Messages in this thread | | | Subject | Re: [RFC][PATCH 2/5] mm: Switch mod_state() to __this_cpu_read() | From | Steven Rostedt <> | Date | Tue, 20 Sep 2011 12:03:28 -0400 |
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On Tue, 2011-09-20 at 10:59 -0500, Christoph Lameter wrote: > On Tue, 20 Sep 2011, Steven Rostedt wrote: > > > On Tue, 2011-09-20 at 09:51 -0500, Christoph Lameter wrote: > > > > > I see that __this_cpu_xx operations may not work as intended in > > > preemptable contexts and there we could have more changes. > > > > Then why do you use it in slub.c? > > > > in mm/slab.c slab_alloc(): > > > > redo: > > > > /* > > * Must read kmem_cache cpu data via this cpu ptr. Preemption is > > * enabled. We may switch back and forth between cpus while > > * reading from one cpu area. That does not matter as long > > * as we end up on the original cpu again when doing the cmpxchg. > > */ > > c = __this_cpu_ptr(s->cpu_slab); > > > > The __this_cpu_*() is in preempt enabled location. In fact, the three > > this_cpu_write()'s in acquire_slab() is done within a spinlock and thus > > with preemption disabled. > > So the this_cpu_writes could become __this_cpu_writes > > The __this_cpu_ptr operation in slab_alloc is special. We explicitly do > address calculation and do not care from which per cpu area we fetch > multiple per cpu data items because we can verify that we are on the same > per cpu area during the this_cpu_cmpxchg_double operation.
ARGH! You now have special cases that break the sanity of it all? This just proves that the design is wrong.
> > > The point is, people get it wrong all the time. In fact, we should > > really require that ALL USES of this_cpu_*() must be with preemption > > disabled. Regardless. Because anytime you touch a per cpu variable, > > NO!! This defeats the whole purpose of this_cpu_ops and make the whole > scheme utterly useless.
The thing is, the whole purpose was broken to begin with. Defeating a broken design is a good thing!
> > > Monkeying around with per cpu data is tricky. If you start doing it in > > preempt enabled code, you are most certainly about to get it wrong. Why > > have this super optimization. A preempt_disable() is a single operation > > that touches cache hot data. > > There are trivial cases like counter increments that are not a problem at > all. Most use cases are those. More complex ones can be developed to avoid > various overhead in performance critical sections of the kernel. >
And adding a preempt_disable; this_cpu_inc(); preempt_enable; is not a bad thing either.
What benchmarks do you have that shows this helped in anything????
-- Steve
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