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[PATCH] Prefetch kernel stacks to speed up context switch

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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 <kenneth.w.chen@intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Chen, Kenneth W 2005-09-09 13:02:02 -07:00 committed by Linus Torvalds
parent b0d62e6d5b
commit 383f2835eb
4 changed files with 30 additions and 0 deletions
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23
arch/ia64/kernel/entry.S
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@ -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

1
include/asm-ia64/system.h
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@ -275,6 +275,7 @@ extern void ia64_load_extra (struct task_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);

5
include/linux/sched.h
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@ -604,6 +604,11 @@ extern int groups_search(struct group_info *group_info, gid_t grp);
#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;

1
kernel/sched.c
View File

@ -2888,6 +2888,7 @@ 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));