Re: [patch 07/10] KVM: introduce kvm->srcu and convert kvm_set_memory_region to SRCU update

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Sep 24, 2009 at 11:06:51AM -0300, Marcelo Tosatti wrote:
> On Mon, Sep 21, 2009 at 08:37:18PM -0300, Marcelo Tosatti wrote:
> > Use two steps for memslot deletion: mark the slot invalid (which stops 
> > instantiation of new shadow pages for that slot, but allows destruction),
> > then instantiate the new empty slot.
> > 
> > Also simplifies kvm_handle_hva locking.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > 
> 
> <snip>
> 
> > -	if (!npages)
> > +	if (!npages) {
> > +		slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
> > +		if (!slots)
> > +			goto out_free;
> > +		memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
> > +		if (mem->slot >= slots->nmemslots)
> > +			slots->nmemslots = mem->slot + 1;
> > +		slots->memslots[mem->slot].flags |= KVM_MEMSLOT_INVALID;
> > +
> > +		old_memslots = kvm->memslots;
> > +		rcu_assign_pointer(kvm->memslots, slots);
> > +		synchronize_srcu(&kvm->srcu);
> > +		/* From this point no new shadow pages pointing to a deleted
> > +		 * memslot will be created.
> > +         	 *
> > +         	 * validation of sp->gfn happens in:
> > +         	 * 	- gfn_to_hva (kvm_read_guest, gfn_to_pfn)
> > +         	 * 	- kvm_is_visible_gfn (mmu_check_roots)
> > +         	 */
> >  		kvm_arch_flush_shadow(kvm);
> > +		kfree(old_memslots);
> > +	}
> >  
> >  	r = kvm_arch_prepare_memory_region(kvm, &new, old, user_alloc);
> >  	if (r)
> >  		goto out_free;
> >  
> > -	spin_lock(&kvm->mmu_lock);
> > -	if (mem->slot >= kvm->memslots->nmemslots)
> > -		kvm->memslots->nmemslots = mem->slot + 1;
> > +#ifdef CONFIG_DMAR
> > +	/* map the pages in iommu page table */
> > +	if (npages)
> > +		r = kvm_iommu_map_pages(kvm, &new);
> > +		if (r)
> > +			goto out_free;
> > +#endif
> >  
> > -	*memslot = new;
> > -	spin_unlock(&kvm->mmu_lock);
> > +	slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
> > +	if (!slots)
> > +		goto out_free;
> > +	memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
> > +	if (mem->slot >= slots->nmemslots)
> > +		slots->nmemslots = mem->slot + 1;
> > +
> > +	/* actual memory is freed via old in kvm_free_physmem_slot below */
> > +	if (!npages) {
> > +		new.rmap = NULL;
> > +		new.dirty_bitmap = NULL;
> > +		for (i = 0; i < KVM_NR_PAGE_SIZES - 1; ++i)
> > +			new.lpage_info[i] = NULL;
> > +	}
> > +
> > +	slots->memslots[mem->slot] = new;
> > +	old_memslots = kvm->memslots;
> > +	rcu_assign_pointer(kvm->memslots, slots);
> > +	synchronize_srcu(&kvm->srcu);
> >  
> >  	kvm_arch_commit_memory_region(kvm, mem, old, user_alloc);
> 
> Paul,
> 
> There is a scenario where this path, which updates KVM memory slots, is
> called relatively often.
> 
> Each synchronize_srcu() call takes about 10ms (avg 3ms per
> synchronize_sched call), so this is hurting us.
> 
> Is this expected? Is there any possibility for synchronize_srcu()
> optimization?
> 
> There are other sides we can work on, such as reducing the memory slot 
> updates, but i'm wondering what can be done regarding SRCU itself.

This is expected behavior, but there is a possible fix currently
in mainline (Linus's git tree).  The idea would be to create a
synchronize_srcu_expedited(), which starts with synchronize_srcu(), and
replaces the synchronize_sched() calls with synchronize_sched_expedited().

This could potentially reduce the overall synchronize_srcu() latency
to well under a microsecond.  The price to be paid is that each instance
of synchronize_sched_expedited() IPIs all the online CPUs, and awakens
the migration thread on each.

Would this approach likely work for you?

							Thanx, Paul