Re: [PATCH] locking/rwsem: Make owner store task pointer of last owning reader

[Peter Zijlstra]

On Mon, Sep 10, 2018 at 10:15:50AM -0700, Davidlohr Bueso wrote:
> On Mon, 10 Sep 2018, Waiman Long wrote:
> 
> > One major issue with a combined count/owner is that we may have to use
> > cmpxchg for reader lock which will certainly impact reader-heavy
> > workloads. I have also thought about ways to compress the task pointer
> > address so that it can use fewer bits and leave the rests for reader
> > count. It is probably doable on 64-bit systems, but likely not on 32-bit
> > system given that there are less bits to play around.
> 
> Yeah we've discussed this before. As a cleanup it would obviously be good,
> but I fear about raw performance loss when using cmpxchg instead of xadd.

Does it really matter though? Last time I looked at something similar
(refcount_t) the "LOCK INCL" vs "LOCK CMPXCHG" was something like 15 vs
23 cycles (and that was with the cmpxchg loop actually doing a lot
more).

Do we really care about the down_read() path _that_ much? I thought that
with the main pain point, pagefaults, the problem was mostly the line
bouncing, not a few extra cycles.