hi all,
I met similar problem to these, while performing live migration or
save-restore test on the kvm platform (qemu:1.4.0, host:suse11sp2,
guest:suse11sp2), running tele-communication software suite in
guest,
https://lists.gnu.org/archive/html/qemu-devel/2013-05/msg00098.html
http://comments.gmane.org/gmane.comp.emulators.kvm.devel/102506
http://thread.gmane.org/gmane.comp.emulators.kvm.devel/100592
https://bugzilla.kernel.org/show_bug.cgi?id=58771
After live migration or virsh restore [savefile], one process's CPU
utilization went up by about 30%, resulted in throughput
degradation of this process.
If EPT disabled, this problem gone.
I suspect that kvm hypervisor has business with this problem.
Based on above suspect, I want to find the two adjacent versions of
kvm-kmod which triggers this problem or not (e.g. 2.6.39, 3.0-rc1),
and analyze the differences between this two versions, or apply the
patches between this two versions by bisection method, finally find the key patches.
Any better ideas?
Thanks,
Zhang Haoyu
I've attempted to duplicate this on a number of machines that are as similar to yours as I am able to get my hands on, and so far have not been able to see any performance degradation. And from what I've read in the above links, huge pages do not seem to be part of the problem.
So, if you are in a position to bisect the kernel changes, that would probably be the best avenue to pursue in my opinion.
Bruce
I found the first bad
commit([612819c3c6e67bac8fceaa7cc402f13b1b63f7e4] KVM: propagate fault r/w information to gup(), allow read-only memory) which triggers this problem by git bisecting the kvm kernel (download from https://git.kernel.org/pub/scm/virt/kvm/kvm.git) changes.
And,
git log 612819c3c6e67bac8fceaa7cc402f13b1b63f7e4 -n 1 -p >
612819c3c6e67bac8fceaa7cc402f13b1b63f7e4.log
git diff
612819c3c6e67bac8fceaa7cc402f13b1b63f7e4~1..612819c3c6e67bac8fceaa7cc4
02f13b1b63f7e4 > 612819c3c6e67bac8fceaa7cc402f13b1b63f7e4.diff
Then, I diffed 612819c3c6e67bac8fceaa7cc402f13b1b63f7e4.log and
612819c3c6e67bac8fceaa7cc402f13b1b63f7e4.diff,
came to a conclusion that all of the differences between
612819c3c6e67bac8fceaa7cc402f13b1b63f7e4~1 and
612819c3c6e67bac8fceaa7cc402f13b1b63f7e4
are contributed by no other than 612819c3c6e67bac8fceaa7cc402f13b1b63f7e4, so this commit is the peace-breaker which directly or indirectly causes the degradation.
Does the map_writable flag passed to mmu_set_spte() function have effect on PTE's PAT flag or increase the VMEXITs induced by that guest tried to write read-only memory?
Thanks,
Zhang Haoyu
There should be no read-only memory maps backing guest RAM.
Can you confirm map_writable = false is being passed to __direct_map? (this should not happen, for guest RAM).
And if it is false, please capture the associated GFN.
I added below check and printk at the start of __direct_map() at the fist bad commit version,
--- kvm-612819c3c6e67bac8fceaa7cc402f13b1b63f7e4/arch/x86/kvm/mmu.c 2013-07-26 18:44:05.000000000 +0800
+++ kvm-612819/arch/x86/kvm/mmu.c 2013-07-31 00:05:48.000000000 +0800
@@ -2223,6 +2223,9 @@ static int __direct_map(struct kvm_vcpu
int pt_write = 0;
gfn_t pseudo_gfn;
+ if (!map_writable)
+ printk(KERN_ERR "%s: %s: gfn = %llu \n", __FILE__, __func__, gfn);
+
for_each_shadow_entry(vcpu, (u64)gfn << PAGE_SHIFT, iterator) {
if (iterator.level == level) {
unsigned pte_access = ACC_ALL;
I virsh-save the VM, and then virsh-restore it, so many GFNs were printed, you can absolutely describe it as flooding.
The flooding you see happens during migrate to file stage because of dirty
page tracking. If you clear dmesg after virsh-save you should not see any
flooding after virsh-restore. I just checked with latest tree, I do not.
I made a verification again.
I virsh-save the VM, during the saving stage, I run 'dmesg', no GFN printed, maybe the switching from running stage to pause stage takes so short time,
no guest-write happens during this switching period.
After the completion of saving operation, I run 'demsg -c' to clear the buffer all the same, then I virsh-restore the VM, so many GFNs are printed by running 'dmesg',
and I also run 'tail -f /var/log/messages' during the restoring stage, so many GFNs are flooded dynamically too.
I'm sure that the flooding happens during the virsh-restore stage, not the migration stage.
On VM's normal starting stage, only very few GFNs are printed, shown as below
gfn = 16
gfn = 604
gfn = 605
gfn = 606
gfn = 607
gfn = 608
gfn = 609
but on the VM's restoring stage, so many GFNs are printed, taking some examples shown as below,
That's really strange. Could you please disable ept and add your trace code to FNAME(fetch)( ), then test again to see what will happen?
If there is still have many !rmap_writable cases, please measure the performance to see if it still has regression.
Many thanks!
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