On Thu, Jul 14, 2016 at 09:43:40AM -0700, John Stultz wrote: > 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.
Well that is weird, did you put a tracepoint/printk in synchronize_sched() to ensure we don't end up calling that?
