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Subject[PATCH] kmap performance
Hi,

Two performance changes against 2.4.3.

flush_all_zero_pkmaps() is guarding against a race which cannot happen,
and thus hurting performance.

It uses the atomic fetch-and-clear "ptep_get_and_clear()" operation,
which is much stronger than needed. No-one has the page mapped, and
cannot get at the page's virtual address without taking the kmap_lock,
which flush_all_zero_pkmaps() holds. Even with speculative execution,
there are no races which need to be closed via an atomic op.

On a two-way, Pentium-III system, flush_all_zero_pkmaps() was taking
over 200K CPU cycles (with the flush_tlb_all() only accounting for ~9K of
those cycles).

This patch replaces ptep_get_and_clear() with a pte_page(), and
pte_clear(). This reduces flush_all_zero_pkmaps() to around 75K cycles.


The second part of this patch adds a conditional guard to the
wake_up() call in kunmap_high().

With most usage patterns, a page will not be simultaneously mapped more
than once, hence the most common case (by far) is for pkmap_count[] to
decrement to 1. This was causing an unconditional call to wake_up(), when
(again) the common case is to have no tasks in the wait-queue.

This patches adds a guard to the wake_up() using an inlined
waitqueue_active(), and so avoids unnecessary function calls.
It also drops the actual wake_up() to be outside of the spinlock. This
is safe, as any waiters will have placed themselves onto the queue under
the kmap_lock, and kunmap_high() tests the queue under this lock.

Mark



diff -urN -X dontdiff linux-2.4.3/mm/highmem.c markhe-2.4.3/mm/highmem.c
--- linux-2.4.3/mm/highmem.c Tue Nov 28 20:31:02 2000
+++ markhe-2.4.3/mm/highmem.c Sat Mar 31 15:03:43 2001
@@ -46,7 +46,7 @@

for (i = 0; i < LAST_PKMAP; i++) {
struct page *page;
- pte_t pte;
+
/*
* zero means we don't have anything to do,
* >1 means that it is still in use. Only
@@ -56,10 +56,21 @@
if (pkmap_count[i] != 1)
continue;
pkmap_count[i] = 0;
- pte = ptep_get_and_clear(pkmap_page_table+i);
- if (pte_none(pte))
+
+ /* sanity check */
+ if (pte_none(pkmap_page_table[i]))
BUG();
- page = pte_page(pte);
+
+ /*
+ * Don't need an atomic fetch-and-clear op here;
+ * no-one has the page mapped, and cannot get at
+ * its virtual address (and hence PTE) without first
+ * getting the kmap_lock (which is held here).
+ * So no dangers, even with speculative execution.
+ */
+ page = pte_page(pkmap_page_table[i]);
+ pte_clear(&pkmap_page_table[i]);
+
page->virtual = NULL;
}
flush_tlb_all();
@@ -139,6 +150,7 @@
{
unsigned long vaddr;
unsigned long nr;
+ int need_wakeup;

spin_lock(&kmap_lock);
vaddr = (unsigned long) page->virtual;
@@ -150,13 +162,31 @@
* A count must never go down to zero
* without a TLB flush!
*/
+ need_wakeup = 0;
switch (--pkmap_count[nr]) {
case 0:
BUG();
case 1:
- wake_up(&pkmap_map_wait);
+ /*
+ * Avoid an unnecessary wake_up() function call.
+ * The common case is pkmap_count[] == 1, but
+ * no waiters.
+ * The tasks queued in the wait-queue are guarded
+ * by both the lock in the wait-queue-head and by
+ * the kmap_lock. As the kmap_lock is held here,
+ * no need for the wait-queue-head's lock. Simply
+ * test if the queue is empty.
+ */
+ need_wakeup = waitqueue_active(&pkmap_map_wait);
}
spin_unlock(&kmap_lock);
+
+ /*
+ * Can do wake-up, if needed, race-free outside of
+ * the spinlock.
+ */
+ if (need_wakeup)
+ wake_up(&pkmap_map_wait);
}

/*
-
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