From mboxrd@z Thu Jan  1 00:00:00 1970
From: Chris Lalancette <clalance@redhat.com>
Subject: Re: Latency spike during page_scrub_softirq
Date: Fri, 03 Jul 2009 09:32:08 +0200
Message-ID: <4A4DB3F8.9020903@redhat.com>
References: <C6728D15.88B5%keir.fraser@eu.citrix.com>
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To: Keir Fraser <keir.fraser@eu.citrix.com>
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Keir Fraser wrote:
> On 02/07/2009 15:47, "Chris Lalancette" <clalance@redhat.com> wrote:
> 
>> There are a couple of solutions that I can think of:
>> 1)  Just clear the pages inside free_domheap_pages().  I tried this with a
>> 64GB
>> guest as mentioned above, and I didn't see any ill effects from doing so.  It
>> seems like this might actually be a valid way to go, although then a single
>> CPU
>> is doing all of the work of freeing the pages (might be a problem on UP
>> systems).
> 
> Now that domain destruction is preemptible all the way back up to libxc, I
> think the page-scrub queue is not so much required. And it seems it never
> worked very well anyway! I will remove it.
> 
> This may make 'xm destroy' operations take a while, but actually this may be
> more sensibly handled by punting the destroy hypercall into another thread
> at dom0 userspace level, rather than doing the shonky 'scheduling' we
> attempt in Xen itself right now.

Yep, agreed, and I see you've committed as c/s 19886.  Except...

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
...
@@ -1247,10 +1220,7 @@ void free_domheap_pages(struct page_info
             for ( i = 0; i < (1 << order); i++ )
             {
                 page_set_owner(&pg[i], NULL);
-                spin_lock(&page_scrub_lock);
-                page_list_add(&pg[i], &page_scrub_list);
-                scrub_pages++;
-                spin_unlock(&page_scrub_lock);
+                scrub_one_page(&pg[i]);
             }
         }
     }

This hunk actually needs to free the page as well, with free_heap_pages().

-- 
Chris Lalancette