Re: Huge perf degradation from missing xen_tlb_flush_all

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Oct 26, 2012 at 03:43:11PM -0700, Mukesh Rathor wrote:
> Hi,
> 
> A customer experienced huge degradation in migration performance moving
> from 2.6.32 based dom0 to 2.6.39 based dom0. We tracked it down to
> missing xen_tlb_flush_all() in 2.6.39/pv-ops kernel.
> 
> To summarize, in 2.6.32,  we had
> 
> #define flush_tlb_all xen_tlb_flush_all
> 
> As a result, when xen_remap_domain_mfn_range called flush_tlb_all(), 
> it made a hypercall to xen: 
> 
> void xen_tlb_flush_all(void)
> {
>         struct mmuext_op op;
> 	op.cmd = MMUEXT_TLB_FLUSH_ALL;
> 	BUG_ON(HYPERVISOR_mmuext_op(&op, 1, NULL, DOMID_SELF) < 0);
> }
> 
> xen optimized IPI to only relevant cpus. But in pvops/2.6.39 kernel,
> the flush_tlb_all will IPI each VCPU whethere it's running or not:
> 
> void flush_tlb_all(void)
> {
>         on_each_cpu(do_flush_tlb_all, NULL, 1);
> }
> 
> This results in each vcpu being scheduled to receive the event channel
> at least. With large number of VCPUs the overhead is significant.
> 
> It seems the best solution would be to restore xen_tlb_flush_all().
> 
> Thoughts?

Like this I presume (not compile tested):


diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 6226c99..dd91c3c 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1288,6 +1288,23 @@ unsigned long xen_read_cr2_direct(void)
 	return this_cpu_read(xen_vcpu_info.arch.cr2);
 }
 
+void xen_flush_tbl_all(void)
+{
+	struct mmuext_op *op;
+	struct multicall_space mcs;
+
+	preempt_disable();
+
+	mcs = xen_mc_entry(sizeof(*op));
+
+	op = mcs.args;
+	op->cmd = MMUEXT_TLB_FLUSH_ALL;
+	MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
+
+	xen_mc_issue(PARAVIRT_LAZY_MMU);
+
+	preempt_enable();
+}
 static void xen_flush_tlb(void)
 {
 	struct mmuext_op *op;
@@ -2518,7 +2535,7 @@ int xen_remap_domain_mfn_range(struct vm_area_struct *vma,
 	err = 0;
 out:
 
-	flush_tlb_all();
+	xen_flush_tbl_all();
 
 	return err;
 }
> 
> thanks
> Mukesh