Re: Soft lockup in __slab_free (SLUB)

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>



[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux]     [Linux OMAP]     [Linux MIPS]     [ECOS]     [Asterisk Internet PBX]     [Linux API]