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authorBaoquan He <bhe@redhat.com>2016-08-24 13:06:45 +0800
committerJonathan Corbet <corbet@lwn.net>2016-08-25 13:14:53 -0600
commit7c142bfee6cbcd90b1847a708e22d461a1dfa38b (patch)
tree3f6340e470b1e264ce508a5d01f51c62e7521748 /Documentation/kernel-parameters.txt
parent6d232c80158ae5a6fb497e0462d52c57c4459439 (diff)
downloadlinux-aarch64-7c142bfee6cbcd90b1847a708e22d461a1dfa38b.tar.gz
docs: kernel-parameter: Improve the description of nr_cpus and maxcpus
From the old description people still can't get what's the exact difference between nr_cpus and maxcpus. Especially in kdump kernel nr_cpus is always suggested if it's implemented in the ARCH. The reason is nr_cpus is used to limit the max number of possible cpu in system, the sum of already plugged cpus and hot plug cpus can't exceed its value. However maxcpus is used to limit how many cpus are allowed to be brought up during bootup. Signed-off-by: Baoquan He <bhe@redhat.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/kernel-parameters.txt')
-rw-r--r--Documentation/kernel-parameters.txt20
1 files changed, 13 insertions, 7 deletions
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 20557efa8bce..1fa28092ec00 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2161,10 +2161,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
than or equal to this physical address is ignored.
maxcpus= [SMP] Maximum number of processors that an SMP kernel
- should make use of. maxcpus=n : n >= 0 limits the
- kernel to using 'n' processors. n=0 is a special case,
- it is equivalent to "nosmp", which also disables
- the IO APIC.
+ will bring up during bootup. maxcpus=n : n >= 0 limits
+ the kernel to bring up 'n' processors. Surely after
+ bootup you can bring up the other plugged cpu by executing
+ "echo 1 > /sys/devices/system/cpu/cpuX/online". So maxcpus
+ only takes effect during system bootup.
+ While n=0 is a special case, it is equivalent to "nosmp",
+ which also disables the IO APIC.
max_loop= [LOOP] The number of loop block devices that get
(loop.max_loop) unconditionally pre-created at init time. The default
@@ -2773,9 +2776,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
nr_cpus= [SMP] Maximum number of processors that an SMP kernel
could support. nr_cpus=n : n >= 1 limits the kernel to
- supporting 'n' processors. Later in runtime you can not
- use hotplug cpu feature to put more cpu back to online.
- just like you compile the kernel NR_CPUS=n
+ support 'n' processors. It could be larger than the
+ number of already plugged CPU during bootup, later in
+ runtime you can physically add extra cpu until it reaches
+ n. So during boot up some boot time memory for per-cpu
+ variables need be pre-allocated for later physical cpu
+ hot plugging.
nr_uarts= [SERIAL] maximum number of UARTs to be registered.