On Mon, Apr 9, 2018 at 9:51 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > On Mon, Apr 9, 2018 at 9:49 AM, Jan Kara <jack@xxxxxxx> wrote: >> On Sat 07-04-18 12:38:24, Dan Williams wrote: > [..] >>> I wonder if this can be trivially solved by using srcu. I.e. we don't >>> need to wait for a global quiescent state, just a >>> get_user_pages_fast() quiescent state. ...or is that an abuse of the >>> srcu api? >> >> Well, I'd rather use the percpu rwsemaphore (linux/percpu-rwsem.h) than >> SRCU. It is a more-or-less standard locking mechanism rather than relying >> on implementation properties of SRCU which is a data structure protection >> method. And the overhead of percpu rwsemaphore for your use case should be >> about the same as that of SRCU. > > I was just about to ask that. Yes, it seems they would share similar > properties and it would be better to use the explicit implementation > rather than a side effect of srcu. ...unfortunately: BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:34 [..] Call Trace: dump_stack+0x85/0xcb ___might_sleep+0x15b/0x240 dax_layout_lock+0x18/0x80 get_user_pages_fast+0xf8/0x140 ...and thinking about it more srcu is a better fit. We don't need the 100% exclusion provided by an rwsem we only need the guarantee that all cpus that might have been running get_user_pages_fast() have finished it at least once. In my tests synchronize_srcu is a bit slower than unpatched for the trivial 100 truncate test, but certainly not the 200x latency you were seeing with syncrhonize_rcu. Elapsed time: 0.006149178 unpatched 0.009426360 srcu
