On Wed, 28 Sep 2016 19:47:05 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Sep 29, 2016 at 12:30:07PM +1000, Nicholas Piggin wrote:
> > On Wed, 28 Sep 2016 19:11:00 -0700
> > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > > On Thu, Sep 29, 2016 at 10:40:24AM +0900, Joonsoo Kim wrote:
> > > > On Wed, Sep 28, 2016 at 10:15:01AM +0300, Nikolay Borisov wrote:
> > > > >
> > > > > I don't think it's an RCU problem per-se since ext4_i_callback is being
> > > > > called from RCU due to the way inodes are being freed.
> > > >
> > > > That doesn't mean that RCU has no problem. IIUC, the fact is that RCU
> > > > has no scheduling point in rcu_process_callbacks() and it would be
> > > > problematic. It just depends on workload.
> > >
> > > You mean rcu_do_batch()? It does limit the callbacks invoked per call
> > > to rcu_do_batch() under normal conditions, see the "++count >= bl" check.
> > >
> > > Now, if you dump a huge number of callbacks down call_rcu()'s throat,
> > > it will stop being Mr. Nice Guy and will start executing the callbacks
> > > as fast as it can for potentially quite some time. But a huge number
> > > will be in the millions. Per CPU. In which case I just might have a
> > > few questions about exactly what you are trying to do.
> > >
> > > Nevertheless, it is entirely possible that RCU's callback-invocation
> > > throttling strategy needs improvement.
> >
> > Would it be useful to have a call_rcu variant that may sleep. Callers would
> > use it preferentially if they can. Implementation might be exactly the same
> > for now, but it would give you more flexibility with throttling strategies
> > in future.
>
> You can specify callback-offloading at build and boot time, which will have
> each CPU's callbacks being processed by a kernel thread:
>
> CONFIG_RCU_NOCB_CPU
> CONFIG_RCU_NOCB_CPU_{NONE,ZERO,ALL}
> rcu_nocbs=
>
> However, this still executes the individual callbacks with bh disabled.
> If you want the actual callbacks themselves to be able to sleep, make
> the callback hand off to a workqueue, wake up a kthread, or some such.
>
> But yes, if enough people were just having the RCU callback immediately
> invoke a workqueue, that could easily be special cased, just as
> kfree_rcu() is now.
>
> Or am I missing your point?
I just meant where the call_rcu() caller can sleep. RCU could block
there to throttle production if necessary.
Thanks,
Nick
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