Re: Soft lockup in __slab_free (SLUB)

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@linux.vnet.ibm.com> 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?

							Thanx, Paul

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