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[PATCH 05/10] percpu: change reserved_size to end page aligned

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  [PATCH 05/10] percpu: change reserved_size to end page aligned Dennis Zhou <dennisz@fb.com> - 2017-07-16 04:30 +0200
    Re: [PATCH 05/10] percpu: change reserved_size to end page aligned Tejun Heo <tj@kernel.org> - 2017-07-17 18:50 +0200

#1688036 — [PATCH 05/10] percpu: change reserved_size to end page aligned

From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>

Preparatory patch to modify the first chunk's static_size +
reserved_size to end page aligned. The first chunk has a unique
allocation scheme overlaying the static, reserved, and dynamic regions.
The other regions of each chunk are reserved or hidden. The bitmap
allocator would have to allocate in the bitmap the static region to
replicate this. By having the reserved region to end page aligned, the
metadata overhead can be saved. The consequence is that up to an
additional page of memory will be allocated to the reserved region that
primarily serves static percpu variables.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
---
 arch/ia64/mm/contig.c    |  3 ++-
 arch/ia64/mm/discontig.c |  3 ++-
 include/linux/percpu.h   | 29 +++++++++++++++++++++++++++++
 mm/percpu.c              |  6 ++++++
 4 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/arch/ia64/mm/contig.c b/arch/ia64/mm/contig.c
index 52715a7..20ee2b2 100644
--- a/arch/ia64/mm/contig.c
+++ b/arch/ia64/mm/contig.c
@@ -164,7 +164,8 @@ setup_per_cpu_areas(void)
 
 	/* set parameters */
 	static_size = __per_cpu_end - __per_cpu_start;
-	reserved_size = PERCPU_MODULE_RESERVE;
+	reserved_size = pcpu_align_reserved_region(static_size,
+						   PERCPU_MODULE_RESERVE);
 	dyn_size = PERCPU_PAGE_SIZE - static_size - reserved_size;
 	if (dyn_size < 0)
 		panic("percpu area overflow static=%zd reserved=%zd\n",
diff --git a/arch/ia64/mm/discontig.c b/arch/ia64/mm/discontig.c
index 8786268..f898b24 100644
--- a/arch/ia64/mm/discontig.c
+++ b/arch/ia64/mm/discontig.c
@@ -214,7 +214,8 @@ void __init setup_per_cpu_areas(void)
 
 	/* set basic parameters */
 	static_size = __per_cpu_end - __per_cpu_start;
-	reserved_size = PERCPU_MODULE_RESERVE;
+	reserved_size = pcpu_align_reserved_region(static_size,
+						   PERCPU_MODULE_RSERVE);
 	dyn_size = PERCPU_PAGE_SIZE - static_size - reserved_size;
 	if (dyn_size < 0)
 		panic("percpu area overflow static=%zd reserved=%zd\n",
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 491b3f5..98a371c 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -130,4 +130,33 @@ extern phys_addr_t per_cpu_ptr_to_phys(void *addr);
 	(typeof(type) __percpu *)__alloc_percpu(sizeof(type),		\
 						__alignof__(type))
 
+/*
+ * pcpu_align_reserved_region - page align the end of the reserved region
+ * @static_size: the static region size
+ * @reserved_size: the minimum reserved region size
+ *
+ * This function calculates the size of the reserved region required to
+ * make the reserved region end page aligned.
+ *
+ * Percpu memory offers a maximum alignment of PAGE_SIZE.  Aligning this
+ * minimizes the metadata overhead of overlapping the static, reserved,
+ * and dynamic regions by allowing the metadata for the static region to
+ * not be allocated.  This lets the base_addr be moved up to a page
+ * aligned address and disregard the static region as offsets are allocated.
+ * The beginning of the reserved region will overlap with the static
+ * region if the end of the static region is not page aligned.
+ *
+ * RETURNS:
+ * Size of reserved region required to make static_size + reserved_size
+ * page aligned.
+ */
+static inline ssize_t pcpu_align_reserved_region(ssize_t static_size,
+						 ssize_t reserved_size)
+{
+	if (!reserved_size)
+		return 0;
+
+	return PFN_ALIGN(static_size + reserved_size) - static_size;
+}
+
 #endif /* __LINUX_PERCPU_H */
diff --git a/mm/percpu.c b/mm/percpu.c
index 5bb90d8..7704db9 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1597,6 +1597,8 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
 	PCPU_SETUP_BUG_ON(ai->unit_size < size_sum);
 	PCPU_SETUP_BUG_ON(offset_in_page(ai->unit_size));
 	PCPU_SETUP_BUG_ON(ai->unit_size < PCPU_MIN_UNIT_SIZE);
+	PCPU_SETUP_BUG_ON(ai->reserved_size &&
+			  !PAGE_ALIGNED(ai->static_size + ai->reserved_size));
 	PCPU_SETUP_BUG_ON(ai->dyn_size < PERCPU_DYNAMIC_EARLY_SIZE);
 	PCPU_SETUP_BUG_ON(pcpu_verify_alloc_info(ai) < 0);
 
@@ -1800,6 +1802,9 @@ early_param("percpu_alloc", percpu_alloc_setup);
  * @atom_size: allocation atom size
  * @cpu_distance_fn: callback to determine distance between cpus, optional
  *
+ * If there is a @reserved_size, it is expanded to ensure the end of the
+ * reserved region is page aligned.
+ *
  * This function determines grouping of units, their mappings to cpus
  * and other parameters considering needed percpu size, allocation
  * atom size and distances between CPUs.
@@ -1835,6 +1840,7 @@ static struct pcpu_alloc_info * __init pcpu_build_alloc_info(
 	memset(group_cnt, 0, sizeof(group_cnt));
 
 	/* calculate size_sum and ensure dyn_size is enough for early alloc */
+	reserved_size = pcpu_align_reserved_region(static_size, reserved_size);
 	size_sum = PFN_ALIGN(static_size + reserved_size +
 			    max_t(size_t, dyn_size, PERCPU_DYNAMIC_EARLY_SIZE));
 	dyn_size = size_sum - static_size - reserved_size;
-- 
2.9.3

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#1689255

Hello,

On Sat, Jul 15, 2017 at 10:23:10PM -0400, Dennis Zhou wrote:
> From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>
> 
> Preparatory patch to modify the first chunk's static_size +
> reserved_size to end page aligned. The first chunk has a unique
> allocation scheme overlaying the static, reserved, and dynamic regions.
> The other regions of each chunk are reserved or hidden. The bitmap
> allocator would have to allocate in the bitmap the static region to
> replicate this. By having the reserved region to end page aligned, the
> metadata overhead can be saved. The consequence is that up to an
> additional page of memory will be allocated to the reserved region that
> primarily serves static percpu variables.
> 
> Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>

Sans the build warnings, generally looks good to me.  Some nits asnd
one question below.

> +/*

Should be /**

> + * pcpu_align_reserved_region - page align the end of the reserved region
> + * @static_size: the static region size
> + * @reserved_size: the minimum reserved region size
> + *
> + * This function calculates the size of the reserved region required to
> + * make the reserved region end page aligned.
> + *
> + * Percpu memory offers a maximum alignment of PAGE_SIZE.  Aligning this
> + * minimizes the metadata overhead of overlapping the static, reserved,
> + * and dynamic regions by allowing the metadata for the static region to
> + * not be allocated.  This lets the base_addr be moved up to a page
> + * aligned address and disregard the static region as offsets are allocated.
> + * The beginning of the reserved region will overlap with the static
> + * region if the end of the static region is not page aligned.

Heh, that was pretty difficult to parse, but here's my question.  So,
we're expanding reserved area so that its end aligns to page boundary
which is completely fine.  We may end up with reserved area which is a
bit larger than specified but no big deal.  However, we can't do the
same thing with the boundary between the static and reserved chunks,
so instead we pull down the start of the reserved area and mark off
the overwrapping area, which is fine too.

My question is why we're doing one thing for the end of reserved area
while we need to do a different thing for the beginning of it.  Can't
we do the same thing in both cases?  ie. for the both boundaries
between static and reserved, and reserved and dynamic, pull down the
start to the page boundary and mark the overlapping areas used?

Thanks.

-- 
tejun

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