Re: [PATCH] KVM: stats: Add VM dirty_pages stats for the number of dirty pages

From: Paolo Bonzini <pbonzini@redhat.com>
To: Oliver Upton <oupton@google.com>, Jing Zhang <jingzhangos@google.com>
Cc: KVM <kvm@vger.kernel.org>, KVMPPC <kvm-ppc@vger.kernel.org>,
	Peter Shier <pshier@google.com>,
	David Matlack <dmatlack@google.com>,
	Ben Gardon <bgardon@google.com>,
	Peter Feiner <pfeiner@google.com>
Subject: Re: [PATCH] KVM: stats: Add VM dirty_pages stats for the number of dirty pages
Date: Wed, 11 Aug 2021 12:20:23 +0200	[thread overview]
Message-ID: <5183cb08-f739-e6a6-f645-3ccbe92d04d8@redhat.com> (raw)
In-Reply-To: <CAOQ_QshcSQWhEUt9d7OV58V=3WrL34xfpFYS-wp6H4rzy_r_4w@mail.gmail.com>

On 11/08/21 00:56, Oliver Upton wrote:
> What if the statistic was 'dirtied_pages', which records the number of
> pages dirtied in the lifetime of a VM? Userspace could just record the
> value each time it blows away the dirty bitmaps and subtract that
> value next time it reads the stat. It would circumvent the need to
> walk the entire dirty bitmap to keep the statistic sane.

Yeah, that'd be much better also because the "number of dirty pages" 
statistic is not well defined in init-all-dirty mode.

Making it a vCPU stat works in fact, because mark_page_dirty_in_slot is 
only called with kvm_get_running_vcpu() != NULL; see 
kvm_dirty_ring_get() in virt/kvm/dirty_ring.c.

>>
>> +               if (kvm->dirty_ring_size) {
>>                         kvm_dirty_ring_push(kvm_dirty_ring_get(kvm),
>>                                             slot, rel_gfn);
>> -               else
>> +               } else {
>>                         set_bit_le(rel_gfn, memslot->dirty_bitmap);
>> +                       ++kvm->stat.generic.dirty_pages;
>> +               }
> 
> Aren't pages being pushed out to the dirty ring just as dirty? 
> 

Yes, they are.

Paolo