Re: [RFC PATCH] writeback: move list_lock down into the for loop

["Shi, Yang" <yang.shi@linaro.org>]

On 2/29/2016 7:06 AM, Michal Hocko wrote:
> On Fri 26-02-16 08:46:25, Yang Shi wrote:
>> The list_lock was moved outside the for loop by commit
>> e8dfc30582995ae12454cda517b17d6294175b07 ("writeback: elevate queue_io()
>> into wb_writeback())", however, the commit log says "No behavior change", so
>> it sounds safe to have the list_lock acquired inside the for loop as it did
>> before.
>> Leave tracepoints outside the critical area since tracepoints already have
>> preempt disabled.
>
> The patch says what but it completely misses the why part.

I'm just wondering the finer grained lock may reach a little better 
performance, i.e. more likely for preempt, lower latency.

Thanks,
Yang

>
>>
>> Signed-off-by: Yang Shi <yang.shi@linaro.org>
>> ---
>> Tested with ltp on 8 cores Cortex-A57 machine.
>>
>>   fs/fs-writeback.c | 12 +++++++-----
>>   1 file changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> index 1f76d89..9b7b5f6 100644
>> --- a/fs/fs-writeback.c
>> +++ b/fs/fs-writeback.c
>> @@ -1623,7 +1623,6 @@ static long wb_writeback(struct bdi_writeback *wb,
>>   	work->older_than_this = &oldest_jif;
>>
>>   	blk_start_plug(&plug);
>> -	spin_lock(&wb->list_lock);
>>   	for (;;) {
>>   		/*
>>   		 * Stop writeback when nr_pages has been consumed
>> @@ -1661,15 +1660,19 @@ static long wb_writeback(struct bdi_writeback *wb,
>>   			oldest_jif = jiffies;
>>
>>   		trace_writeback_start(wb, work);
>> +
>> +		spin_lock(&wb->list_lock);
>>   		if (list_empty(&wb->b_io))
>>   			queue_io(wb, work);
>>   		if (work->sb)
>>   			progress = writeback_sb_inodes(work->sb, wb, work);
>>   		else
>>   			progress = __writeback_inodes_wb(wb, work);
>> -		trace_writeback_written(wb, work);
>>
>>   		wb_update_bandwidth(wb, wb_start);
>> +		spin_unlock(&wb->list_lock);
>> +
>> +		trace_writeback_written(wb, work);
>>
>>   		/*
>>   		 * Did we write something? Try for more
>> @@ -1693,15 +1696,14 @@ static long wb_writeback(struct bdi_writeback *wb,
>>   		 */
>>   		if (!list_empty(&wb->b_more_io))  {
>>   			trace_writeback_wait(wb, work);
>> +			spin_lock(&wb->list_lock);
>>   			inode = wb_inode(wb->b_more_io.prev);
>> -			spin_lock(&inode->i_lock);
>>   			spin_unlock(&wb->list_lock);
>> +			spin_lock(&inode->i_lock);
>>   			/* This function drops i_lock... */
>>   			inode_sleep_on_writeback(inode);
>> -			spin_lock(&wb->list_lock);
>>   		}
>>   	}
>> -	spin_unlock(&wb->list_lock);
>>   	blk_finish_plug(&plug);
>>
>>   	return nr_pages - work->nr_pages;
>> --
>> 2.0.2
>>
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