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

[Yu Zhao <yuzhao@google.com>]

On Thu, Jun 16, 2022 at 5:29 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Thu, Jun 16, 2022 at 4:33 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > On Fri, Jun 17, 2022 at 9:56 AM Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > On Wed, Jun 8, 2022 at 4:46 PM Barry Song <21cnbao@gmail.com> wrote:
> > > >
> > > > On Thu, Jun 9, 2022 at 3:52 AM Linus Torvalds
> > > > <torvalds@linux-foundation.org> wrote:
> > > > >
> > > > > On Tue, Jun 7, 2022 at 5:43 PM Barry Song <21cnbao@gmail.com> 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@vger.kernel.org/msg1793830.html
> > > > but arm64 still sent a nosync tlbi and depent on a deferred to dsb :
> > > > https://www.mail-archive.com/linux-kernel@vger.kernel.org/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@mail.gmail.com/
> > >
> > > 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@BYAPR12MB2712.namprd12.prod.outlook.com/
> 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")