[lustre-devel] [PATCH 22/37] lustre: lprocfs: use log2.h macros instead of shift loop.

[NeilBrown]

On Tue, Feb 26 2019, James Simmons wrote:

>> These shift loops seem to be trying to avoid doing a
>> multiplication.
>> We same effect can be achieved more transparently using
>> rounddown_pow_of_two().  Even though there is a multiplication
>> in the C code, the resulting machine code just does a single shift.
>
> Be aware rounddown_pow_of_two(n) is undefined when "n == 0". 

Hmm... can os_bsize ever be less than 1024?  I guess we still need to be
careful of the possibility.

The current code treats anything less than 1024 as though it were 1024,
so I could achieve the same thing with 

		result *= rounddown_pow_of_two(blk_size ?: 1);

Is that too obscure?

Thanks,
NeilBrown


>  
>> Signed-off-by: NeilBrown <neilb at suse.com>
>> ---
>>  .../lustre/lustre/obdclass/lprocfs_status.c        |   10 +++-------
>>  1 file changed, 3 insertions(+), 7 deletions(-)
>> 
>> diff --git a/drivers/staging/lustre/lustre/obdclass/lprocfs_status.c b/drivers/staging/lustre/lustre/obdclass/lprocfs_status.c
>> index a179b0d6979e..637aaca96888 100644
>> --- a/drivers/staging/lustre/lustre/obdclass/lprocfs_status.c
>> +++ b/drivers/staging/lustre/lustre/obdclass/lprocfs_status.c
>> @@ -341,9 +341,7 @@ static ssize_t kbytestotal_show(struct kobject *kobj, struct attribute *attr,
>>  		u32 blk_size = osfs.os_bsize >> 10;
>>  		u64 result = osfs.os_blocks;
>>  
>> -		while (blk_size >>= 1)
>> -			result <<= 1;
>> -
>> +		result *= rounddown_pow_of_two(blk_size);
>>  		return sprintf(buf, "%llu\n", result);
>>  	}
>>  
>> @@ -364,8 +362,7 @@ static ssize_t kbytesfree_show(struct kobject *kobj, struct attribute *attr,
>>  		u32 blk_size = osfs.os_bsize >> 10;
>>  		u64 result = osfs.os_bfree;
>>  
>> -		while (blk_size >>= 1)
>> -			result <<= 1;
>> +		result *= rounddown_pow_of_two(blk_size);
>>  
>>  		return sprintf(buf, "%llu\n", result);
>>  	}
>> @@ -387,8 +384,7 @@ static ssize_t kbytesavail_show(struct kobject *kobj, struct attribute *attr,
>>  		u32 blk_size = osfs.os_bsize >> 10;
>>  		u64 result = osfs.os_bavail;
>>  
>> -		while (blk_size >>= 1)
>> -			result <<= 1;
>> +		result *= rounddown_pow_of_two(blk_size);
>>  
>>  		return sprintf(buf, "%llu\n", result);
>>  	}
>> 
>> 
>> 
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