On Tue, Aug 14, 2012 at 09:06:51AM +0900, Takuya Yoshikawa wrote:
> On Mon, 13 Aug 2012 19:15:23 -0300
> Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
>
> > On Fri, Aug 10, 2012 at 05:16:12PM +0900, Takuya Yoshikawa wrote:
> > > The following commit changed mmu_shrink() so that it would skip VMs
> > > whose n_used_mmu_pages was not zero and try to free pages from others:
> > >
> > > commit 1952639665e92481c34c34c3e2a71bf3e66ba362
> > > KVM: MMU: do not iterate over all VMs in mmu_shrink()
> > >
> > > This patch fixes the function so that it can free mmu pages as before.
> > > Note that "if (!nr_to_scan--)" check is removed since we do not try to
> > > free mmu pages from more than one VM.
> > >
> > > Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>
> > > Cc: Gleb Natapov <gleb@xxxxxxxxxx>
> > > ---
> > > This patch just recovers the original behaviour and is not related
> > > to how to improve mmu_shrink() further; so please apply.
> >
> > Before 1952639665e92481c34 the code was maxed at nr_to_scan loops. So
> > removing if (!nr_to_scan--) patch does change behaviour.
> >
> > Am i missing something here?
>
> No. You are right about that.
>
> But as Gleb and I confirmed when I first sent this patch, the possiblity
> that we see "n_used_mmu_pages == 0" 128 times is quite low that it is
> almost impossible to see the effect.
>
> If you prefer to have the check, I will do so.
"kvm->arch.n_used_mmu_pages == 0" is an unlikely scenario, agree
with that.
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 9651c2c..4aeec72 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4154,11 +4154,8 @@ static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
* want to shrink a VM that only started to populate its
* MMU
* anyway.
*/
- if (kvm->arch.n_used_mmu_pages > 0) {
- if (!nr_to_scan--)
- break;
+ if (!kvm->arch.n_used_mmu_pages)
continue;
- }
idx = srcu_read_lock(&kvm->srcu);
spin_lock(&kvm->mmu_lock);
This patch removes the maximum (successful) loops, which is nr_scan ==
sc->nr_to_scan.
The description above where you say 'possibility that we see
"n_used_mmu_pages == 0" 128 times' does not match the patch above.
If the patch is correct, then please explain it clearly in the
changelog.
What is the reasoning to remove nr_to_scan? What tests did you perform?
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