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Date: Thu, 24 Oct 2019 11:59:43 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
cc: Michal Hocko <mhocko@...nel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Mel Gorman <mgorman@...e.de>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>
Subject: Re: [patch for-5.3 0/4] revert immediate fallback to remote
hugepages
On Wed, 23 Oct 2019, Vlastimil Babka wrote:
> From 8bd960e4e8e7e99fe13baf0d00b61910b3ae8d23 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@...e.cz>
> Date: Tue, 1 Oct 2019 14:20:58 +0200
> Subject: [PATCH] mm, thp: tweak reclaim/compaction effort of local-only and
> all-node allocations
>
> THP page faults now attempt a __GFP_THISNODE allocation first, which should
> only compact existing free memory, followed by another attempt that can
> allocate from any node using reclaim/compaction effort specified by global
> defrag setting and madvise.
>
> This patch makes the following changes to the scheme:
>
> - before the patch, the first allocation relies on a check for pageblock order
> and __GFP_IO to prevent excessive reclaim. This however affects also the
> second attempt, which is not limited to single node. Instead of that, reuse
> the existing check for costly order __GFP_NORETRY allocations, and make sure
> the first THP attempt uses __GFP_NORETRY. As a side-effect, all costly order
> __GFP_NORETRY allocations will bail out if compaction needs reclaim, while
> previously they only bailed out when compaction was deferred due to previous
> failures. This should be still acceptable within the __GFP_NORETRY semantics.
>
> - before the patch, the second allocation attempt (on all nodes) was passing
> __GFP_NORETRY. This is redundant as the check for pageblock order (discussed
> above) was stronger. It's also contrary to madvise(MADV_HUGEPAGE) which means
> some effort to allocate THP is requested. After this patch, the second
> attempt doesn't pass __GFP_THISNODE nor __GFP_NORETRY.
>
> To sum up, THP page faults now try the following attempt:
>
> 1. local node only THP allocation with no reclaim, just compaction.
> 2. THP allocation from any node with effort determined by global defrag setting
> and VMA madvise
> 3. fallback to base pages on any node
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> mm/mempolicy.c | 16 +++++++++-------
> mm/page_alloc.c | 24 +++++-------------------
> 2 files changed, 14 insertions(+), 26 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 4ae967bcf954..2c48146f3ee2 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2129,18 +2129,20 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
> nmask = policy_nodemask(gfp, pol);
> if (!nmask || node_isset(hpage_node, *nmask)) {
> mpol_cond_put(pol);
> + /*
> + * First, try to allocate THP only on local node, but
> + * don't reclaim unnecessarily, just compact.
> + */
> page = __alloc_pages_node(hpage_node,
> - gfp | __GFP_THISNODE, order);
> + gfp | __GFP_THISNODE | __GFP_NORETRY, order);
>
> /*
> - * If hugepage allocations are configured to always
> - * synchronous compact or the vma has been madvised
> - * to prefer hugepage backing, retry allowing remote
> - * memory as well.
> + * If that fails, allow both compaction and reclaim,
> + * but on all nodes.
> */
> - if (!page && (gfp & __GFP_DIRECT_RECLAIM))
> + if (!page)
> page = __alloc_pages_node(hpage_node,
> - gfp | __GFP_NORETRY, order);
> + gfp, order);
>
> goto out;
> }
Hi Vlastimil,
For the default case where thp enabled is not set to "always" and the VMA
is not madvised for MADV_HUGEPAGE, how does this prefer to return node
local pages rather than remote hugepages? The idea is to optimize for
access latency when the vma has not been explicitly madvised.
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ecc3dbad606b..36d7d852f7b1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4473,8 +4473,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> if (page)
> goto got_pg;
>
> - if (order >= pageblock_order && (gfp_mask & __GFP_IO) &&
> - !(gfp_mask & __GFP_RETRY_MAYFAIL)) {
> + /*
> + * Checks for costly allocations with __GFP_NORETRY, which
> + * includes some THP page fault allocations
> + */
> + if (costly_order && (gfp_mask & __GFP_NORETRY)) {
> /*
> * If allocating entire pageblock(s) and compaction
> * failed because all zones are below low watermarks
> @@ -4495,23 +4498,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> if (compact_result == COMPACT_SKIPPED ||
> compact_result == COMPACT_DEFERRED)
> goto nopage;
> - }
> -
> - /*
> - * Checks for costly allocations with __GFP_NORETRY, which
> - * includes THP page fault allocations
> - */
> - if (costly_order && (gfp_mask & __GFP_NORETRY)) {
> - /*
> - * If compaction is deferred for high-order allocations,
> - * it is because sync compaction recently failed. If
> - * this is the case and the caller requested a THP
> - * allocation, we do not want to heavily disrupt the
> - * system, so we fail the allocation instead of entering
> - * direct reclaim.
> - */
> - if (compact_result == COMPACT_DEFERRED)
> - goto nopage;
>
> /*
> * Looks like reclaim/compaction is worth trying, but
> --
> 2.23.0
>
>
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