fOn 17/02/2025 17:58, Matthew Wilcox wrote:
> On Mon, Feb 17, 2025 at 05:09:34PM +0000, Colin Ian King wrote:
>> Adding an unlikely() hint on the masked start comparison error
>> return path improves run-time performance of the mincore system call.
>>
>> Benchmarking on an i9-12900 shows an improvement of 7ns on mincore calls
>> on a 256KB mmap'd region where 50% of the pages we resident.
>>
>> Results based on running 20 tests with turbo disabled (to reduce
>> clock freq turbo changes), with 10 second run per test and comparing
>> the number of mincores calls per second. The % standard deviation of
>> the 20 tests was ~0.10%, so results are reliable.
> 
> I think you've elided _just_ enough information here that nobody can
> judge whether your stats skills are any good ;-)  You've told us 7ns
> (per call, presumably) and you've told us 0.10% standard deviation,
> but you haven't told us how long the syscall takes, so nobody can tell
> whether 7ns is within 0.10% or not ;-)

Ugh, my bad.

Improvement was from ~970 down to 963 ns, so small ~0.7% improvement.

Colin

> 
>> Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
>> ---
>>   mm/mincore.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/mincore.c b/mm/mincore.c
>> index d6bd19e520fc..832f29f46767 100644
>> --- a/mm/mincore.c
>> +++ b/mm/mincore.c
>> @@ -239,7 +239,7 @@ SYSCALL_DEFINE3(mincore, unsigned long, start, size_t, len,
>>   	start = untagged_addr(start);
>>   
>>   	/* Check the start address: needs to be page-aligned.. */
>> -	if (start & ~PAGE_MASK)
>> +	if (unlikely(start & ~PAGE_MASK))
>>   		return -EINVAL;
> 
> We might get even more advantage by moving the EINVAL test before
> untagged_addr() since we know that the tags are all in the high bits and
> we don't need to have the test be dependent on the previous arithmetic.