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authorWaiman Long <longman@redhat.com>2023-07-24 17:38:26 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2023-08-05 09:31:41 +0300
commit1fd7ab3facfc793c3f99d86752f8eb82db18e03d (patch)
tree5b476c29b83ae15ba9f08febf8bfd5c5e24ab177 /drivers/base
parent33568553b3fc0afe99389ac4c6954c8b6c50e948 (diff)
downloadlinux-1fd7ab3facfc793c3f99d86752f8eb82db18e03d.tar.xz
driver/base/cpu: Retry online operation if -EBUSY
Booting the kernel with "maxcpus=1" is a common technique for CPU partitioning and isolation. It delays the CPU bringup process until when the bootup scripts are ready to bring CPUs online by writing 1 to /sys/device/system/cpu/cpu<X>/online. However, it was found that not all the CPUs were online after bootup. The collection of offline CPUs are different after every reboot. Further investigation reveals that some "online" write operations fail with an -EBUSY error. This error is returned when CPU hotplug is temporiarly disabled when cpu_hotplug_disable() is called. During bootup, the main caller of cpu_hotplug_disable() is pci_call_probe() for PCI device initialization. By measuring the times spent with cpu_hotplug_disabled set in a typical 2-socket server, most of them last less than 10ms. However, there are a few that can last hundreds of ms. Note that the cpu_hotplug_disabled period of different devices can overlap leading to longer cpu_hotplug_disabled hold time. Since the CPU hotplug disable condition is transient and it is not that easy to modify all the existing bootup scripts to handle this condition, the kernel can help by retrying the online operation when an -EBUSY error is returned. This patch retries the online operation in cpu_subsys_online() when an -EBUSY error is returned for up to 5 times after an exponentially increasing delay that can last a total of at least 620ms of waiting time by calling msleep(). With this patch in place, booting up the patched kernel with "maxcpus=1" does not leave any CPU in an offline state in 10 reboot attempts. Reported-by: Vishal Agrawal <vagrawal@redhat.com> Signed-off-by: Waiman Long <longman@redhat.com> Link: https://lore.kernel.org/r/20230724143826.3996163-1-longman@redhat.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/base')
-rw-r--r--drivers/base/cpu.c19
1 files changed, 19 insertions, 0 deletions
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index c1815b9dae68..4b828f54f9f4 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -19,6 +19,7 @@
#include <linux/cpufeature.h>
#include <linux/tick.h>
#include <linux/pm_qos.h>
+#include <linux/delay.h>
#include <linux/sched/isolation.h>
#include "base.h"
@@ -50,12 +51,30 @@ static int cpu_subsys_online(struct device *dev)
int cpuid = dev->id;
int from_nid, to_nid;
int ret;
+ int retries = 0;
from_nid = cpu_to_node(cpuid);
if (from_nid == NUMA_NO_NODE)
return -ENODEV;
+retry:
ret = cpu_device_up(dev);
+
+ /*
+ * If -EBUSY is returned, it is likely that hotplug is temporarily
+ * disabled when cpu_hotplug_disable() was called. This condition is
+ * transient. So we retry after waiting for an exponentially
+ * increasing delay up to a total of at least 620ms as some PCI
+ * device initialization can take quite a while.
+ */
+ if (ret == -EBUSY) {
+ retries++;
+ if (retries > 5)
+ return ret;
+ msleep(10 * (1 << retries));
+ goto retry;
+ }
+
/*
* When hot adding memory to memoryless node and enabling a cpu
* on the node, node number of the cpu may internally change.