Messages in this thread | | | Date | Fri, 15 Jul 2016 21:47:03 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 1/2] locking/percpu-rwsem: Optimize readers and reduce global impact |
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On Fri, Jul 15, 2016 at 06:30:54PM +0200, Oleg Nesterov wrote: > On 07/14, Peter Zijlstra wrote: > > > > Currently the percpu-rwsem switches to (global) atomic ops while a > > writer is waiting; which could be quite a while and slows down > > releasing the readers. > > > > This patch cures this problem by ordering the reader-state vs > > reader-count (see the comments in __percpu_down_read() and > > percpu_down_write()). This changes a global atomic op into a full > > memory barrier, which doesn't have the global cacheline contention. > > I've applied this patch + another change you sent on top of it. > > Everything looks good to me except the __this_cpu_inc() in > __percpu_down_read(), > > > + __down_read(&sem->rw_sem); > > + __this_cpu_inc(*sem->read_count); > > + __up_read(&sem->rw_sem); > > Preemption is already enabled, don't we need this_cpu_inc() ?
Ah indeed. This mistake is quite old it seems, good catch.
> > -EXPORT_SYMBOL_GPL(percpu_up_write); > > +EXPORT_SYMBOL(percpu_up_write); > > and this one ;) I do not really care, but it seems you did this change > by accident.
Yep, oops ;-)
> Actually, I _think_ we can do some cleanups/improvements on top of this > change, but we can do this later. In particular, _perhaps_ we can avoid > the unconditional wakeup in __percpu_up_read(), but I am not sure and in > any case this needs another change. > > Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Thanks!
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