Messages in this thread | | | Date | Fri, 16 Apr 2010 19:22:24 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation |
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On Fri, Apr 16, 2010 at 06:29:02PM +1000, Benjamin Herrenschmidt wrote: > On Fri, 2010-04-16 at 18:18 +1000, Nick Piggin wrote: > > On Fri, Apr 16, 2010 at 04:51:34PM +1000, Benjamin Herrenschmidt wrote: > > > On Mon, 2010-04-12 at 20:43 -0700, Paul E. McKenney wrote: > > > > > So we might have to support the interrupt assumption, at least in > > > > some > > > > > form, with those guys... > > > > > > > > One way to make the interrupt assumption official is to use > > > > synchronize_sched() rather than synchronize_rcu(). > > > > > > Ok, so I'm a bit of a RCU newbie as you may know :-) Right now, we use > > > neither, we use call_rcu and we free the pages from the callback. > > > > BTW. you currently have an interesting page table freeing path where > > you usually free by RCU, but (occasionally) free by IPI. This means > > you need to disable both RCU and interrupts to walk page tables. > > Well, the point is we use interrupts to synchronize. The fact that RCU > used to do the job was an added benefit. I may need to switch to rcu > _sched variants tho to keep that. The IPI case is a slow path in case we > are out of memory and cannot allocate our page of RCU batch.
It is the slowpath but it forces all lookup paths to do irq disable too.
> > If you change it to always use RCU, then you wouldn't need to disable > > interrupts. Whether this actually matters anywhere in your mm code, I > > don't know (it's probably not terribly important for gup_fast). But > > rcu disable is always preferable for latency and performance. > > Well, the main case is the hash miss and that always runs with IRQs off.
Probably not a big deal then.
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