From: Tetsu Yamamoto <tyamamot@redhat.com>
Date: Tue, 12 Aug 2008 16:07:32 -0400
Subject: [xen] serialize scrubbing pages
Message-id: 20080812200732.4832.81645.sendpatchset@pq0-1.lab.bos.redhat.com
O-Subject: [RHEL5.3 PATCH 6/7] xen: Serialize scrubbing pages
Bugzilla: 456171
RH-Acked-by: Chris Lalancette <clalance@redhat.com>
RH-Acked-by: Bill Burns <bburns@redhat.com>

bz456171
# HG changeset patch
# User Keir Fraser <keir.fraser@citrix.com>
# Date 1218449930 -3600
# Node ID f10d436d9a9ef413135374e73b32659fc2da9cfb
# Parent  3d5515f40b9b2bfee339cc223f891bd5c61d6855
Serialize scrubbing pages

I noticed that parallel scrubbing pages is not efficient
due to spinlock contention. (also memory bandwidth?)
Actually heap_lock becomes a bottleneck.

In my investigation, it often takes 1 millisec just to acquire
the lock on ia64 with 8cpus. It's very wasteful.

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>

diff --git a/common/page_alloc.c b/common/page_alloc.c
index 81baf22..7a54dbe 100644
--- a/common/page_alloc.c
+++ b/common/page_alloc.c
@@ -1018,10 +1018,15 @@ static void page_scrub_softirq(void)
     int               i;
     s_time_t          start = NOW();
 
+    /* free_heap_pages() does not parallelise well. Serialise this function. */
+    if ( !spin_trylock(&page_scrub_lock) )
+    {
+        set_timer(&this_cpu(page_scrub_timer), NOW() + MILLISECS(1));
+        return;
+    }
+
     /* Aim to do 1ms of work every 10ms. */
     do {
-        spin_lock(&page_scrub_lock);
-
         if ( unlikely((ent = page_scrub_list.next) == &page_scrub_list) )
         {
             spin_unlock(&page_scrub_lock);
@@ -1041,8 +1046,6 @@ static void page_scrub_softirq(void)
         page_scrub_list.next = ent->next;
         scrub_pages -= (i+1);
 
-        spin_unlock(&page_scrub_lock);
-
         /* Working backwards, scrub each page in turn. */
         while ( ent != &page_scrub_list )
         {
@@ -1055,6 +1058,8 @@ static void page_scrub_softirq(void)
         }
     } while ( (NOW() - start) < MILLISECS(1) );
 
+    spin_unlock(&page_scrub_lock);
+
     set_timer(&this_cpu(page_scrub_timer), NOW() + MILLISECS(10));
 }