arch/i386/kernel/ptrace.c 2006-06-17 18:49:35.000000000 -0700
+++ linux-2.6.17.2-tif/arch/i386/kernel/ptrace.c 2006-07-04 00:19:59.000000000 -0700
@@ -468,8 +468,11 @@ long arch_ptrace(struct task_struct *chi
for(i=0; i<4; i++)
if ((0x5f54 >> ((data >> (16 + 4*i)) & 0xf)) & 1)
goto out_tsk;
+ if (data)
+ set_tsk_thread_flag(child, TIF_DEBUG);
+ else
+ clear_tsk_thread_flag(child, TIF_DEBUG);
}
-
addr -= (long) &dummy->u_debugreg;
addr = addr >> 2;
child->thread.debugreg[addr] = data;
diff -urNp linux-2.6.17.2.orig/include/asm-i386/thread_info.h linux-2.6.17.2-tif/include/asm-i386/thread_info.h
--- linux-2.6.17.2.orig/include/asm-i386/thread_info.h 2006-06-17 18:49:35.000000000 -0700
+++ linux-2.6.17.2-tif/include/asm-i386/thread_info.h 2006-07-03 05:51:25.000000000 -0700
@@ -143,6 +143,8 @@ register unsigned long current_stack_poi
#define TIF_RESTORE_SIGMASK 9 /* restore signal mask in do_signal() */
#define TIF_POLLING_NRFLAG 16 /* true if poll_idle() is polling TIF_NEED_RESCHED */
#define TIF_MEMDIE 17
+#define TIF_DEBUG 18 /* uses debug registers */
+#define TIF_IO_BITMAP 19 /* uses I/O bitmap */

#define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
#define _TIF_NOTIFY_RESUME (1<<TIF_NOTIFY_RESUME)
@@ -155,6 +157,8 @@ register unsigned long current_stack_poi
#define _TIF_SECCOMP (1<<TIF_SECCOMP)
#define _TIF_RESTORE_SIGMASK (1<<TIF_RESTORE_SIGMASK)
#define _TIF_POLLING_NRFLAG (1<<TIF_POLLING_NRFLAG)
+#define _TIF_DEBUG (1<<TIF_DEBUG)
+#define _TIF_IO_BITMAP (1<<TIF_IO_BITMAP)

/* work to do on interrupt/exception return */
#define _TIF_WORK_MASK \
@@ -163,6 +167,9 @@ register unsigned long current_stack_poi
/* work to do on any return to u-space */
#define _TIF_ALLWORK_MASK (0x0000FFFF & ~_TIF_SECCOMP)

+/* flags to check in __switch_to() */
+#define _TIF_WORK_CTXSW (_TIF_DEBUG|_TIF_IO_BITMAP)
+
/*
* Thread-synchronous status.
*

-
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Next message: Ingo Molnar: "Re: 2.6.17-mm5 + pcmcia/hostap/8139too patches -- inconsistent {hardirq-on-W} -> {in-hardirq-W} usage"
Previous message: Stephane Eranian: "[PATCH 1/2] x86-64 TIF flags for debug regs and io bitmap in ctxsw"
Next in thread: Stephane Eranian: "[PATCH] i386 TIF flags for debug regs and io bitmap in ctxsw (v2)"
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