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SubjectRe: Gang scheduling

On 10/12/2018 11:01 AM, Tim Chen wrote:
> On 10/10/2018 05:09 PM, Subhra Mazumdar wrote:
>> Hi,
>> I was following the Coscheduling patch discussion on lkml and Peter mentioned he had a patch series. I found the following on github.
>> I would like to test this with KVMs. Are the commits from 38d5acb to f019876 sufficient? Also is there any documentaion on how to use it (any knobs I need to turn on for gang scheduling to happen?) or is it enabled by default for KVMs?
>> Thanks,
>> Subhra
> I would suggest you try
> without the load balancing part of gang scheduling.
> It is enabled by default for KVMs.
> Due to the constant change in gang scheduling status of the QEMU thread
> depending on whether vcpu is loaded or unloaded,
> the load balancing part of the code doesn't work very well.
Thanks. Does this mean each vcpu thread need to be affinitized to a cpu?
> The current version of the code need to be optimized further. Right now
> the QEMU thread constantly does vcpu load and unload during VM enter and exit.
> We gang schedule only after vcpu load and register the thread to be gang
> scheduled. When we do vcpu unload, the thread is removed from the set
> to be gang scheduled. Each time there's a synchronization with the
> sibling thread that's expensive.
> However, for QEMU, there's a one to one correspondence between the QEMU
> thread and vcpu. So we don't have to change the gang scheduling status
> for such thread to avoid the church and sync with the sibling. That should
> be helpful for VM with lots of I/O causing constant VM exits. We're
> still working on this optimization. And the load balancing should be
> better after this change.
> Tim

Also FYI I get the following error while building sched_1.23-base:

ERROR: "sched_ttwu_pending" [arch/x86/kvm/kvm-intel.ko] undefined!
scripts/Makefile.modpost:92: recipe for target '__modpost' failed

Adding the following fixed it:

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 46807dc..302b77d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -21,6 +21,7 @@
 #include <trace/events/sched.h>

 DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);

 #if defined(CONFIG_SCHED_DEBUG) && defined(HAVE_JUMP_LABEL)
 \ /
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