Repost previously discussed patch (on Jul 27, 2005). Ingo did
the same thing for all arch with 471 lines of patch. I'm still
advocating this little 30 lines patch, of 6 lines introduces
prefetch_stack() generic interface.
Andrew, please consider -mm inclusion. Or advise me what I need
to do to take this forward. Thanks.
- Ken
------
For architecture like ia64, the switch stack structure is fairly large
(currently 528 bytes). For context switch intensive application, we
found that significant amount of cache misses occurs in switch_to()
function. The following patch adds a hook in the schedule() function to
prefetch switch stack structure as soon as 'next' task is determined.
This allows maximum overlap in prefetch cache lines for that structure.
Signed-off-by: Ken Chen <[email protected]>
arch/ia64/kernel/entry.S | 23 +++++++++++++++++++++++
include/asm-ia64/system.h | 1 +
include/linux/sched.h | 5 +++++
kernel/sched.c | 1 +
4 files changed, 30 insertions(+)
--- ./arch/ia64/kernel/entry.S.orig 2005-08-09 15:32:47.778894000 -0700
+++ ./arch/ia64/kernel/entry.S 2005-08-09 16:14:17.839410590 -0700
@@ -470,6 +470,29 @@ ENTRY(load_switch_stack)
br.cond.sptk.many b7
END(load_switch_stack)
+GLOBAL_ENTRY(prefetch_stack)
+ add r14 = -IA64_SWITCH_STACK_SIZE, sp
+ add r15 = IA64_TASK_THREAD_KSP_OFFSET, in0
+ ;;
+ ld8 r16 = [r15] // load next's stack pointer
+ lfetch.fault.excl [r14], 128
+ ;;
+ lfetch.fault.excl [r14], 128
+ lfetch.fault [r16], 128
+ ;;
+ lfetch.fault.excl [r14], 128
+ lfetch.fault [r16], 128
+ ;;
+ lfetch.fault.excl [r14], 128
+ lfetch.fault [r16], 128
+ ;;
+ lfetch.fault.excl [r14], 128
+ lfetch.fault [r16], 128
+ ;;
+ lfetch.fault [r16], 128
+ br.ret.sptk.many rp
+END(prefetch_switch_stack)
+
GLOBAL_ENTRY(execve)
mov r15=__NR_execve // put syscall number in place
break __BREAK_SYSCALL
--- ./include/asm-ia64/system.h.orig 2005-08-09 15:32:51.100183000 -0700
+++ ./include/asm-ia64/system.h 2005-08-09 16:10:08.942929264 -0700
@@ -274,6 +274,7 @@ extern void ia64_load_extra (struct task
*/
#define __ARCH_WANT_UNLOCKED_CTXSW
+#define ARCH_HAS_PREFETCH_SWITCH_STACK
#define ia64_platform_is(x) (strcmp(x, platform_name) == 0)
void cpu_idle_wait(void);
--- ./include/linux/sched.h.orig 2005-08-09 15:32:51.339441000 -0700
+++ ./include/linux/sched.h 2005-08-09 16:02:35.378481695 -0700
@@ -592,6 +592,11 @@ extern int groups_search(struct group_in
#define GROUP_AT(gi, i) \
((gi)->blocks[(i)/NGROUPS_PER_BLOCK][(i)%NGROUPS_PER_BLOCK])
+#ifdef ARCH_HAS_PREFETCH_SWITCH_STACK
+extern void prefetch_stack(struct task_struct*);
+#else
+static inline void prefetch_stack(struct task_struct *t) { }
+#endif
struct audit_context; /* See audit.c */
struct mempolicy;
--- ./kernel/sched.c.orig 2005-08-09 15:32:51.429284000 -0700
+++ ./kernel/sched.c 2005-08-09 16:02:51.929262743 -0700
@@ -2887,6 +2887,7 @@ switch_tasks:
if (next == rq->idle)
schedstat_inc(rq, sched_goidle);
prefetch(next);
+ prefetch_stack(next);
clear_tsk_need_resched(prev);
rcu_qsctr_inc(task_cpu(prev));
-
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