linux-kernel - Re: RT sched: cpupri_vec lock contention with def_root

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Date:	Mon, 24 Nov 2008 13:46:14 -0800
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Li Zefan <lizf@...fujitsu.com>
CC:	Dimitri Sivanich <sivanich@....com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Derek Fults <dfults@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: RT sched: cpupri_vec lock contention with def_root_domain and
 no load balance

Li Zefan wrote:
> Max Krasnyansky wrote:
>> Dimitri Sivanich wrote:
>>> kernel: CPU3 root domain e0000069ecb20000
>>> kernel: CPU3 attaching sched-domain:
>>> kernel:  domain 0: span 3 level NODE
>>> kernel:   groups: 3
>>> kernel: CPU2 root domain e000006884a00000
>>> kernel: CPU2 attaching sched-domain:
>>> kernel:  domain 0: span 2 level NODE
>>> kernel:   groups: 2
>>> kernel: CPU1 root domain e000006884a20000
>>> kernel: CPU1 attaching sched-domain:
>>> kernel:  domain 0: span 1 level NODE
>>> kernel:   groups: 1
>>> kernel: CPU0 root domain e000006884a40000
>>> kernel: CPU0 attaching sched-domain:
>>> kernel:  domain 0: span 0 level NODE
>>> kernel:   groups: 0
>>>
>>> Which is the way sched_load_balance is supposed to work. You need to set
>>> sched_load_balance=0 for all cpusets containing any cpu you want to disable
>>> balancing on, otherwise some balancing will happen.
>> It won't be much of a balancing in this case because this just one cpu per
>> domain.
>> In other words no that's not how it supposed to work. There is code in
>> cpu_attach_domain() that is supposed to remove redundant levels
>> (sd_degenerate() stuff). There is an explicit check in there for numcpus == 1.
>> btw The reason you got a different result that I did is because you have a
>> NUMA box where is mine is UMA. I was able to reproduce the problem though by
>> enabling multi-core scheduler. In which case I also get one redundant domain
>> level CPU, with a single CPU in it.
>> So we definitely need to fix this. I'll try to poke around tomorrow and figure
>> out why redundant level is not dropped.
>>
> 
> You were not using latest kernel, were you?
> 
> There was a bug in sd degenerate code, and it has already been fixed:
> 	http://lkml.org/lkml/2008/11/8/10
Ah, makes sense.
The funny part is that I did see the patch before but completely forgot 
about it :).

>>> So when we do that for just par3, we get the following:
>>> echo 0 > par3/cpuset.sched_load_balance
>>> kernel: cpusets: rebuild ndoms 3
>>> kernel: cpuset: domain 0 cpumask
>>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>>> 0000000,00000000,00000000,00000000,0
>>> kernel: cpuset: domain 1 cpumask
>>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>>> 0000000,00000000,00000000,00000000,0
>>> kernel: cpuset: domain 2 cpumask
>>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>>> 0000000,00000000,00000000,00000000,0
>>> kernel: CPU3 root domain default
>>> kernel: CPU3 attaching NULL sched-domain.
>>>
>>> So the def_root_domain is now attached for CPU 3.  And we do have a NULL
>>> sched-domain, which we expect for a cpu with load balancing turned off.  If
>>> we turn sched_load_balance off ('0') on each of the other cpusets (par0-2),
>>> each of those cpus would also have a NULL sched-domain attached.
>> Ok. This one is a bug in cpuset.c:generate_sched_domains(). Sched domain
>> generator in cpusets should not drop domains with single cpu in them when
>> sched_load_balance==0. I'll look at that tomorrow too.
>>
> 
> Do you mean the correct behavior should be as following?
> 	kernel: cpusets: rebuild ndoms 4
Yes.

> But why do you think this is a bug? In generate_sched_domains(), cpusets with
> sched_load_balance==0 will be skippped:
> 
> 	list_add(&top_cpuset.stack_list, &q);
> 	while (!list_empty(&q)) {
> 		...
> 		if (is_sched_load_balance(cp)) {
> 			csa[csn++] = cp;
> 			continue;
> 		}
> 		...
>   	}
> 
> Correct me if I misunderstood your point.
The problem is that all cpus in cpusets with sched_load_balance==0 end 
up in the default root_domain which causes lock contention.
We can fix it either in sched.c:partition_sched_domains() or in 
cpusets.c:generate_sched_domains(). I'd rather fix cpusets because 
sched.c fix will be sub-optimal. See my answer to Greg on the same 
thread. Basically the scheduler code would have to allocate a 
root_domain for each CPU even on transitional states. So I'd rather fix 
cpusets to generate domain for each non-overlapping cpuset regardless of 
the sched_load_balance flag.

Max
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