On Tue, 1 Aug 2017 21:07:35 +0900
leesioh <solee@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> ---
> 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;
--
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