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

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Date:	Tue, 14 Jul 2015 13:27:22 -0700
From:	Andres Lagar-Cavilla <andreslc@...gle.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jonathan Corbet <corbet@....net>, linux-api@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm v7 5/6] proc: add kpageidle file

On Tue, Jul 14, 2015 at 4:05 AM, Vladimir Davydov
<vdavydov@...allels.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@...allels.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@...gle.com
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