linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Thu, 18 Dec 2014 23:03:08 -0500
From:	Dave Jones <davej@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Suresh Siddha <sbsiddha@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Anvin <hpa@...ux.intel.com>
Subject: Re: frequent lockups in 3.18rc4

On Thu, Dec 18, 2014 at 10:58:59PM -0500, Dave Jones wrote:
 >  > lock debugging and other overheads (does this still have
 >  > DEBUG_PAGEALLOC?) you really are getting into a "real" softlockup
 >  > because things are scaling so horribly badly.
 >  > 
 >  > If you now disable spinlock debugging and lockdep, hopefully that page
 >  > table lock now doesn't always get hung up on the lockdep locking, so
 >  > it starts scaling much better, and maybe you'd not see this...
 > 
 > I can give it a shot.  Hopefully there's some further mitigation that
 > could be done to allow a workload like this to survive under a debug
 > build though, as we've caught *so many* bugs with this stuff in the past.
 
Turns out also that this build didn't have PROVE_LOCKING enabled.
CONFIG_LOCKDEP was, but that just bloats the structures a little, and
afaik doesn't incur the same runtime overhead.

I also forgot to answer the question above, PAGEALLOC is also off.

So the only thing that was on that could cause spinlock overhead
was DEBUG_SPINLOCK (and LOCK_STAT, though iirc that's not huge either)

	Dave
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