Re: [PATCH v2 1/2] mm: clear pte for folios that are zero filled

[Usama Arif <usamaarif642@xxxxxxxxx>]

On 05/06/2024 09:55, Shakeel Butt wrote:
> On Tue, Jun 04, 2024 at 11:58:24AM GMT, Usama Arif wrote:
> [...]
> >   
> > +static bool is_folio_page_zero_filled(struct folio *folio, int i)
> > +{
> > +	unsigned long *data;
> > +	unsigned int pos, last_pos = PAGE_SIZE / sizeof(*data) - 1;
> > +	bool ret = false;
> > +
> > +	data = kmap_local_folio(folio, i * PAGE_SIZE);
> > +
> > +	if (data[last_pos])
> > +		goto out;
> > +
> Use memchr_inv() instead of the following.

I had done some benchmarking before sending v1 and this version is 35% 
faster than using memchr_inv(). Its likely because this does long 
comparison, while memchr_inv does a byte comparison using check_bytes8 
[1]. I will stick with the current version for my next revision. I have 
added the kernel module I used for benchmarking below:

[308797.975269] Time taken for orig: 2850 ms
[308801.911439] Time taken for memchr_inv: 3936 ms

[1] https://elixir.bootlin.com/linux/v6.9.3/source/lib/string.c#L800


#include <linux/time.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/string.h>

#define ITERATIONS 10000000
static int is_page_zero_filled(void *ptr, unsigned long *value)
{
    unsigned long *page;
    unsigned long val;
    unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1;

    page = (unsigned long *)ptr;
    val = page[0];

    if (page[last_pos] != 0)
        return 0;

    for (pos = 1; pos < last_pos; pos++) {
        if (page[pos] != 0)
            return 0;
    }

    *value = val;

    return 1;
}

static int is_page_zero_filled_memchr_inv(void *ptr, unsigned long *value)
{
    unsigned long *page;
    unsigned long val;
    unsigned long *ret;
    page = (unsigned long *)ptr;

    val = page[0];
    *value = val;

    ret = memchr_inv(ptr, 0, PAGE_SIZE);

    return ret == NULL ? 1: 0;
}

static int __init zsmalloc_test_init(void)
{
    unsigned long *src;
    unsigned long value;
    ktime_t start_time, end_time;
    volatile int res = 0;
    unsigned long milliseconds;

    src = kmalloc(PAGE_SIZE, GFP_KERNEL);
    if (!src)
        return -ENOMEM;

    for (unsigned int pos = 0; pos <= PAGE_SIZE / sizeof(*src) - 1; 
pos++) {
        src[pos] = 0x0;
    }

    start_time = ktime_get();
    for (int i = 0; i < ITERATIONS; i++)
        res = is_page_zero_filled(src, &value);
    end_time = ktime_get();
    milliseconds = ktime_ms_delta(end_time, start_time);
    // printk(KERN_INFO "Result: %d, Value: %lu\n", res, value);
    printk(KERN_INFO "Time taken for orig: %lu ms\n", milliseconds);

    start_time = ktime_get();
    for (int i = 0; i < ITERATIONS; i++)
        res = is_page_zero_filled_memchr_inv(src, &value);
    end_time = ktime_get();
    milliseconds = ktime_ms_delta(end_time, start_time);
    // printk(KERN_INFO "Result: %d, Value: %lu\n", res, value);
    printk(KERN_INFO "Time taken for memchr_inv: %lu ms\n", milliseconds);

    kfree(src);
    // Dont insmod so that you can re-run
    return -1;
}

module_init(zsmalloc_test_init);
MODULE_LICENSE("GPL");