Re: [PATCH] arm64: mm: fix linear mapping mem access performace degradation
From: guanghui.fgh
Date: Tue Jun 28 2022 - 03:52:59 EST
在 2022/6/28 11:06, guanghui.fgh 写道:
Thanks.
在 2022/6/28 9:34, Leizhen (ThunderTown) 写道:
On 2022/6/27 20:25, guanghui.fgh wrote:
Thanks.
在 2022/6/27 20:06, Leizhen (ThunderTown) 写道:
On 2022/6/27 18:46, guanghui.fgh wrote:
在 2022/6/27 17:49, Mike Rapoport 写道:
Please don't post HTML.
On Mon, Jun 27, 2022 at 05:24:10PM +0800, guanghui.fgh wrote:
Thanks.
在 2022/6/27 14:34, Mike Rapoport 写道:
On Sun, Jun 26, 2022 at 07:10:15PM +0800, Guanghui Feng
wrote:
The arm64 can build 2M/1G block/sectiion mapping. When
using DMA/DMA32 zone
(enable crashkernel, disable rodata full, disable
kfence), the mem_map will
use non block/section mapping(for crashkernel requires
to shrink the region
in page granularity). But it will degrade performance
when doing larging
continuous mem access in kernel(memcpy/memmove, etc).
There are many changes and discussions:
commit 031495635b46
commit 1a8e1cef7603
commit 8424ecdde7df
commit 0a30c53573b0
commit 2687275a5843
Please include oneline summary of the commit. (See section
"Describe your
changes" in Documentation/process/submitting-patches.rst)
OK, I will add oneline summary in the git commit messages.
This patch changes mem_map to use block/section
mapping with crashkernel.
Firstly, do block/section mapping(normally 2M or 1G)
for all avail mem at
mem_map, reserve crashkernel memory. And then walking
pagetable to split
block/section mapping to non block/section
mapping(normally 4K) [[[only]]]
for crashkernel mem.
This already happens when ZONE_DMA/ZONE_DMA32 are
disabled. Please explain
why is it Ok to change the way the memory is mapped with
ZONE_DMA/ZONE_DMA32 enabled.
In short:
1.building all avail mem with block/section mapping（normally
1G/2M） without
inspecting crashkernel
2. Reserve crashkernel mem as same as previous doing
3. only change the crashkernle mem mapping to normal
mapping(normally 4k).
With this method, there are block/section mapping as more as
possible.
This does not answer the question why changing the way the memory
is mapped
when there is ZONE_DMA/DMA32 and crashkernel won't cause a
regression.
1.Quoted messages from arch/arm64/mm/init.c
"Memory reservation for crash kernel either done early or deferred
depending on DMA memory zones configs (ZONE_DMA) --
In absence of ZONE_DMA configs arm64_dma_phys_limit initialized
here instead of max_zone_phys(). This lets early reservation of
crash kernel memory which has a dependency on arm64_dma_phys_limit.
Reserving memory early for crash kernel allows linear creation of
block
mappings (greater than page-granularity) for all the memory bank
rangs.
In this scheme a comparatively quicker boot is observed.
If ZONE_DMA configs are defined, crash kernel memory reservation
is delayed until DMA zone memory range size initialization
performed in
zone_sizes_init(). The defer is necessary to steer clear of DMA zone
memory range to avoid overlap allocation. So crash kernel memory
boundaries are not known when mapping all bank memory ranges, which
otherwise means not possible to exclude crash kernel range from
creating block mappings so page-granularity mappings are created
for the entire memory range."
Namely, the init order: memblock init--->linear mem mapping(4k
mapping for crashkernel, requirinig page-granularity
changing))--->zone dma limit--->reserve crashkernel.
So when enable ZONE DMA and using crashkernel, the mem mapping
using 4k mapping.
2.As mentioned above, when linear mem use 4k mapping simply, there
is high dtlb miss(degrade performance).
This patch use block/section mapping as far as possible with
performance improvement.
3.This patch reserve crashkernel as same as the history(ZONE DMA &
crashkernel reserving order), and only change the linear mem
mapping to block/section mapping.
.
I think Mike Rapoport's probably asking you to answer whether you've
taken into account such as BBM. For example, the following code:
we should prepare the next level pgtable first, then change 2M block
mapping to 4K page mapping, and flush TLB at the end.
+static void init_crashkernel_pmd(pud_t *pudp, unsigned long addr,
+ unsigned long end, phys_addr_t phys,
+ pgprot_t prot,
+ phys_addr_t (*pgtable_alloc)(int), int flags)
+{
+ phys_addr_t map_offset;
+ unsigned long next;
+ pmd_t *pmdp;
+ pmdval_t pmdval;
+
+ pmdp = pmd_offset(pudp, addr);
+ do {
+ next = pmd_addr_end(addr, end);
+ if (!pmd_none(*pmdp) && pmd_sect(*pmdp)) {
+ phys_addr_t pte_phys = pgtable_alloc(PAGE_SHIFT);
+ pmd_clear(pmdp);
+ pmdval = PMD_TYPE_TABLE | PMD_TABLE_UXN;
+ if (flags & NO_EXEC_MAPPINGS)
+ pmdval |= PMD_TABLE_PXN;
+ __pmd_populate(pmdp, pte_phys, pmdval);
+ flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
The pgtable is empty now. However, memory other than crashkernel may
be being accessed.
1.When reserving crashkernel and remapping linear mem mapping, there
is only one boot cpu running. There is no other cpu/thread running at
the same time.
So, put this in the code comment?
OK.
If scalability is considered and unpredictable changes occur in the
future, for example,
other modules also need this mapping function. It would be better to
deal with the BBM now,
and make this public.
OK, could you give me some advice?
2.When clearing block/section mapping, I have flush tlb by
flush_tlb_kernel_range. Afterwards rebuilt 4k mapping(I think it's no
need flush tlb).
+
+ map_offset = addr - (addr & PMD_MASK);
+ if (map_offset)
+ alloc_init_cont_pte(pmdp, addr & PMD_MASK, addr,
+ phys - map_offset, prot,
+ pgtable_alloc, flags);
+
+ if (next < (addr & PMD_MASK) + PMD_SIZE)
+ alloc_init_cont_pte(pmdp, next, (addr & PUD_MASK) +
+ PUD_SIZE, next - addr + phys,
+ prot, pgtable_alloc, flags);
Here and alloc_crashkernel_pud() should use the raw flags. It may not
contain (NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS)
Yes. the mem out of crashkernel should use block/section mapping as far
as possible including the LeftMargin and RightMargin.
But I had test it on HiSilicon Kunpeng 920-6426 with it and get
performacne degrade(without NO_BLOCK_MAPPINGS/NO_CONT_MAPPINGS flags for
the left/right margin)
It's strange, could you give some advice? Maybe it's good for other arm
platform except for HiSilicon Kunpeng 920-6426.
There should split non-crashkernel mem [[[ without ]]]
NO_BLOCK_MAPPINGS/NO_CONT_MAPPINGS flags
I had test it on other arm platform [[[ non HiSilicon arm platform ]]]
and also get performance improvement greatly.
Could you help me to check the difference betweent HiSilicon Kunpeng
920-6426 and other arm platform for the block/section mapping TLB support?
+ }
+ alloc_crashkernel_cont_pte(pmdp, addr, next, phys, prot,
+ pgtable_alloc, flags);
+ phys += next - addr;
+ } while (pmdp++, addr = next, addr != end);
+}
.