diff options
| author | Robert Love <rml@tech9.net> | 2002-06-13 20:51:48 -0700 |
|---|---|---|
| committer | Dave Kleikamp <shaggy@kleikamp.austin.ibm.com> | 2002-06-13 20:51:48 -0700 |
| commit | 4d0b85ea4610b51beeb97cd0cc43edb1be42d063 (patch) | |
| tree | 94ee171a0673171fd474555a2d1b9a4529b72a41 | |
| parent | d257b778c1c21ebb99c400e0ffa9f3af215da18a (diff) | |
[PATCH] kernel preemption bits (2/2)
In both preempt_schedule in sched.c and resume_kernel in entry.S, it is
possible to return with need_resched set and thus a pending preemption
but not service that preemption for some time.
Consider:
- return from schedule() to preempt_schedule
- interrupt occurs, sets need_resched
- we cannot preempt since preempt_count = PREEMPT_ACTIVE
- back in preempt_schedule, set preempt_count = 0
Now we again can preempt, but we will not. Instead we return and
continue executing. On the next interrupt, we will redo the whole
fiasco which is a waste since we could of reentered schedule while we
were there. Worse, if we acquire a lock before the next interrupt we
can potentially delay the pending reschedule a very long time. This is
not acceptable.
The solution is to check for and loop on need_resched on resume_kernel
and preempt_schedule like schedule itself does.
| -rw-r--r-- | arch/i386/kernel/entry.S | 4 | ||||
| -rw-r--r-- | kernel/sched.c | 6 |
2 files changed, 9 insertions, 1 deletions
diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S index 6a42ae06ccbe..09d303454378 100644 --- a/arch/i386/kernel/entry.S +++ b/arch/i386/kernel/entry.S @@ -211,6 +211,7 @@ ENTRY(resume_userspace) ENTRY(resume_kernel) cmpl $0,TI_PRE_COUNT(%ebx) # non-zero preempt_count ? jnz restore_all +need_resched: movl TI_FLAGS(%ebx), %ecx # need_resched set ? testb $_TIF_NEED_RESCHED, %cl jz restore_all @@ -220,7 +221,8 @@ ENTRY(resume_kernel) sti call schedule movl $0,TI_PRE_COUNT(%ebx) - jmp restore_all + cli + jmp need_resched #endif # system call handler stub diff --git a/kernel/sched.c b/kernel/sched.c index 1254ad6df71f..7dc3cd10f3b8 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -893,9 +893,15 @@ asmlinkage void preempt_schedule(void) if (unlikely(ti->preempt_count)) return; +need_resched: ti->preempt_count = PREEMPT_ACTIVE; schedule(); ti->preempt_count = 0; + + /* we can miss a preemption opportunity between schedule and now */ + barrier(); + if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) + goto need_resched; } #endif /* CONFIG_PREEMPT */ |