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Subject[PATCH] low-latency zap_page_range()
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Andrew,

Attached patch implements a low latency version of "zap_page_range()".

Calls with even moderately large page ranges result in very long lock
held times and consequently very long periods of non-preemptibility.
This function is in my list of the top 3 worst offenders. It is gross.

This new version reimplements zap_page_range() as a loop over
ZAP_BLOCK_SIZE chunks. After each iteration, if a reschedule is
pending, we drop page_table_lock and automagically preempt. Note we can
not blindly drop the locks and reschedule (e.g. for the non-preempt
case) since there is a possibility to enter this codepath holding other
locks.

... I am sure you are familar with all this, its the same deal as your
low-latency work. This patch implements the "cond_resched_lock()" as we
discussed sometime back. I think this solution should be acceptable to
you and Linus.

There are other misc. cleanups, too.

This new zap_page_range() yields latency too-low-to-benchmark: <<1ms.

Please, Andrew, add this to your ever-growing list.

Robert Love

diff -urN linux-2.5.32/include/linux/sched.h linux/include/linux/sched.h
--- linux-2.5.32/include/linux/sched.h Tue Aug 27 15:26:34 2002
+++ linux/include/linux/sched.h Wed Aug 28 18:04:41 2002
@@ -898,6 +898,34 @@
__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 does not assume the given lock is the _only_ lock held.
+ * The kernel preemption counter gives us "free" checking that we are
+ * atomic -- 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-2.5.32/mm/memory.c linux/mm/memory.c
--- linux-2.5.32/mm/memory.c Tue Aug 27 15:26:42 2002
+++ linux/mm/memory.c Wed Aug 28 18:03:11 2002
@@ -389,8 +389,8 @@
{
pgd_t * dir;

- if (address >= end)
- BUG();
+ BUG_ON(address >= end);
+
dir = pgd_offset(vma->vm_mm, address);
tlb_start_vma(tlb, vma);
do {
@@ -401,30 +401,43 @@
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;
- unsigned long start = address, end = address + size;
+ unsigned long end, block;

- /*
- * 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.
- */
- if (address >= end)
- BUG();
spin_lock(&mm->page_table_lock);
- flush_cache_range(vma, address, end);

- tlb = tlb_gather_mmu(mm, 0);
- unmap_page_range(tlb, vma, address, end);
- tlb_finish_mmu(tlb, start, end);
+ /*
+ * 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.
+ */
+ 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;
+ }
+
spin_unlock(&mm->page_table_lock);
}



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