Re: [PATCH RFC v2 12/27] arm64: mte: Add tag storage pages to the MIGRATE_CMA migratetype

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Tue, Nov 28, 2023 at 06:03:52PM +0100, David Hildenbrand wrote:
> 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 have been looking at the CMA code, and nothing stands out. I'll try changing
the code to use cma_alloc/cma_release for the next iteration.

> 
> > 
> > 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? :)

Sounds good, I'll try to integrate the fast path code to cma_alloc(), that way
existing callers can benefit from it immediately.

Thanks,
Alex

> 
> -- 
> Cheers,
> 
> David / dhildenb
>