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

[Shi, Yang]

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@xxxxxxxxxx>
> > ---
> > 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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