Re: [PATCH 2/2] Revert "mm, thp: restore node-local hugepage allocations"

Mel Gorman
2019-05-09T08:38:16Z

On Fri, May 03, 2019 at 06:31:46PM -0400, Andrea Arcangeli wrote:
> This reverts commit 2f0799a0ffc033bf3cc82d5032acc3ec633464c2.
>
> commit 2f0799a0ffc033bf3cc82d5032acc3ec633464c2 was rightfully applied
> to avoid the risk of a severe regression that was reported by the
> kernel test robot at the end of the merge window. Now we understood
> the regression was a false positive and was caused by a significant
> increase in fairness during a swap trashing benchmark. So it's safe to
> re-apply the fix and continue improving the code from there. The
> benchmark that reported the regression is very useful, but it provides
> a meaningful result only when there is no significant alteration in
> fairness during the workload. The removal of __GFP_THISNODE increased
> fairness.
>
> __GFP_THISNODE cannot be used in the generic page faults path for new
> memory allocations under the MPOL_DEFAULT mempolicy, or the allocation
> behavior significantly deviates from what the MPOL_DEFAULT semantics
> are supposed to be for THP and 4k allocations alike.
>
> Setting THP defrag to "always" or using MADV_HUGEPAGE (with THP defrag
> set to "madvise") has never meant to provide an implicit MPOL_BIND on
> the "current" node the task is running on, causing swap storms and
> providing a much more aggressive behavior than even zone_reclaim_node
> = 3.
>
> Any workload who could have benefited from __GFP_THISNODE has now to
> enable zone_reclaim_mode=1||2||3. __GFP_THISNODE implicitly provided
> the zone_reclaim_mode behavior, but it only did so if THP was enabled:
> if THP was disabled, there would have been no chance to get any 4k
> page from the current node if the current node was full of pagecache,
> which further shows how this __GFP_THISNODE was misplaced in
> MADV_HUGEPAGE. MADV_HUGEPAGE has never been intended to provide any
> zone_reclaim_mode semantics, in fact the two are orthogonal,
> zone_reclaim_mode = 1|2|3 must work exactly the same with
> MADV_HUGEPAGE set or not.
>
> The performance characteristic of memory depends on the hardware
> details. The numbers below are obtained on Naples/EPYC architecture
> and the N/A projection extends them to show what we should aim for in
> the future as a good THP NUMA locality default. The benchmark used
> exercises random memory seeks (note: the cost of the page faults is
> not part of the measurement).
>
> D0 THP | D0 4k | D1 THP | D1 4k | D2 THP | D2 4k | D3 THP | D3 4k | ...
> 0% | +43% | +45% | +106% | +131% | +224% | N/A | N/A
>
> D0 means distance zero (i.e. local memory), D1 means distance
> one (i.e. intra socket memory), D2 means distance two (i.e. inter
> socket memory), etc...
>
> For the guest physical memory allocated by qemu and for guest mode kernel
> the performance characteristic of RAM is more complex and an ideal
> default could be:
>
> D0 THP | D1 THP | D0 4k | D2 THP | D1 4k | D3 THP | D2 4k | D3 4k | ...
> 0% | +58% | +101% | N/A | +222% | N/A | N/A | N/A
>
> NOTE: the N/A are projections and haven't been measured yet, the
> measurement in this case is done on a 1950x with only two NUMA nodes.
> The THP case here means THP was used both in the host and in the
> guest.
>
> After applying this commit the THP NUMA locality order that we'll get
> out of MADV_HUGEPAGE is this:
>
> D0 THP | D1 THP | D2 THP | D3 THP | ... | D0 4k | D1 4k | D2 4k | D3 4k | ...
>
> Before this commit it was:
>
> D0 THP | D0 4k | D1 4k | D2 4k | D3 4k | ...
>
> Even if we ignore the breakage of large workloads that can't fit in a
> single node that the __GFP_THISNODE implicit "current node" mbind
> caused, the THP NUMA locality order provided by __GFP_THISNODE was
> still not the one we shall aim for in the long term (i.e. the first
> one at the top).
>
> After this commit is applied, we can introduce a new allocator multi
> order API and to replace those two alloc_pages_vmas calls in the page
> fault path, with a single multi order call:
>
> unsigned int order = (1 << HPAGE_PMD_ORDER) | (1 << 0);
> page = alloc_pages_multi_order(..., &order);
> if (!page)
> goto out;
> if (!(order & (1 << 0))) {
> VM_WARN_ON(order != 1 << HPAGE_PMD_ORDER);
> /* THP fault */
> } else {
> VM_WARN_ON(order != 1 << 0);
> /* 4k fallback */
> }
>
> The page allocator logic has to be altered so that when it fails on
> any zone with order 9, it has to try again with a order 0 before
> falling back to the next zone in the zonelist.
>
> After that we need to do more measurements and evaluate if adding an
> opt-in feature for guest mode is worth it, to swap "DN 4k | DN+1 THP"
> with "DN+1 THP | DN 4k" at every NUMA distance crossing.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

Acked-by: Mel Gorman <mgorman@suse.de>

--
Mel Gorman
SUSE Labs