Messages in this thread | | | Date | Tue, 11 Sep 2018 10:17:55 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/rwsem: Make owner store task pointer of last owning reader |
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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.
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