Re: [PATCH -mm v7 5/6] proc: add kpageidle file

Andres Lagar-Cavilla
2015-07-14T20:27:26Z

On Tue, Jul 14, 2015 at 4:05 AM, Vladimir Davydov
<vdavydov@parallels.com> wrote:
> On Mon, Jul 13, 2015 at 12:02:57PM -0700, Andres Lagar-Cavilla wrote:
>> On Sat, Jul 11, 2015 at 7:48 AM, Vladimir Davydov
>> <vdavydov@parallels.com> wrote:
> [...]
>> > +static struct page *kpageidle_get_page(unsigned long pfn)
>> > +{
>> > + struct page *page;
>> > + struct zone *zone;
>> > +
>> > + if (!pfn_valid(pfn))
>> > + return NULL;
>> > +
>> > + page = pfn_to_page(pfn);
>> > + if (!page || PageTail(page) || !PageLRU(page) ||
>> > + !get_page_unless_zero(page))
>>
>> get_page_unless_zero does not succeed for Tail pages.
>
> True. So we don't seem to need the PageTail checks here at all, because
> if kpageidle_get_page succeeds, the page must be a head, so that we
> won't dive into expensive rmap_walk for tail pages. Will remove it then.
>
>>
>> > + return NULL;
>> > +
>> > + if (unlikely(PageTail(page))) {
>> > + put_page(page);
>> > + return NULL;
>> > + }
>> > +
>> > + zone = page_zone(page);
>> > + spin_lock_irq(&zone->lru_lock);
>> > + if (unlikely(!PageLRU(page))) {
>> > + put_page(page);
>> > + page = NULL;
>> > + }
>> > + spin_unlock_irq(&zone->lru_lock);
>> > + return page;
>> > +}
>> > +
>> > +static int kpageidle_clear_pte_refs_one(struct page *page,
>> > + struct vm_area_struct *vma,
>> > + unsigned long addr, void *arg)
>> > +{
>> > + struct mm_struct *mm = vma->vm_mm;
>> > + spinlock_t *ptl;
>> > + pmd_t *pmd;
>> > + pte_t *pte;
>> > + bool referenced = false;
>> > +
>> > + if (unlikely(PageTransHuge(page))) {
>>
>> VM_BUG_ON(!PageHead)?
>
> Don't think it's necessary, because PageTransHuge already does this sort
> of check:
>
> : static inline int PageTransHuge(struct page *page)
> : {
> : VM_BUG_ON_PAGE(PageTail(page), page);
> : return PageHead(page);
> : }
>
>>
>> > + pmd = page_check_address_pmd(page, mm, addr,
>> > + PAGE_CHECK_ADDRESS_PMD_FLAG, &ptl);
>> > + if (pmd) {
>> > + referenced = pmdp_test_and_clear_young(vma, addr, pmd);
>>
>> For any workload using MMU notifiers, this will lose significant
>> information by not querying the secondary PTE. The most
>> straightforward case is KVM. Once mappings are setup, all access
>> activity is recorded through shadow PTEs. This interface will say
>> "idle" even though the VM is blasting memory.
>
> Hmm, interesting. It seems we have to introduce
> mmu_notifier_ops.clear_young then, which, in contrast to
> clear_flush_young, won't flush TLB. Looking back at your comment to v6,
> now I see that you already mentioned it, but I missed your point :-(
> OK, will do it in the next iteration.

There's clearly value in fixing things for KVM, but I don't have
knowledge of the other MMU notifiers. I like clear_young, maybe other
mmu notifiers will turn this into a no-op().

mmmmhh. What about TLB flushing in the mmu notifier? I guess that can
be internal to each implementation.

Andres
>
> Thanks a lot for the review!
>
> Vladimir

--
Andres Lagar-Cavilla | Google Kernel Team | andreslc@google.com