Re: Re: [PATCH v6 02/14] mm/damon: Implement region based sampling

[SeongJae Park]

On Tue, 10 Mar 2020 15:55:10 +0000 Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:

> On Tue, 10 Mar 2020 12:52:33 +0100
> SeongJae Park <sjpark@xxxxxxxxxx> 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);
> > > > +}
> > > > +
> 
>