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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