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

[yalin wang <yalin.wang2010@gmail.com>]

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;
                        }
                }
-               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@redhat.com> 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@gmail.com>
>> 
>> 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@gmail.com>
>> ---
>> 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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