x86/mm: New tunable for single vs full TLB flush

When the kernel unmaps or modified the attributes of a range of

memory, it has two choices:

 1. Flush the entire TLB with a two-instruction sequence.  This is

    a quick operation, but it causes collateral damage: TLB entries

    from areas other than the one we are trying to flush will be

    destroyed and must be refilled later, at some cost.

 2. Use the invlpg instruction to invalidate a single page at a

    time.  This could potentialy cost many more instructions, but

    it is a much more precise operation, causing no collateral

    damage to other TLB entries.

Which method to do depends on a few things:

 1. The size of the flush being performed.  A flush of the entire

    address space is obviously better performed by flushing the

    entire TLB than doing 2^48/PAGE_SIZE individual flushes.

 2. The contents of the TLB.  If the TLB is empty, then there will

    be no collateral damage caused by doing the global flush, and

    all of the individual flush will have ended up being wasted

    work.

 3. The size of the TLB.  The larger the TLB, the more collateral

    damage we do with a full flush.  So, the larger the TLB, the

    more attrative an individual flush looks.  Data and

    instructions have separate TLBs, as do different page sizes.

 4. The microarchitecture.  The TLB has become a multi-level

    cache on modern CPUs, and the global flushes have become more

    expensive relative to single-page flushes.

There is obviously no way the kernel can know all these things,

especially the contents of the TLB during a given flush.  The

sizes of the flush will vary greatly depending on the workload as

well.  There is essentially no "right" point to choose.

You may be doing too many individual invalidations if you see the

invlpg instruction (or instructions _near_ it) show up high in

profiles.  If you believe that individual invalidations being

called too often, you can lower the tunable:

	/sys/debug/kernel/x86/tlb_single_page_flush_ceiling

This will cause us to do the global flush for more cases.

Lowering it to 0 will disable the use of the individual flushes.

Setting it to 1 is a very conservative setting and it should

never need to be 0 under normal circumstances.

Despite the fact that a single individual flush on x86 is

guaranteed to flush a full 2MB [1], hugetlbfs always uses the full

flushes.  THP is treated exactly the same as normal memory.

You might see invlpg inside of flush_tlb_mm_range() show up in

profiles, or you can use the trace_tlb_flush() tracepoints. to

determine how long the flush operations are taking.

Essentially, you are balancing the cycles you spend doing invlpg

with the cycles that you spend refilling the TLB later.

You can measure how expensive TLB refills are by using

performance counters and 'perf stat', like this:

perf stat -e

	cpu/event=0x8,umask=0x84,name=dtlb_load_misses_walk_duration/,

	cpu/event=0x8,umask=0x82,name=dtlb_load_misses_walk_completed/,

	cpu/event=0x49,umask=0x4,name=dtlb_store_misses_walk_duration/,

	cpu/event=0x49,umask=0x2,name=dtlb_store_misses_walk_completed/,

	cpu/event=0x85,umask=0x4,name=itlb_misses_walk_duration/,

	cpu/event=0x85,umask=0x2,name=itlb_misses_walk_completed/

That works on an IvyBridge-era CPU (i5-3320M).  Different CPUs

may have differently-named counters, but they should at least

be there in some form.  You can use pmu-tools 'ocperf list'

(https://github.com/andikleen/pmu-tools) to find the right

counters for a given CPU.

1. A footnote in Intel's SDM "4.10.4.2 Recommended Invalidation"

   says: "One execution of INVLPG is sufficient even for a page

   with size greater than 4 KBytes."

		on_each_cpu(do_kernel_range_flush, &info, 1);

	}

}

static ssize_t tlbflush_read_file(struct file *file, char __user *user_buf,

			     size_t count, loff_t *ppos)

{

	char buf[32];

	unsigned int len;

	len = sprintf(buf, "%ld\n", tlb_single_page_flush_ceiling);

	return simple_read_from_buffer(user_buf, count, ppos, buf, len);

}

static ssize_t tlbflush_write_file(struct file *file,

		 const char __user *user_buf, size_t count, loff_t *ppos)

{

	char buf[32];

	ssize_t len;

	int ceiling;

	len = min(count, sizeof(buf) - 1);

	if (copy_from_user(buf, user_buf, len))

		return -EFAULT;

	buf[len] = '\0';

	if (kstrtoint(buf, 0, &ceiling))

		return -EINVAL;

	if (ceiling < 0)

		return -EINVAL;

	tlb_single_page_flush_ceiling = ceiling;

	return count;

}

static const struct file_operations fops_tlbflush = {

	.read = tlbflush_read_file,

	.write = tlbflush_write_file,

	.llseek = default_llseek,

};

static int __init create_tlb_single_page_flush_ceiling(void)

{

	debugfs_create_file("tlb_single_page_flush_ceiling", S_IRUSR | S_IWUSR,

			    arch_debugfs_dir, NULL, &fops_tlbflush);

	return 0;

}

late_initcall(create_tlb_single_page_flush_ceiling);