kern/172166: Deadlock in the networking code, possible due to a bug in the SCHED_ULE

[Alexander Motin]

The following reply was made to PR kern/172166; it has been noted by GNATS.

From: Alexander Motin <mav at FreeBSD.org>
To: Eugene Grosbein <egrosbein at rdtc.ru>
Cc: bug-followup at FreeBSD.org, eugen at eg.sd.rdtc.ru, 
 Andriy Gapon <avg at FreeBSD.org>
Subject: Re: kern/172166: Deadlock in the networking code, possible due to
 a bug in the SCHED_ULE
Date: Tue, 02 Oct 2012 11:45:23 +0300

 On 02.10.2012 10:59, Eugene Grosbein wrote:
 > 02.10.2012 14:53, Alexander Motin ÐÉÛÅÔ:
 >> On 02.10.2012 10:48, Eugene Grosbein wrote:
 >>> 02.10.2012 13:58, Alexander Motin ÐÉÛÅÔ:
 >>>> About rw_lock priority propagation locking(9) tells:
 >>>> The rw_lock locks have priority propagation like mutexes, but priority
 >>>> can be propagated only to an exclusive holder.  This limitation comes
 >>>> from the fact that shared owners are anonymous.
 >>>>
 >>>> What's about idle stealing threshold, it was fixed in HEAD at r239194,
 >>>> but wasn't merged yet. It should be trivial to merge it.
 >>>
 >>> Would it fix my problem with 6-CPU box?
 >>> Your commit log talks about "8 or more cores".
 >>
 >> Hmm. Then I see no reason why threads were not stolen, unless they are
 >> bound to specific CPU. Check `sysctl kern.sched.steal_thresh` output to
 >> be sure.
 >
 > All NIC's threads and dummynet are bound in my boxes.
 > igb(4) in RELENG_8 bounds its threads by default in very wrong way,
 > so I rebound them. dummynet(8) in RELENG_8 goes wild under severe load
 > unless bound to single or two cores.
 
 That can be an answer. Active thread can never never stolen and if it 
 has high absolute priority and never sleeps voluntary -- it will run 
 there forever. If all other threads are bound to that CPU, they also can 
 not be stolen and will wait forever.
 
 > kern.sched.steal_thresh: 2
 
 This should not prevent stealing.
 
 PS: I've just noticed that for some reason I haven't merged my scheduler 
 improvements to 8-STABLE branch. So behavior may differ from one in HEAD 
 or 9-STABLE. I will recheck commits history to recall what stopped me 
 from merge. But I don't remember all details to predict whether it may 
 affect your problem somehow.
 
 -- 
 Alexander Motin