A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
taking an excessive amount of time for large amounts of memory. Further
testing allocating huge pages that the cost is linear i.e. if allocating
1G pages in batches of 10 then the time to allocate nr_hugepages from
10->20->30->etc increases linearly even though 10 pages are allocated at
each step. Profiles indicated that much of the time is spent checking the
validity within already existing huge pages and then attempting a migration
that fails after isolating the range, draining pages and a whole lot of
other useless work.
Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
pfn_range_valid_contig") removed two checks, one which ignored huge pages
for contiguous allocations as huge pages can sometimes migrate. While
there may be value on migrating a 2M page to satisfy a 1G allocation, it's
potentially expensive if the 1G allocation fails and it's pointless to
try moving a 1G page for a new 1G allocation or scan the tail pages for
valid PFNs.
Reintroduce the PageHuge check and assume any contiguous region with
hugetlbfs pages is unsuitable for a new 1G allocation.
The hpagealloc test allocates huge pages in batches and reports the
average latency per page over time. This test happens just after boot when
fragmentation is not an issue. Units are in milliseconds.
hpagealloc
6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
vanilla hugeallocrevert-v1r1 hugeallocsimple-v1r2
Min Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
1st-qrtle Latency 356.61 ( 0.00%) 5.34 ( 98.50%) 19.85 ( 94.43%)
2nd-qrtle Latency 697.26 ( 0.00%) 5.47 ( 99.22%) 20.44 ( 97.07%)
3rd-qrtle Latency 972.94 ( 0.00%) 5.50 ( 99.43%) 20.81 ( 97.86%)
Max-1 Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
Max-5 Latency 82.14 ( 0.00%) 5.11 ( 93.78%) 19.31 ( 76.49%)
Max-10 Latency 150.54 ( 0.00%) 5.20 ( 96.55%) 19.43 ( 87.09%)
Max-90 Latency 1164.45 ( 0.00%) 5.53 ( 99.52%) 20.97 ( 98.20%)
Max-95 Latency 1223.06 ( 0.00%) 5.55 ( 99.55%) 21.06 ( 98.28%)
Max-99 Latency 1278.67 ( 0.00%) 5.57 ( 99.56%) 22.56 ( 98.24%)
Max Latency 1310.90 ( 0.00%) 8.06 ( 99.39%) 26.62 ( 97.97%)
Amean Latency 678.36 ( 0.00%) 5.44 * 99.20%* 20.44 * 96.99%*
6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
vanilla revert-v1 hugeallocfix-v2
Duration User 0.28 0.27 0.30
Duration System 808.66 17.77 35.99
Duration Elapsed 830.87 18.08 36.33
The vanilla kernel is poor, taking up to 1.3 second to allocate a huge page
and almost 10 minutes in total to run the test. Reverting the problematic
commit reduces it to 8ms at worst and the patch takes 26ms. This patch
fixes the main issue with skipping huge pages but leaves the page_count()
out because a page with an elevated count potentially can migrate.
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from pfn_range_valid_contig")
Reported-by: Yuanxi Liu <[email protected]>
Signed-off-by: Mel Gorman <[email protected]>
---
mm/page_alloc.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7136c36c5d01..b47f520c3051 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -9450,6 +9450,9 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
if (PageReserved(page))
return false;
+
+ if (PageHuge(page))
+ return false;
}
return true;
}
On 4/14/23 16:14, Mel Gorman wrote:
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
> taking an excessive amount of time for large amounts of memory. Further
> testing allocating huge pages that the cost is linear i.e. if allocating
> 1G pages in batches of 10 then the time to allocate nr_hugepages from
> 10->20->30->etc increases linearly even though 10 pages are allocated at
> each step. Profiles indicated that much of the time is spent checking the
> validity within already existing huge pages and then attempting a migration
> that fails after isolating the range, draining pages and a whole lot of
> other useless work.
>
> Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig") removed two checks, one which ignored huge pages
> for contiguous allocations as huge pages can sometimes migrate. While
> there may be value on migrating a 2M page to satisfy a 1G allocation, it's
> potentially expensive if the 1G allocation fails and it's pointless to
> try moving a 1G page for a new 1G allocation or scan the tail pages for
> valid PFNs.
>
> Reintroduce the PageHuge check and assume any contiguous region with
> hugetlbfs pages is unsuitable for a new 1G allocation.
>
> The hpagealloc test allocates huge pages in batches and reports the
> average latency per page over time. This test happens just after boot when
> fragmentation is not an issue. Units are in milliseconds.
>
> hpagealloc
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla hugeallocrevert-v1r1 hugeallocsimple-v1r2
> Min Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> 1st-qrtle Latency 356.61 ( 0.00%) 5.34 ( 98.50%) 19.85 ( 94.43%)
> 2nd-qrtle Latency 697.26 ( 0.00%) 5.47 ( 99.22%) 20.44 ( 97.07%)
> 3rd-qrtle Latency 972.94 ( 0.00%) 5.50 ( 99.43%) 20.81 ( 97.86%)
> Max-1 Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> Max-5 Latency 82.14 ( 0.00%) 5.11 ( 93.78%) 19.31 ( 76.49%)
> Max-10 Latency 150.54 ( 0.00%) 5.20 ( 96.55%) 19.43 ( 87.09%)
> Max-90 Latency 1164.45 ( 0.00%) 5.53 ( 99.52%) 20.97 ( 98.20%)
> Max-95 Latency 1223.06 ( 0.00%) 5.55 ( 99.55%) 21.06 ( 98.28%)
> Max-99 Latency 1278.67 ( 0.00%) 5.57 ( 99.56%) 22.56 ( 98.24%)
> Max Latency 1310.90 ( 0.00%) 8.06 ( 99.39%) 26.62 ( 97.97%)
> Amean Latency 678.36 ( 0.00%) 5.44 * 99.20%* 20.44 * 96.99%*
>
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla revert-v1 hugeallocfix-v2
> Duration User 0.28 0.27 0.30
> Duration System 808.66 17.77 35.99
> Duration Elapsed 830.87 18.08 36.33
>
> The vanilla kernel is poor, taking up to 1.3 second to allocate a huge page
> and almost 10 minutes in total to run the test. Reverting the problematic
> commit reduces it to 8ms at worst and the patch takes 26ms. This patch
> fixes the main issue with skipping huge pages but leaves the page_count()
> out because a page with an elevated count potentially can migrate.
>
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
> Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from pfn_range_valid_contig")
> Reported-by: Yuanxi Liu <[email protected]>
> Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
> ---
> mm/page_alloc.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7136c36c5d01..b47f520c3051 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -9450,6 +9450,9 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>
> if (PageReserved(page))
> return false;
> +
> + if (PageHuge(page))
> + return false;
> }
> return true;
> }
On 14.04.23 16:14, Mel Gorman wrote:
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
> taking an excessive amount of time for large amounts of memory. Further
> testing allocating huge pages that the cost is linear i.e. if allocating
> 1G pages in batches of 10 then the time to allocate nr_hugepages from
> 10->20->30->etc increases linearly even though 10 pages are allocated at
> each step. Profiles indicated that much of the time is spent checking the
> validity within already existing huge pages and then attempting a migration
> that fails after isolating the range, draining pages and a whole lot of
> other useless work.
>
> Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig") removed two checks, one which ignored huge pages
> for contiguous allocations as huge pages can sometimes migrate. While
> there may be value on migrating a 2M page to satisfy a 1G allocation, it's
> potentially expensive if the 1G allocation fails and it's pointless to
> try moving a 1G page for a new 1G allocation or scan the tail pages for
> valid PFNs.
>
> Reintroduce the PageHuge check and assume any contiguous region with
> hugetlbfs pages is unsuitable for a new 1G allocation.
>
> The hpagealloc test allocates huge pages in batches and reports the
> average latency per page over time. This test happens just after boot when
> fragmentation is not an issue. Units are in milliseconds.
>
> hpagealloc
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla hugeallocrevert-v1r1 hugeallocsimple-v1r2
> Min Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> 1st-qrtle Latency 356.61 ( 0.00%) 5.34 ( 98.50%) 19.85 ( 94.43%)
> 2nd-qrtle Latency 697.26 ( 0.00%) 5.47 ( 99.22%) 20.44 ( 97.07%)
> 3rd-qrtle Latency 972.94 ( 0.00%) 5.50 ( 99.43%) 20.81 ( 97.86%)
> Max-1 Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> Max-5 Latency 82.14 ( 0.00%) 5.11 ( 93.78%) 19.31 ( 76.49%)
> Max-10 Latency 150.54 ( 0.00%) 5.20 ( 96.55%) 19.43 ( 87.09%)
> Max-90 Latency 1164.45 ( 0.00%) 5.53 ( 99.52%) 20.97 ( 98.20%)
> Max-95 Latency 1223.06 ( 0.00%) 5.55 ( 99.55%) 21.06 ( 98.28%)
> Max-99 Latency 1278.67 ( 0.00%) 5.57 ( 99.56%) 22.56 ( 98.24%)
> Max Latency 1310.90 ( 0.00%) 8.06 ( 99.39%) 26.62 ( 97.97%)
> Amean Latency 678.36 ( 0.00%) 5.44 * 99.20%* 20.44 * 96.99%*
>
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla revert-v1 hugeallocfix-v2
> Duration User 0.28 0.27 0.30
> Duration System 808.66 17.77 35.99
> Duration Elapsed 830.87 18.08 36.33
>
> The vanilla kernel is poor, taking up to 1.3 second to allocate a huge page
> and almost 10 minutes in total to run the test. Reverting the problematic
> commit reduces it to 8ms at worst and the patch takes 26ms. This patch
> fixes the main issue with skipping huge pages but leaves the page_count()
> out because a page with an elevated count potentially can migrate.
>
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
> Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from pfn_range_valid_contig")
> Reported-by: Yuanxi Liu <[email protected]>
> Signed-off-by: Mel Gorman <[email protected]>
> ---
> mm/page_alloc.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7136c36c5d01..b47f520c3051 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -9450,6 +9450,9 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>
> if (PageReserved(page))
> return false;
> +
> + if (PageHuge(page))
> + return false;
> }
> return true;
> }
>
Reviewed-by: David Hildenbrand <[email protected]>
--
Thanks,
David / dhildenb
On Fri 14-04-23 15:14:29, Mel Gorman wrote:
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
> taking an excessive amount of time for large amounts of memory. Further
> testing allocating huge pages that the cost is linear i.e. if allocating
> 1G pages in batches of 10 then the time to allocate nr_hugepages from
> 10->20->30->etc increases linearly even though 10 pages are allocated at
> each step. Profiles indicated that much of the time is spent checking the
> validity within already existing huge pages and then attempting a migration
> that fails after isolating the range, draining pages and a whole lot of
> other useless work.
>
> Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig") removed two checks, one which ignored huge pages
> for contiguous allocations as huge pages can sometimes migrate. While
> there may be value on migrating a 2M page to satisfy a 1G allocation, it's
> potentially expensive if the 1G allocation fails and it's pointless to
> try moving a 1G page for a new 1G allocation or scan the tail pages for
> valid PFNs.
>
> Reintroduce the PageHuge check and assume any contiguous region with
> hugetlbfs pages is unsuitable for a new 1G allocation.
>
> The hpagealloc test allocates huge pages in batches and reports the
> average latency per page over time. This test happens just after boot when
> fragmentation is not an issue. Units are in milliseconds.
>
> hpagealloc
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla hugeallocrevert-v1r1 hugeallocsimple-v1r2
> Min Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> 1st-qrtle Latency 356.61 ( 0.00%) 5.34 ( 98.50%) 19.85 ( 94.43%)
> 2nd-qrtle Latency 697.26 ( 0.00%) 5.47 ( 99.22%) 20.44 ( 97.07%)
> 3rd-qrtle Latency 972.94 ( 0.00%) 5.50 ( 99.43%) 20.81 ( 97.86%)
> Max-1 Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> Max-5 Latency 82.14 ( 0.00%) 5.11 ( 93.78%) 19.31 ( 76.49%)
> Max-10 Latency 150.54 ( 0.00%) 5.20 ( 96.55%) 19.43 ( 87.09%)
> Max-90 Latency 1164.45 ( 0.00%) 5.53 ( 99.52%) 20.97 ( 98.20%)
> Max-95 Latency 1223.06 ( 0.00%) 5.55 ( 99.55%) 21.06 ( 98.28%)
> Max-99 Latency 1278.67 ( 0.00%) 5.57 ( 99.56%) 22.56 ( 98.24%)
> Max Latency 1310.90 ( 0.00%) 8.06 ( 99.39%) 26.62 ( 97.97%)
> Amean Latency 678.36 ( 0.00%) 5.44 * 99.20%* 20.44 * 96.99%*
>
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla revert-v1 hugeallocfix-v2
> Duration User 0.28 0.27 0.30
> Duration System 808.66 17.77 35.99
> Duration Elapsed 830.87 18.08 36.33
>
> The vanilla kernel is poor, taking up to 1.3 second to allocate a huge page
> and almost 10 minutes in total to run the test. Reverting the problematic
> commit reduces it to 8ms at worst and the patch takes 26ms. This patch
> fixes the main issue with skipping huge pages but leaves the page_count()
> out because a page with an elevated count potentially can migrate.
>
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
> Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from pfn_range_valid_contig")
> Reported-by: Yuanxi Liu <[email protected]>
> Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Thanks!
> ---
> mm/page_alloc.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7136c36c5d01..b47f520c3051 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -9450,6 +9450,9 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>
> if (PageReserved(page))
> return false;
> +
> + if (PageHuge(page))
> + return false;
> }
> return true;
> }
--
Michal Hocko
SUSE Labs
On Fri, 14 Apr 2023 15:14:29 +0100 Mel Gorman <[email protected]> wrote:
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
> taking an excessive amount of time for large amounts of memory.
>
> ...
>
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla revert-v1 hugeallocfix-v2
> Duration User 0.28 0.27 0.30
> Duration System 808.66 17.77 35.99
> Duration Elapsed 830.87 18.08 36.33
>
I'll put a cc:stable on this.
On 4/14/2023 7:14 AM, Mel Gorman wrote:
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
> taking an excessive amount of time for large amounts of memory. Further
> testing allocating huge pages that the cost is linear i.e. if allocating
> 1G pages in batches of 10 then the time to allocate nr_hugepages from
> 10->20->30->etc increases linearly even though 10 pages are allocated at
> each step. Profiles indicated that much of the time is spent checking the
> validity within already existing huge pages and then attempting a migration
> that fails after isolating the range, draining pages and a whole lot of
> other useless work.
>
> Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig") removed two checks, one which ignored huge pages
> for contiguous allocations as huge pages can sometimes migrate. While
> there may be value on migrating a 2M page to satisfy a 1G allocation, it's
> potentially expensive if the 1G allocation fails and it's pointless to
> try moving a 1G page for a new 1G allocation or scan the tail pages for
> valid PFNs.
>
> Reintroduce the PageHuge check and assume any contiguous region with
> hugetlbfs pages is unsuitable for a new 1G allocation.
>
> The hpagealloc test allocates huge pages in batches and reports the
> average latency per page over time. This test happens just after boot when
> fragmentation is not an issue. Units are in milliseconds.
>
> hpagealloc
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla hugeallocrevert-v1r1 hugeallocsimple-v1r2
> Min Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> 1st-qrtle Latency 356.61 ( 0.00%) 5.34 ( 98.50%) 19.85 ( 94.43%)
> 2nd-qrtle Latency 697.26 ( 0.00%) 5.47 ( 99.22%) 20.44 ( 97.07%)
> 3rd-qrtle Latency 972.94 ( 0.00%) 5.50 ( 99.43%) 20.81 ( 97.86%)
> Max-1 Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> Max-5 Latency 82.14 ( 0.00%) 5.11 ( 93.78%) 19.31 ( 76.49%)
> Max-10 Latency 150.54 ( 0.00%) 5.20 ( 96.55%) 19.43 ( 87.09%)
> Max-90 Latency 1164.45 ( 0.00%) 5.53 ( 99.52%) 20.97 ( 98.20%)
> Max-95 Latency 1223.06 ( 0.00%) 5.55 ( 99.55%) 21.06 ( 98.28%)
> Max-99 Latency 1278.67 ( 0.00%) 5.57 ( 99.56%) 22.56 ( 98.24%)
> Max Latency 1310.90 ( 0.00%) 8.06 ( 99.39%) 26.62 ( 97.97%)
> Amean Latency 678.36 ( 0.00%) 5.44 * 99.20%* 20.44 * 96.99%*
>
> 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> vanilla revert-v1 hugeallocfix-v2
> Duration User 0.28 0.27 0.30
> Duration System 808.66 17.77 35.99
> Duration Elapsed 830.87 18.08 36.33
>
> The vanilla kernel is poor, taking up to 1.3 second to allocate a huge page
> and almost 10 minutes in total to run the test. Reverting the problematic
> commit reduces it to 8ms at worst and the patch takes 26ms. This patch
> fixes the main issue with skipping huge pages but leaves the page_count()
> out because a page with an elevated count potentially can migrate.
>
A while ago I submitted this patch set that should significantly improve
the chances of a 2MB Huge Page being successfully migrated:
https://lore.kernel.org/linux-mm/[email protected]/
Unfortunately, it is collecting dust and needs to be updated to support
Folios, but I would be curious how it affects the performance of this test.
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
> Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from pfn_range_valid_contig")
> Reported-by: Yuanxi Liu <[email protected]>
> Signed-off-by: Mel Gorman <[email protected]>
> ---
> mm/page_alloc.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7136c36c5d01..b47f520c3051 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -9450,6 +9450,9 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
>
> if (PageReserved(page))
> return false;
> +
> + if (PageHuge(page))
> + return false;
> }
> return true;
> }
>
I am OK with this patch too, but I could resubmit my patch with Mike's
suggested variant and Folio support if anyone wants to try it instead of
this approach.
Regards,
Doug
On 2023-04-14 16:14, Mel Gorman wrote:
> A bug was reported by Yuanxi Liu where allocating 1G pages at runtime
> is
> taking an excessive amount of time for large amounts of memory. Further
> testing allocating huge pages that the cost is linear i.e. if
> allocating
> 1G pages in batches of 10 then the time to allocate nr_hugepages from
> 10->20->30->etc increases linearly even though 10 pages are allocated
> at
> each step. Profiles indicated that much of the time is spent checking
> the
> validity within already existing huge pages and then attempting a
> migration
> that fails after isolating the range, draining pages and a whole lot of
> other useless work.
>
> Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig") removed two checks, one which ignored huge
> pages
> for contiguous allocations as huge pages can sometimes migrate. While
> there may be value on migrating a 2M page to satisfy a 1G allocation,
> it's
> potentially expensive if the 1G allocation fails and it's pointless to
> try moving a 1G page for a new 1G allocation or scan the tail pages for
> valid PFNs.
>
> Reintroduce the PageHuge check and assume any contiguous region with
> hugetlbfs pages is unsuitable for a new 1G allocation.
>
> The hpagealloc test allocates huge pages in batches and reports the
> average latency per page over time. This test happens just after boot
> when
> fragmentation is not an issue. Units are in milliseconds.
>
...
>
> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
> Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> pfn_range_valid_contig")
> Reported-by: Yuanxi Liu <[email protected]>
> Signed-off-by: Mel Gorman <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Thanks Mel!
--
Oscar Salvador
SUSE Labs
On Fri, Apr 14, 2023 at 04:20:33PM -0700, Doug Berger wrote:
> On 4/14/2023 7:14 AM, Mel Gorman wrote:
> > A bug was reported by Yuanxi Liu where allocating 1G pages at runtime is
> > taking an excessive amount of time for large amounts of memory. Further
> > testing allocating huge pages that the cost is linear i.e. if allocating
> > 1G pages in batches of 10 then the time to allocate nr_hugepages from
> > 10->20->30->etc increases linearly even though 10 pages are allocated at
> > each step. Profiles indicated that much of the time is spent checking the
> > validity within already existing huge pages and then attempting a migration
> > that fails after isolating the range, draining pages and a whole lot of
> > other useless work.
> >
> > Commit eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from
> > pfn_range_valid_contig") removed two checks, one which ignored huge pages
> > for contiguous allocations as huge pages can sometimes migrate. While
> > there may be value on migrating a 2M page to satisfy a 1G allocation, it's
> > potentially expensive if the 1G allocation fails and it's pointless to
> > try moving a 1G page for a new 1G allocation or scan the tail pages for
> > valid PFNs.
> >
> > Reintroduce the PageHuge check and assume any contiguous region with
> > hugetlbfs pages is unsuitable for a new 1G allocation.
> >
> > The hpagealloc test allocates huge pages in batches and reports the
> > average latency per page over time. This test happens just after boot when
> > fragmentation is not an issue. Units are in milliseconds.
> >
> > hpagealloc
> > 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> > vanilla hugeallocrevert-v1r1 hugeallocsimple-v1r2
> > Min Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> > 1st-qrtle Latency 356.61 ( 0.00%) 5.34 ( 98.50%) 19.85 ( 94.43%)
> > 2nd-qrtle Latency 697.26 ( 0.00%) 5.47 ( 99.22%) 20.44 ( 97.07%)
> > 3rd-qrtle Latency 972.94 ( 0.00%) 5.50 ( 99.43%) 20.81 ( 97.86%)
> > Max-1 Latency 26.42 ( 0.00%) 5.07 ( 80.82%) 18.94 ( 28.30%)
> > Max-5 Latency 82.14 ( 0.00%) 5.11 ( 93.78%) 19.31 ( 76.49%)
> > Max-10 Latency 150.54 ( 0.00%) 5.20 ( 96.55%) 19.43 ( 87.09%)
> > Max-90 Latency 1164.45 ( 0.00%) 5.53 ( 99.52%) 20.97 ( 98.20%)
> > Max-95 Latency 1223.06 ( 0.00%) 5.55 ( 99.55%) 21.06 ( 98.28%)
> > Max-99 Latency 1278.67 ( 0.00%) 5.57 ( 99.56%) 22.56 ( 98.24%)
> > Max Latency 1310.90 ( 0.00%) 8.06 ( 99.39%) 26.62 ( 97.97%)
> > Amean Latency 678.36 ( 0.00%) 5.44 * 99.20%* 20.44 * 96.99%*
> >
> > 6.3.0-rc6 6.3.0-rc6 6.3.0-rc6
> > vanilla revert-v1 hugeallocfix-v2
> > Duration User 0.28 0.27 0.30
> > Duration System 808.66 17.77 35.99
> > Duration Elapsed 830.87 18.08 36.33
> >
> > The vanilla kernel is poor, taking up to 1.3 second to allocate a huge page
> > and almost 10 minutes in total to run the test. Reverting the problematic
> > commit reduces it to 8ms at worst and the patch takes 26ms. This patch
> > fixes the main issue with skipping huge pages but leaves the page_count()
> > out because a page with an elevated count potentially can migrate.
> >
> A while ago I submitted this patch set that should significantly improve
> the chances of a 2MB Huge Page being successfully migrated:
> https://lore.kernel.org/linux-mm/[email protected]/
>
> Unfortunately, it is collecting dust and needs to be updated to support
> Folios, but I would be curious how it affects the performance of this test.
>
It would not affect the specific test reported in the changelog as it's
1G allocations early in boot so no other hugetlb pages present. To be
representative there would have to an initialisation step to allocate 2M
pages first, place them awkwardly and then attempt the 1G allocations
to force the migrations to occur. That's not impossible as huge pages
could be allocated while base pages are being faulted in a mmap() region
to try interleaving 2M and base pages in the physical address space (only
realistic during boot and the same process faulting base pages should also
increment nr_hugepages so it's reproducible). That would be sufficient
to demonstrate that migrating the 2M pages allows a 1G to succeed where
it would have failed otherwise.
> > BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=217022
> > Fixes: eb14d4eefdc4 ("mm,page_alloc: drop unnecessary checks from pfn_range_valid_contig")
> > Reported-by: Yuanxi Liu <[email protected]>
> > Signed-off-by: Mel Gorman <[email protected]>
> > ---
> > mm/page_alloc.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 7136c36c5d01..b47f520c3051 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -9450,6 +9450,9 @@ static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
> > if (PageReserved(page))
> > return false;
> > +
> > + if (PageHuge(page))
> > + return false;
> > }
> > return true;
> > }
> >
>
> I am OK with this patch too, but I could resubmit my patch with Mike's
> suggested variant and Folio support if anyone wants to try it instead of
> this approach.
>
I have no objection to a patch checking compound_order(page) > MAX_ORDER but
I wouldn't consider it to be a stable candidate. Ideally, there would be some
data showing it actually works (1G pages succeed that would otherwise fail)
and that it doesn't take a silly amount of time to succeed *or* fail a 1G
allocation. The test case is an ideal based on commit eb14d4eefdc4 implying
that 2M migrations were expected to improve 1G allocations but I don't
know if that's actually true or what the cost is. Maybe someone else knows.
--
Mel Gorman
SUSE Labs