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

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Date:	Tue, 14 Jul 2015 14:05:28 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Andres Lagar-Cavilla <andreslc@...gle.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 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.

Thanks a lot for the review!

Vladimir
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