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authorSrivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>2012-05-24 19:46:26 +0530
committerIngo Molnar <mingo@kernel.org>2012-07-24 13:53:14 +0200
commitd35be8bab9b0ce44bed4b9453f86ebf64062721e (patch)
tree295823c4f0d50be5b30714a083de658e1a4185e6 /kernel/cpuset.c
parentee08d1284ea9235b29bd2d9b7493b4b4cf3da09c (diff)
CPU hotplug, cpusets, suspend: Don't modify cpusets during suspend/resume
In the event of CPU hotplug, the kernel modifies the cpusets' cpus_allowed masks as and when necessary to ensure that the tasks belonging to the cpusets have some place (online CPUs) to run on. And regular CPU hotplug is destructive in the sense that the kernel doesn't remember the original cpuset configurations set by the user, across hotplug operations. However, suspend/resume (which uses CPU hotplug) is a special case in which the kernel has the responsibility to restore the system (during resume), to exactly the same state it was in before suspend. In order to achieve that, do the following: 1. Don't modify cpusets during suspend/resume. At all. In particular, don't move the tasks from one cpuset to another, and don't modify any cpuset's cpus_allowed mask. So, simply ignore cpusets during the CPU hotplug operations that are carried out in the suspend/resume path. 2. However, cpusets and sched domains are related. We just want to avoid altering cpusets alone. So, to keep the sched domains updated, build a single sched domain (containing all active cpus) during each of the CPU hotplug operations carried out in s/r path, effectively ignoring the cpusets' cpus_allowed masks. (Since userspace is frozen while doing all this, it will go unnoticed.) 3. During the last CPU online operation during resume, build the sched domains by looking up the (unaltered) cpusets' cpus_allowed masks. That will bring back the system to the same original state as it was in before suspend. Ultimately, this will not only solve the cpuset problem related to suspend resume (ie., restores the cpusets to exactly what it was before suspend, by not touching it at all) but also speeds up suspend/resume because we avoid running cpuset update code for every CPU being offlined/onlined. Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20120524141611.3692.20155.stgit@srivatsabhat.in.ibm.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/cpuset.c')
-rw-r--r--kernel/cpuset.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 8c8bd652dd12..746d1eeb5dbe 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -2054,6 +2054,9 @@ static void scan_for_empty_cpusets(struct cpuset *root)
* (of no affect) on systems that are actively using CPU hotplug
* but making no active use of cpusets.
*
+ * The only exception to this is suspend/resume, where we don't
+ * modify cpusets at all.
+ *
* This routine ensures that top_cpuset.cpus_allowed tracks
* cpu_active_mask on each CPU hotplug (cpuhp) event.
*