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    SubjectRe: Re: [PATCH v6 02/14] mm/damon: Implement region based sampling
    Date
    On Tue, 10 Mar 2020 15:55:10 +0000 Jonathan Cameron <Jonathan.Cameron@Huawei.com> wrote:

    > On Tue, 10 Mar 2020 12:52:33 +0100
    > SeongJae Park <sjpark@amazon.com> wrote:
    >
    > > Added replies to your every comment in line below. I agree to your whole
    > > opinions, will apply those in next spin! :)
    > >
    >
    > One additional question inline that came to mind. Using a single statistic
    > to monitor huge page and normal page hits is going to give us problems
    > I think.

    Ah, you're right!!! This is indeed a critical bug!

    >
    > Perhaps I'm missing something?
    >
    > > > > +/*
    > > > > + * Check whether the given region has accessed since the last check
    > > >
    > > > Should also make clear that this sets us up for the next access check at
    > > > a different memory address it the region.
    > > >
    > > > Given the lack of connection between activities perhaps just split this into
    > > > two functions that are always called next to each other.
    > >
    > > Will make the description more clearer as suggested.
    > >
    > > Also, I found that I'm not clearing *pte and *pmd before going 'mkold', thanks
    > > to this comment. Will fix it, either.
    > >
    > > >
    > > > > + *
    > > > > + * mm 'mm_struct' for the given virtual address space
    > > > > + * r the region to be checked
    > > > > + */
    > > > > +static void kdamond_check_access(struct damon_ctx *ctx,
    > > > > + struct mm_struct *mm, struct damon_region *r)
    > > > > +{
    > > > > + pte_t *pte = NULL;
    > > > > + pmd_t *pmd = NULL;
    > > > > + spinlock_t *ptl;
    > > > > +
    > > > > + if (follow_pte_pmd(mm, r->sampling_addr, NULL, &pte, &pmd, &ptl))
    > > > > + goto mkold;
    > > > > +
    > > > > + /* Read the page table access bit of the page */
    > > > > + if (pte && pte_young(*pte))
    > > > > + r->nr_accesses++;
    > > > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > > >
    > > > Is it worth having this protection? Seems likely to have only a very small
    > > > influence on performance and makes it a little harder to reason about the code.
    > >
    > > It was necessary for addressing 'implicit declaration' problem of 'pmd_young()'
    > > and 'pmd_mkold()' for build of DAMON on several architectures including User
    > > Mode Linux.
    > >
    > > Will modularize the code for better readability.
    > >
    > > >
    > > > > + else if (pmd && pmd_young(*pmd))
    > > > > + r->nr_accesses++;
    >
    > So we increment a region count by one if we have an access in a huge page, or
    > in a normal page.
    >
    > If we get a region that has a mixture of the two, this seems likely to give a
    > bad approximation.
    >
    > Assume the region is accessed 'evenly' but each " 4k page" is only hit 10% of the time
    > (where a hit is in one check period)
    >
    > If our address in a page, then we'll hit 10% of the time, but if it is in a 2M
    > huge page then we'll hit a much higher percentage of the time.
    > 1 - (0.9^512) ~= 1
    >
    > Should we look to somehow account for this?

    Yes, this is really critical bug and we should fix this! Thank you so much for
    finding this!

    >
    > > > > +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
    > > > > +
    > > > > + spin_unlock(ptl);
    > > > > +
    > > > > +mkold:
    > > > > + /* mkold next target */
    > > > > + r->sampling_addr = damon_rand(ctx, r->vm_start, r->vm_end);
    > > > > +
    > > > > + if (follow_pte_pmd(mm, r->sampling_addr, NULL, &pte, &pmd, &ptl))
    > > > > + return;
    > > > > +
    > > > > + if (pte) {
    > > > > + if (pte_young(*pte)) {
    > > > > + clear_page_idle(pte_page(*pte));
    > > > > + set_page_young(pte_page(*pte));
    > > > > + }
    > > > > + *pte = pte_mkold(*pte);
    > > > > + }
    > > > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > > > > + else if (pmd) {
    > > > > + if (pmd_young(*pmd)) {
    > > > > + clear_page_idle(pmd_page(*pmd));
    > > > > + set_page_young(pmd_page(*pmd));
    > > > > + }
    > > > > + *pmd = pmd_mkold(*pmd);
    > > > > + }

    This is also very problematic if several regions are backed by a single huge
    page, as only one region in the huge page will be checked as accessed.

    Will address these problems in next spin!


    Thanks,
    SeongJae Park

    > > > > +#endif
    > > > > +
    > > > > + spin_unlock(ptl);
    > > > > +}
    > > > > +
    >
    >

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