Messages in this thread | | | Date | Tue, 28 Nov 2023 18:03:52 +0100 | Subject | Re: [PATCH RFC v2 12/27] arm64: mte: Add tag storage pages to the MIGRATE_CMA migratetype | From | David Hildenbrand <> |
| |
On 27.11.23 16:01, Alexandru Elisei wrote: > Hi David, > > On Fri, Nov 24, 2023 at 08:40:55PM +0100, David Hildenbrand wrote: >> On 19.11.23 17:57, Alexandru Elisei wrote: >>> Add the MTE tag storage pages to the MIGRATE_CMA migratetype, which allows >>> the page allocator to manage them like regular pages. >>> >>> Ths migratype lends the pages some very desirable properties: >>> >>> * They cannot be longterm pinned, meaning they will always be migratable. >>> >>> * The pages can be allocated explicitely by using their PFN (with >>> alloc_contig_range()) when they are needed to store tags. >>> >>> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com> >>> --- >>> arch/arm64/Kconfig | 1 + >>> arch/arm64/kernel/mte_tag_storage.c | 68 +++++++++++++++++++++++++++++ >>> include/linux/mmzone.h | 5 +++ >>> mm/internal.h | 3 -- >>> 4 files changed, 74 insertions(+), 3 deletions(-) >>> >>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig >>> index fe8276fdc7a8..047487046e8f 100644 >>> --- a/arch/arm64/Kconfig >>> +++ b/arch/arm64/Kconfig >>> @@ -2065,6 +2065,7 @@ config ARM64_MTE >>> if ARM64_MTE >>> config ARM64_MTE_TAG_STORAGE >>> bool "Dynamic MTE tag storage management" >>> + select CONFIG_CMA >>> help >>> Adds support for dynamic management of the memory used by the hardware >>> for storing MTE tags. This memory, unlike normal memory, cannot be >>> diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c >>> index fa6267ef8392..427f4f1909f3 100644 >>> --- a/arch/arm64/kernel/mte_tag_storage.c >>> +++ b/arch/arm64/kernel/mte_tag_storage.c >>> @@ -5,10 +5,12 @@ >>> * Copyright (C) 2023 ARM Ltd. >>> */ >>> +#include <linux/cma.h> >>> #include <linux/memblock.h> >>> #include <linux/mm.h> >>> #include <linux/of_device.h> >>> #include <linux/of_fdt.h> >>> +#include <linux/pageblock-flags.h> >>> #include <linux/range.h> >>> #include <linux/string.h> >>> #include <linux/xarray.h> >>> @@ -189,6 +191,14 @@ static int __init fdt_init_tag_storage(unsigned long node, const char *uname, >>> return ret; >>> } >>> + /* Pages are managed in pageblock_nr_pages chunks */ >>> + if (!IS_ALIGNED(tag_range->start | range_len(tag_range), pageblock_nr_pages)) { >>> + pr_err("Tag storage region 0x%llx-0x%llx not aligned to pageblock size 0x%llx", >>> + PFN_PHYS(tag_range->start), PFN_PHYS(tag_range->end), >>> + PFN_PHYS(pageblock_nr_pages)); >>> + return -EINVAL; >>> + } >>> + >>> ret = tag_storage_get_memory_node(node, &mem_node); >>> if (ret) >>> return ret; >>> @@ -254,3 +264,61 @@ void __init mte_tag_storage_init(void) >>> pr_info("MTE tag storage region management disabled"); >>> } >>> } >>> + >>> +static int __init mte_tag_storage_activate_regions(void) >>> +{ >>> + phys_addr_t dram_start, dram_end; >>> + struct range *tag_range; >>> + unsigned long pfn; >>> + int i, ret; >>> + >>> + if (num_tag_regions == 0) >>> + return 0; >>> + >>> + dram_start = memblock_start_of_DRAM(); >>> + dram_end = memblock_end_of_DRAM(); >>> + >>> + for (i = 0; i < num_tag_regions; i++) { >>> + tag_range = &tag_regions[i].tag_range; >>> + /* >>> + * Tag storage region was clipped by arm64_bootmem_init() >>> + * enforcing addressing limits. >>> + */ >>> + if (PFN_PHYS(tag_range->start) < dram_start || >>> + PFN_PHYS(tag_range->end) >= dram_end) { >>> + pr_err("Tag storage region 0x%llx-0x%llx outside addressable memory", >>> + PFN_PHYS(tag_range->start), PFN_PHYS(tag_range->end)); >>> + ret = -EINVAL; >>> + goto out_disabled; >>> + } >>> + } >>> + >>> + /* >>> + * MTE disabled, tag storage pages can be used like any other pages. The >>> + * only restriction is that the pages cannot be used by kexec because >>> + * the memory remains marked as reserved in the memblock allocator. >>> + */ >>> + if (!system_supports_mte()) { >>> + for (i = 0; i< num_tag_regions; i++) { >>> + tag_range = &tag_regions[i].tag_range; >>> + for (pfn = tag_range->start; pfn <= tag_range->end; pfn++) >>> + free_reserved_page(pfn_to_page(pfn)); >>> + } >>> + ret = 0; >>> + goto out_disabled; >>> + } >>> + >>> + for (i = 0; i < num_tag_regions; i++) { >>> + tag_range = &tag_regions[i].tag_range; >>> + for (pfn = tag_range->start; pfn <= tag_range->end; pfn += pageblock_nr_pages) >>> + init_cma_reserved_pageblock(pfn_to_page(pfn)); >>> + totalcma_pages += range_len(tag_range); >>> + } >> >> You shouldn't be doing that manually in arm code. Likely you want some cma.c >> helper for something like that. > > If you referring to the last loop (the one that does > ini_cma_reserved_pageblock()), indeed, there's already a function which > does that, cma_init_reserved_areas() -> cma_activate_area(). > >> >> But, can you elaborate on why you took this hacky (sorry) approach as >> documented in the cover letter: > > No worries, it is indeed a bit hacky :) > >> >> "The arm64 code manages this memory directly instead of using >> cma_declare_contiguous/cma_alloc for performance reasons." >> >> What is the exact problem? > > I am referring to the performance degredation that is fixed in patch #26, > "arm64: mte: Fast track reserving tag storage when the block is free" [1]. > The issue is that alloc_contig_range() -> __alloc_contig_migrate_range() > calls lru_cache_disable(), which IPIs all the CPUs in the system, and that > leads to a 10-20% performance degradation on Chrome. It has been observed > that most of the time the tag storage pages are free, and the > lru_cache_disable() calls are unnecessary.
This sounds like something eventually worth integrating into CMA/alloc_contig_range(). Like, a fast path to check if we are only allocating something small (e.g., falls within a single pageblock), and if the page is free.
> > The performance degradation is almost entirely eliminated by having the code > take the tag storage page directly from the free list if it's free, instead > of calling alloc_contig_range(). > > Do you believe it would be better to use the cma code, and modify it to use > this fast path to take the page drectly from the buddy allocator?
That sounds reasonable yes. Do you see any blockers for that?
> > I can definitely try to integrate the code with cma_alloc(), but I think > keeping the fast path for reserving tag storage is extremely desirable, > since it makes such a huge difference to performance.
Yes, but let's try finding a way to optimize common code, to eventually improve some CMA cases as well? :)
-- Cheers,
David / dhildenb
| |