On Wed, 2002-07-24 at 18:21, Linus Torvalds wrote:
> > Then we get "if (0 && ..)" which should hopefully be evaluated away.
>
> I think preempt_count() is not unconditionally 0 for non-preemptible
> kernels, so I don't think this is a compile-time constant.
>
> That may be a bug in preempt_count(), of course.
Oh, it was until Ingo's IRQ rewrite... we do not want it unconditional
anymore. Here is a new version which defines an empty
"cond_resched_lock()" for !CONFIG_PREEMPT [1].
Good for you guys?
Robert Love
[1] You may ask, why not just have it drop the lock and reschedule
unconditionally if !CONFIG_PREEMPT? The answer is because in this
function, as in many others, we do not know the call chain and the locks
held. Most callers of zap_page_range() hold no locks on call -- but one
does. This is one reason I would prefer to just unconditionally drop
the locks and have each kernel do the right thing.
diff -urN linux-2.5.28/include/linux/sched.h linux/include/linux/sched.h
--- linux-2.5.28/include/linux/sched.h Wed Jul 24 14:03:20 2002
+++ linux/include/linux/sched.h Wed Jul 24 18:26:06 2002
@@ -888,6 +888,34 @@
__cond_resched();
}
+#ifdef CONFIG_PREEMPT
+
+/*
+ * cond_resched_lock() - if a reschedule is pending, drop the given lock,
+ * call schedule, and on return reacquire the lock.
+ *
+ * Note: this assumes the given lock is the _only_ held lock and otherwise
+ * you are not atomic. The kernel preemption counter gives us "free"
+ * checking that this is really the only lock held -- let's use it.
+ */
+static inline void cond_resched_lock(spinlock_t * lock)
+{
+ if (need_resched() && preempt_count() == 1) {
+ _raw_spin_unlock(lock);
+ preempt_enable_no_resched();
+ __cond_resched();
+ spin_lock(lock);
+ }
+}
+
+#else
+
+static inline void cond_resched_lock(spinlock_t * lock)
+{
+}
+
+#endif
+
/* Reevaluate whether the task has signals pending delivery.
This is required every time the blocked sigset_t changes.
Athread cathreaders should have t->sigmask_lock. */
diff -urN linux-2.5.28/mm/memory.c linux/mm/memory.c
--- linux-2.5.28/mm/memory.c Wed Jul 24 14:03:27 2002
+++ linux/mm/memory.c Wed Jul 24 18:24:48 2002
@@ -390,8 +390,8 @@
{
pgd_t * dir;
- if (address >= end)
- BUG();
+ BUG_ON(address >= end);
+
dir = pgd_offset(vma->vm_mm, address);
tlb_start_vma(tlb, vma);
do {
@@ -402,33 +402,43 @@
tlb_end_vma(tlb, vma);
}
-/*
- * remove user pages in a given range.
+#define ZAP_BLOCK_SIZE (256 * PAGE_SIZE) /* how big a chunk we loop over */
+
+/**
+ * zap_page_range - remove user pages in a given range
+ * @vma: vm_area_struct holding the applicable pages
+ * @address: starting address of pages to zap
+ * @size: number of bytes to zap
*/
void zap_page_range(struct vm_area_struct *vma, unsigned long address, unsigned long size)
{
struct mm_struct *mm = vma->vm_mm;
mmu_gather_t *tlb;
- pgd_t * dir;
- unsigned long start = address, end = address + size;
+ unsigned long end, block;
- dir = pgd_offset(mm, address);
+ spin_lock(&mm->page_table_lock);
/*
- * This is a long-lived spinlock. That's fine.
- * There's no contention, because the page table
- * lock only protects against kswapd anyway, and
- * even if kswapd happened to be looking at this
- * process we _want_ it to get stuck.
+ * This was once a long-held spinlock. Now we break the
+ * work up into ZAP_BLOCK_SIZE units and relinquish the
+ * lock after each interation. This drastically lowers
+ * lock contention and allows for a preemption point.
*/
- if (address >= end)
- BUG();
- spin_lock(&mm->page_table_lock);
- flush_cache_range(vma, address, end);
+ while (size) {
+ block = (size > ZAP_BLOCK_SIZE) ? ZAP_BLOCK_SIZE : size;
+ end = address + block;
+
+ flush_cache_range(vma, address, end);
+ tlb = tlb_gather_mmu(mm, 0);
+ unmap_page_range(tlb, vma, address, end);
+ tlb_finish_mmu(tlb, address, end);
+
+ cond_resched_lock(&mm->page_table_lock);
+
+ address += block;
+ size -= block;
+ }
- tlb = tlb_gather_mmu(mm, 0);
- unmap_page_range(tlb, vma, address, end);
- tlb_finish_mmu(tlb, start, end);
spin_unlock(&mm->page_table_lock);
}
-
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This archive was generated by hypermail 2b29 : Tue Jul 30 2002 - 14:00:18 EST