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

[Michal Hocko]

On Mon 29-02-16 09:27:44, Shi, Yang wrote:
> 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.

If this is supposed to be a performance enhancement then some numbers
would definitely make it easier to get in. Or even an arguments to back
your theory. Basing your argument on 4+ years commit doesn't really seem
sound... Just to make it clear, I am not opposing the patch I just
stumbled over it and the changelog was just too terrible which made me
response.
-- 
Michal Hocko
SUSE Labs