[RFC] munmap(64k) is much slower with patch set reduce synchronous TLB invalidation on ARM64 merged

From: Hanjun Guo <guohanjun@huawei.com>
To: Will Deacon <Will.Deacon@arm.com>
Cc: "wangxiongfeng \(C\)" <wangxiongfeng2@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Linuxarm <linuxarm@huawei.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"Zhouguanghui \(OS Kernel\)" <zhouguanghui1@huawei.com>
Subject: [RFC] munmap(64k) is much slower with patch set reduce synchronous TLB invalidation on ARM64 merged
Date: Sat, 4 May 2019 19:44:59 +0800	[thread overview]
Message-ID: <3a0e9ffc-315b-079f-bc0b-3ee3dca88905@huawei.com> (raw)

Hi Will,

With patch set "Avoid synchronous TLB invalidation for intermediate
page-table entries on arm64" was merged, munmap() for hugepage such as
2M will be much faster, but with following case, munmap(64k) will take
much longer time than before:

#define MEM_LEN_TEST			(0x10000UL)
#define MEM_ADD_START			(0x8000fffef000UL)
#define MEM1_ADD_START			(0x800000090000UL)

mem = (unsigned char *)mmap((void *)MEM_ADD_START, MEM_LEN_TEST, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

mem1 = (unsigned char *)mmap((void *)MEM1_ADD_START, MEM_LEN_TEST, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

munmap(mem, MEM_LEN_TEST); // will take much longer time (from 50us to 230us on A55) than before

munmap(mem1, MEM_LEN_TEST);

munmap(mem, MEM_LEN_TEST) will finally call unmap_region() in the kernel,
and in free_pgtables() will adjust the start and end in mmu_gather, so when
running to __flush_tlb_range(), the (end - start) will more that 1G but less
than 2G in this case, on the other hand, (MAX_TLBI_OPS * stride) will be 2G
as the stride is 2M.

So in the end if ((end - start) > (MAX_TLBI_OPS * stride)) will not valid,
and will take loops to invalidate the TLB for many times which is time consuming.

Can we make the (MAX_TLBI_OPS * stride) as constant such as 4M (which is the
value before "Avoid synchronous TLB invalidation for intermediate page-table
entries on arm64" merged)? or a smaller value for MAX_TLBI_OPS?

Looking forward your insight on this.

Best regards
Hanjun

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