On Thu, Jul 14, 2016 at 6:18 AM, Peter Zijlstra <pet...@infradead.org> wrote: > On Wed, Jul 13, 2016 at 10:51:02PM +0200, Peter Zijlstra wrote: >> So, IIRC, the trade-off is a full memory barrier in read_lock and >> read_unlock() vs sync_sched() in write. >> >> Full memory barriers are expensive and while the combined cost might >> well exceed the cost of the sync_sched() it doesn't suffer the latency >> issues. >> >> Not sure if we can frob the two in a single codebase, but I can have a >> poke if Oleg or Paul doesn't beat me to it. > > OK, not too horrible if I say so myself :-) > > The below is a compile tested only first draft so far. I'll go give it > some runtime next.
Unfortunately it didn't apply cleanly to the 4.4 based tree I'm working with, so I had to manually apply the entirety of the percpu-rwsem.c changes myself. Hopefully I didn't screw it up. So running with this, I'm still seeing some pretty large delays. 80ms peak, with lots of >20ms values as well. So it doesn't seem to have the positive effect that Paul's change provided. thanks -john