Re: [PATCH] fs/buffer: simplify the code flow of LRU management algorithm

[Minfei Huang]

On 09/28/15 at 02:52pm, yalin wang wrote:
> why not change like this:
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 82283ab..d6769f1 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1287,40 +1287,31 @@ static inline void check_irqs_on(void)
>   */
>  static void bh_lru_install(struct buffer_head *bh)
>  {
> -       struct buffer_head *evictee = NULL;
> +       struct buffer_head *old = NULL;
>  
>         check_irqs_on();
>         bh_lru_lock();
>         if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
> -               struct buffer_head *bhs[BH_LRU_SIZE];
> -               int in;
> +               struct buffer_head *temp;
>                 int out = 0;
>  
> +               old = __this_cpu_read(bh_lrus.bhs[0]);
>                 get_bh(bh);
> -               bhs[out++] = bh;
> -               for (in = 0; in < BH_LRU_SIZE; in++) {
> -                       struct buffer_head *bh2 =
> -                               __this_cpu_read(bh_lrus.bhs[in]);
> -
> -                       if (bh2 == bh) {
> -                               __brelse(bh2);
> +               __this_cpu_write(bh_lrus.bhs[out++], bh);
> +               for (; out < BH_LRU_SIZE; out++) {
> +                       if (old == bh || old == NULL) {
> +                               break;
>                         } else {
> -                               if (out >= BH_LRU_SIZE) {
> -                                       BUG_ON(evictee != NULL);
> -                                       evictee = bh2;
> -                               } else {
> -                                       bhs[out++] = bh2;
> -                               }
> +                               temp = __this_cpu_read(bh_lrus.bhs[out]);
> +                               __this_cpu_write(bh_lrus.bhs[out], old);
> +                               old = temp;

If we should copy the successive struct buffer_head, it is appropriate
to use memcpy to copy a bunch of struct buffer_head.

Thanks
Minfei

>                         }
>                 }
> -               while (out < BH_LRU_SIZE)
> -                       bhs[out++] = NULL;
> -               memcpy(this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
>         }
>         bh_lru_unlock();
>  
> -       if (evictee)
> -               __brelse(evictee);
> +       if (old)
> +               __brelse(old);
>  }
>  
>  /*
> 
> 
> more simple to understand and have better performance .
> am i understanding correctly ?
> 
> > On Sep 28, 2015, at 13:36, Minfei Huang <mhuang@xxxxxxxxxx> wrote:
> > 
> > Ping, Could you someone help to review this patch?
> > 
> > Thanks
> > Minfei
> > 
> > On 09/10/15 at 04:09pm, Minfei Huang wrote:
> >> From: Minfei Huang <mnfhuang@xxxxxxxxx>
> >> 
> >> There is a buffer_head lru list cache in local cpu to accelerate the
> >> speed. The LRU management algorithm is simple enough in
> >> bh_lru_install().
> >> 
> >> There are three situtaions we should deal with.
> >> 1) All/part of the lru cache is NULL.
> >> 2) The new buffer_head hitts the lru cache.
> >> 3) The new buffer_head does hit the lru cache.
> >> 
> >> We put the new buffer_head at the head of lru cache, then copy the
> >> buffer_head from the original lru cache, and drop the spare.
> >> 
> >> Signed-off-by: Minfei Huang <mnfhuang@xxxxxxxxx>
> >> ---
> >> fs/buffer.c | 32 ++++++++++++++++++++------------
> >> 1 file changed, 20 insertions(+), 12 deletions(-)
> >> 
> >> diff --git a/fs/buffer.c b/fs/buffer.c
> >> index 1cf7a53..2139574 100644
> >> --- a/fs/buffer.c
> >> +++ b/fs/buffer.c
> >> @@ -1287,8 +1287,6 @@ static inline void check_irqs_on(void)
> >>  */
> >> static void bh_lru_install(struct buffer_head *bh)
> >> {
> >> -	struct buffer_head *evictee = NULL;
> >> -
> >> 	check_irqs_on();
> >> 	bh_lru_lock();
> >> 	if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
> >> @@ -1302,25 +1300,35 @@ static void bh_lru_install(struct buffer_head *bh)
> >> 			struct buffer_head *bh2 =
> >> 				__this_cpu_read(bh_lrus.bhs[in]);
> >> 
> >> -			if (bh2 == bh) {
> >> +			if (bh2 == NULL) {
> >> +				/* Rest value in bh_lrus.bhs always is NULL */
> >> +				break;
> >> +			} else if (bh2 == bh) {
> >> 				__brelse(bh2);
> >> 			} else {
> >> -				if (out >= BH_LRU_SIZE) {
> >> -					BUG_ON(evictee != NULL);
> >> -					evictee = bh2;
> >> +				if (out == BH_LRU_SIZE) {
> >> +					/*
> >> +					 * this condition will be happened,
> >> +					 * only if none of entry in
> >> +					 * bh_lrus.bhs hits the new bh,
> >> +					 * so the last bh should be released.
> >> +					 */
> >> +					BUG_ON(in != BH_LRU_SIZE - 1);
> >> +					__brelse(bh2);
> >> +					break;
> >> 				} else {
> >> 					bhs[out++] = bh2;
> >> 				}
> >> 			}
> >> 		}
> >> -		while (out < BH_LRU_SIZE)
> >> -			bhs[out++] = NULL;
> >> -		memcpy(this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
> >> +		/*
> >> +		 * it is fine that the value out may be smaller than
> >> +		 * BH_LRU_SIZE. The rest of the value in bh_lrus.bhs is NULL.
> >> +		 */
> >> +		memcpy(this_cpu_ptr(&bh_lrus.bhs), bhs,
> >> +				sizeof(struct buffer_head *) * out);
> >> 	}
> >> 	bh_lru_unlock();
> >> -
> >> -	if (evictee)
> >> -		__brelse(evictee);
> >> }
> >> 
> >> /*
> >> -- 
> >> 2.1.0
> >> 
> >> --
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