From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: x86/dom0: limit dom0_max_vcpus value
Date: Mon, 12 Mar 2012 15:56:21 +0000
Message-ID: <4F5E1CA5.1060103@citrix.com>
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This caused particularly poor performance when booting a server in
uniprocessor mode for debugging reasons, and had 4 dom0 vcpus competing
for 1pcpus worth of time.

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com


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# HG changeset patch
# Parent 5d20d2f6ffed0a49f030f04a8870f1926babbcbf
x86/dom0: limit dom0_max_vcpus value

The command line parameter "dom0_max_vcpus" is useful on large servers where
dom0 does not want many vcpus, as an alternative to booting and hot-unplugging
down to a sensible number (which is poor memory-wize as many drivers allocate
large per-cpu buffers which will never be used after the hot-unplug).

However, as the code currently stands, the value for "dom0_max_vcpus" is used as
an absolute value rather than a maximum, resulting in dom0 getting more vcpus
than pcpus.  This can lead to unexpected poor performance.

Therefore, augment the "opt_dom0_max_vcpus == 0" test to also check whether the
user specified parameter is greater than the number of available pcpus.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff -r 5d20d2f6ffed xen/arch/x86/domain_build.c
--- a/xen/arch/x86/domain_build.c
+++ b/xen/arch/x86/domain_build.c
@@ -87,8 +87,9 @@ integer_param("dom0_max_vcpus", opt_dom0
 
 struct vcpu *__init alloc_dom0_vcpu0(void)
 {
-    if ( opt_dom0_max_vcpus == 0 )
-        opt_dom0_max_vcpus = num_cpupool_cpus(cpupool0);
+    int nr_pcpus = num_cpupool_cpus(cpupool0);
+    if ( opt_dom0_max_vcpus == 0 || opt_dom0_max_vcpus > nr_pcpus)
+        opt_dom0_max_vcpus = nr_pcpus;
     if ( opt_dom0_max_vcpus > MAX_VIRT_CPUS )
         opt_dom0_max_vcpus = MAX_VIRT_CPUS;
 

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