Re: [PATCH] mm/ksm : Checksum calculation function change (jhash2 -> crc32)

[Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>]

On Tue,  1 Aug 2017 21:07:35 +0900
leesioh <solee@os.korea.ac.kr> wrote:

> In ksm, the checksum values are used to check changes in page content
> and keep the unstable tree more stable. KSM implements checksum
> calculation with jhash2 hash function. However, because jhash2 is
> implemented in software, it consumes high CPU cycles (about 26%,
> according to KSM thread profiling results)
> 
> To reduce CPU consumption, this commit applies the crc32 hash function
> which is included in the SSE4.2 CPU instruction set.
> This can significantly reduce the page checksum overhead as follows.

This is really a nice idea, but you are assuming that everything is x86.

> I measured checksum computation 300 times to see how fast crc32 is
> compared to jhash2. With jhash2, the average checksum calculation
> time is about 3460ns, and with crc32, the average checksum
> calculation time is 888ns. This is about 74% less than jhash2.

This could be different on other architectures.


I really like the idea, though. Can you maybe extract the jhash part
and make it generic, and then add an arch-specific override for x86?

that way other architectures will be able to plug in their preferred
checksum function.


best regards


Claudio Imbrenda


> Signed-off-by: leesioh <solee@os.korea.ac.kr>
> ---
>  mm/ksm.c | 36 ++++++++++++++++++++++++++++++++----
>  1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 0c927e3..390a3cb 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -42,7 +42,8 @@
> 
>  #include <asm/tlbflush.h>
>  #include "internal.h"
> -
> +#include <linux/crypto.h>
> +#include <crypto/hash.h>
>  #ifdef CONFIG_NUMA
>  #define NUMA(x)		(x)
>  #define DO_NUMA(x)	do { (x); } while (0)
> @@ -260,6 +261,9 @@ static unsigned int zero_checksum __read_mostly;
>  /* Whether to merge empty (zeroed) pages with actual zero pages */
>  static bool ksm_use_zero_pages __read_mostly;
> 
> +/* Whether to support crc32 hash function */
> +static bool crc32_support;
> +
>  #ifdef CONFIG_NUMA
>  /* Zeroed when merging across nodes is not allowed */
>  static unsigned int ksm_merge_across_nodes = 1;
> @@ -279,11 +283,27 @@ static void wait_while_offlining(void);
>  static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
>  static DEFINE_MUTEX(ksm_thread_mutex);
>  static DEFINE_SPINLOCK(ksm_mmlist_lock);
> -
> +static struct shash_desc desc;
> +static struct crypto_shash *tfm;
>  #define KSM_KMEM_CACHE(__struct, __flags)
> kmem_cache_create("ksm_"#__struct,\ sizeof(struct __struct),
> __alignof__(struct __struct),\ (__flags), NULL)
> 
> +static void __init ksm_crc32_init(void)
> +{
> +	tfm = crypto_alloc_shash("crc32", 0, CRYPTO_ALG_ASYNC);
> +
> +	if (IS_ERR(tfm) || tfm->base.__crt_alg->cra_priority < 200) {
> +		pr_warn("not support crc32 instruction, use jhash2
> \n");
> +		crc32_support = false;
> +		crypto_free_shash(tfm);
> +		return;
> +	}
> +	desc.tfm = tfm;
> +	desc.flags = 0;
> +	crc32_support = true;
> +}
> +
>  static int __init ksm_slab_init(void)
>  {
>  	rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
> @@ -986,7 +1006,14 @@ static u32 calc_checksum(struct page *page)
>  {
>  	u32 checksum;
>  	void *addr = kmap_atomic(page);
> -	checksum = jhash2(addr, PAGE_SIZE / 4, 17);
> +	/*
> +	* If crc32 is supported, the use crc32 to calculate the
> checksum
> +	* otherwise use jhash2
> +	*/
> +	if (crc32_support)
> +		crypto_shash_digest(&desc, addr, PAGE_SIZE, (u8
> *)&checksum);
> +	else
> +		checksum = jhash2(addr, PAGE_SIZE / 4, 17);
>  	kunmap_atomic(addr);
>  	return checksum;
>  }
> @@ -3057,7 +3084,8 @@ static int __init ksm_init(void)
>  	zero_checksum = calc_checksum(ZERO_PAGE(0));
>  	/* Default to false for backwards compatibility */
>  	ksm_use_zero_pages = false;
> -
> +
> +	ksm_crc32_init();
>  	err = ksm_slab_init();
>  	if (err)
>  		goto out;

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>