Re: [rfc] mm, hugetlb: allow hugepage allocations to excessively reclaim

[Michal Hocko <mhocko@kernel.org>]

On Wed 02-10-19 16:03:03, David Rientjes wrote:
> Hugetlb allocations use __GFP_RETRY_MAYFAIL to aggressively attempt to get 
> hugepages that the user needs.  Commit b39d0ee2632d ("mm, page_alloc: 
> avoid expensive reclaim when compaction may not succeed") intends to 
> improve allocator behind for thp allocations to prevent excessive amounts 
> of reclaim especially when constrained to a single node.
> 
> Since hugetlb allocations have explicitly preferred to loop and do reclaim 
> and compaction, exempt them from this new behavior at least for the time 
> being.  It is not shown that hugetlb allocation success rate has been 
> impacted by commit b39d0ee2632d but hugetlb allocations are admittedly 
> beyond the scope of what the patch is intended to address (thp 
> allocations).

It has become pretty clear that b39d0ee2632d has regressed hugetlb
allocation success rate for any non-trivial case (complately free
memory) http://lkml.kernel.org/r/20191001054343.GA15624@dhcp22.suse.cz.
And this really is not just about hugetlb requests, really. They are
likely the most obvious example but __GFP_RETRY_MAYFAIL in general is
supposed to try as hard as feasible to success the allocation. The
decision to bail out is done at a different spot and b39d0ee2632d is
effectively bypassing that logic.

Now to the patch itself. I didn't get to test it on my testing
workload but hey steps are clearly documented and easily to set up and
reproduce. I am at a training for today and unlikely to get to test by
the end of the week infortunatelly. Anyway the patch should be fixing
the problem because it explicitly opts out for __GFP_RETRY_MAYFAIL.

I am pretty sure we will need more follow ups because the bail out logic
is simply behaving quite randomly as my measurements show (I would really
appreciate a feedback there). We need a more systematic solution because
the current logic has been rushed through without a proper analysis and
without any actual workloads to verify the effect.

> Cc: Mike Kravetz <mike.kravetz@oracle.com>
Fixes: b39d0ee2632d ("mm, page_alloc: avoid expensive reclaim when compaction may not succeed")

> Signed-off-by: David Rientjes <rientjes@google.com>

I am willing to give my ack by considering that this is a clear
regression and this is probably the simplest fix but the changelog
should be explicit about the effect (feel free to borrow my numbers and
explanation in this thread).

> ---
>  Mike, you eluded that you may want to opt hugetlbfs out of this for the
>  time being in https://marc.info/?l=linux-kernel&m=156771690024533 --
>  not sure if you want to allow this excessive amount of reclaim for 
>  hugetlb allocations or not given the swap storms Andrea has shown is
>  possible (and nr_hugepages_mempolicy does exist), but hugetlbfs was not
>  part of the problem we are trying to address here so no objection to
>  opting it out.  
> 
>  You might want to consider how expensive hugetlb allocations can become
>  and disruptive to the system if it does not yield additional hugepages,
>  but that can be done at any time later as a general improvement rather
>  than part of a series aimed at thp.
> 
>  mm/page_alloc.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4467,12 +4467,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  		if (page)
>  			goto got_pg;
>  
> -		 if (order >= pageblock_order && (gfp_mask & __GFP_IO)) {
> +		 if (order >= pageblock_order && (gfp_mask & __GFP_IO) &&
> +		     !(gfp_mask & __GFP_RETRY_MAYFAIL)) {
>  			/*
>  			 * If allocating entire pageblock(s) and compaction
>  			 * failed because all zones are below low watermarks
>  			 * or is prohibited because it recently failed at this
> -			 * order, fail immediately.
> +			 * order, fail immediately unless the allocator has
> +			 * requested compaction and reclaim retry.
>  			 *
>  			 * Reclaim is
>  			 *  - potentially very expensive because zones are far

-- 
Michal Hocko
SUSE Labs