Re: [PATCH v11 07/14] mm: multi-gen LRU: exploit locality in rmap

From: Yu Zhao
Date: Sun Jun 19 2022 - 16:37:12 EST

On Thu, Jun 16, 2022 at 9:17 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> On Thu, Jun 16, 2022 at 9:03 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> >
> > On Thu, Jun 16, 2022 at 8:01 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > >
> > > On Fri, Jun 17, 2022 at 1:43 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Jun 16, 2022 at 5:29 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Thu, Jun 16, 2022 at 4:33 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > > > > >
> > > > > > On Fri, Jun 17, 2022 at 9:56 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Wed, Jun 8, 2022 at 4:46 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > On Thu, Jun 9, 2022 at 3:52 AM Linus Torvalds
> > > > > > > > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > > > > > > >
> > > > > > > > > On Tue, Jun 7, 2022 at 5:43 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> > > > > > > > > >
> > > > > > > > > > Given we used to have a flush for clear pte young in LRU, right now we are
> > > > > > > > > > moving to nop in almost all cases for the flush unless the address becomes
> > > > > > > > > > young exactly after look_around and before ptep_clear_flush_young_notify.
> > > > > > > > > > It means we are actually dropping flush. So the question is, were we
> > > > > > > > > > overcautious? we actually don't need the flush at all even without mglru?
> > > > > > > > >
> > > > > > > > > We stopped flushing the TLB on A bit clears on x86 back in 2014.
> > > > > > > > >
> > > > > > > > > See commit b13b1d2d8692 ("x86/mm: In the PTE swapout page reclaim case
> > > > > > > > > clear the accessed bit instead of flushing the TLB").
> > > > > > > >
> > > > > > > > This is true for x86, RISC-V, powerpc and S390. but it is not true for
> > > > > > > > most platforms.
> > > > > > > >
> > > > > > > > There was an attempt to do the same thing in arm64:
> > > > > > > > https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1793830.html
> > > > > > > > but arm64 still sent a nosync tlbi and depent on a deferred to dsb :
> > > > > > > > https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1794484.html
> > > > > > >
> > > > > > > Barry, you've already answered your own question.
> > > > > > >
> > > > > > > Without commit 07509e10dcc7 arm64: pgtable: Fix pte_accessible():
> > > > > > > #define pte_accessible(mm, pte) \
> > > > > > > - (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte))
> > > > > > > + (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))
> > > > > > >
> > > > > > > You missed all TLB flushes for PTEs that have gone through
> > > > > > > ptep_test_and_clear_young() on the reclaim path. But most of the time,
> > > > > > > you got away with it, only occasional app crashes:
> > > > > > > https://lore.kernel.org/r/CAGsJ_4w6JjuG4rn2P=d974wBOUtXUUnaZKnx+-G6a8_mSROa+Q@xxxxxxxxxxxxxx/
> > > > > > >
> > > > > > > Why?
> > > > > >
> > > > > > Yes. On the arm64 platform, ptep_test_and_clear_young() without flush
> > > > > > can cause random
> > > > > > App to crash.
> > > > > > ptep_test_and_clear_young() + flush won't have this kind of crashes though.
> > > > > > But after applying commit 07509e10dcc7 arm64: pgtable: Fix
> > > > > > pte_accessible(), on arm64,
> > > > > > ptep_test_and_clear_young() without flush won't cause App to crash.
> > > > > >
> > > > > > ptep_test_and_clear_young(), with flush, without commit 07509e10dcc7: OK
> > > > > > ptep_test_and_clear_young(), without flush, with commit 07509e10dcc7: OK
> > > > > > ptep_test_and_clear_young(), without flush, without commit 07509e10dcc7: CRASH
> > > > >
> > > > > I agree -- my question was rhetorical :)
> > > > >
> > > > > I was trying to imply this logic:
> > > > > 1. We cleared the A-bit in PTEs with ptep_test_and_clear_young()
> > > > > 2. We missed TLB flush for those PTEs on the reclaim path, i.e., case
> > > > > 3 (case 1 & 2 guarantee flushes)
> > > > > 3. We saw crashes, but only occasionally
> > > > >
> > > > > Assuming TLB cached those PTEs, we would have seen the crashes more
> > > > > often, which contradicts our observation. So the conclusion is TLB
> > > > > didn't cache them most of the time, meaning flushing TLB just for the
> > > > > sake of the A-bit isn't necessary.
> > > > >
> > > > > > do you think it is safe to totally remove the flush code even for
> > > > > > the original
> > > > > > LRU?
> > > > >
> > > > > Affirmative, based on not only my words, but 3rd parties':
> > > > > 1. Your (indirect) observation
> > > > > 2. Alexander's benchmark:
> > > > > https://lore.kernel.org/r/BYAPR12MB271295B398729E07F31082A7CFAA0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > > > > 3. The fundamental hardware limitation in terms of the TLB scalability
> > > > > (Fig. 1): https://www.usenix.org/legacy/events/osdi02/tech/full_papers/navarro/navarro.pdf
> > > >
> > > > 4. Intel's commit b13b1d2d8692 ("x86/mm: In the PTE swapout page
> > > > reclaim case clear the accessed bit instead of flushing the TLB")
> > >
> > > Hi Yu,
> > > I am going to send a RFC based on the above discussion.
> > >
> > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > index 5bcb334cd6f2..7ce6f0b6c330 100644
> > > --- a/mm/rmap.c
> > > +++ b/mm/rmap.c
> > > @@ -830,7 +830,7 @@ static bool folio_referenced_one(struct folio *folio,
> > > }
> > >
> > > if (pvmw.pte) {
> > > - if (ptep_clear_flush_young_notify(vma, address,
> > > + if (ptep_clear_young_notify(vma, address,
> > > pvmw.pte)) {
> > > /*
> > > * Don't treat a reference through
> >
> > Thanks!
> >
> > This might make a difference on my 64 core Altra -- I'll test after
> > you post the RFC.
>
> Also, IIRC, it made no difference on POWER9 because POWER9
> flushes TBL regardless which variant is used.
^^^^^^^ doesn't flush

I just verified this on POWER9. So on top of exhibit 1-4, we got:
5. 3cb1aa7aa3940 ("powerpc/64s: Implement ptep_clear_flush_young
that does not flush TLBs")