Re: [PATCH v6 6/9] mm: multigenerational lru: aging

[Yu Zhao]

On Mon, Jan 10, 2022 at 03:37:28PM +0100, Michal Hocko wrote:
> On Sun 09-01-22 20:58:02, Yu Zhao wrote:
> > On Fri, Jan 07, 2022 at 10:00:31AM +0100, Michal Hocko wrote:
> > > On Fri 07-01-22 09:55:09, Michal Hocko wrote:
> > > [...]
> > > > > In this case, lru_gen_mm_walk is small (160 bytes); it's per direct
> > > > > reclaimer; and direct reclaimers rarely come here, i.e., only when
> > > > > kswapd can't keep up in terms of the aging, which is similar to the
> > > > > condition where the inactive list is empty for the active/inactive
> > > > > lru.
> > > > 
> > > > Well, this is not a strong argument to be honest. Kswapd being stuck
> > > > and the majority of the reclaim being done in the direct reclaim
> > > > context is a situation I have seen many many times.
> > > 
> > > Also do not forget that memcg reclaim is effectivelly only direct
> > > reclaim. Not that the memcg reclaim indicates a global memory shortage
> > > but it can add up and race with the global reclaim as well.
> > 
> > I don't dispute any of the above, and I probably don't like this code
> > more than you do.
> > 
> > But let's not forget the purposes of PF_MEMALLOC, besides preventing
> > recursive reclaims, include letting reclaim dip into reserves so that
> > it can make more free memory. So I think it's acceptable if the
> > following conditions are met:
> > 1. The allocation size is small.
> > 2. The number of allocations is bounded.
> > 3. Its failure doesn't stall reclaim.
> > And it'd be nice if
> > 4. The allocation happens rarely, e.g., slow path only.
> 
> I would add 
>   0. The allocation should be done only if absolutely _necessary_.
> 
> Please keep in mind that whatever you allocate from that context will be
> consuming a very precious memory reserves which are shared with other
> components of the system. Even worse these can go all the way to
> depleting memory completely where other things can fall apart.

I agree but I also see a distinction:
   1,2,3 are objective;
   0,4 are subjective.

For some users, page reclaim itself could be not absolutely necessary
because they are okay with OOM kills. But for others, the situation
could be reversed.

> > The code in question meets all of them.
> > 
> > 1. This allocation is 160 bytes.
> > 2. It's bounded by the number of page table walkers which, in the
> >    worst, is same as the number of mm_struct's.
> > 3. Most importantly, its failure doesn't stall the aging. The aging
> >    will fallback to the rmap-based function lru_gen_look_around().
> >    But this function only gathers the accessed bit from at most 64
> >    PTEs, meaning it's less efficient (retains ~80% performance gains).
> > 4. This allocation is rare, i.e., only when the aging is required,
> >    which is similar to the low inactive case for the active/inactive
> >    lru.
> 
> I think this fallback behavior deserves much more detailed explanation
> in changelogs.

Will do.

> > The bottom line is I can try various optimizations, e.g., preallocate
> > a few buffers for a limited number of page walkers and if this number
> > has been reached, fallback to the rmap-based function. But I have yet
> > to see evidence that calls for additional complexity.
> 
> I would disagree here. This is not an optimization. You should be
> avoiding allocations from the memory reclaim because any allocation just
> add a runtime behavior complexity and potential corner cases.

Would __GFP_NOMEMALLOC address your concern? It prevents allocations
from accessing the reserves even under PF_MEMALLOC.