linux-kernel - Re: [ISSUE] sched/cgroup: Does cpu-cgroup still works fine nowadays?

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Date:	Thu, 15 May 2014 11:46:06 +0800
From:	Michael wang <wangyun@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
	Alex Shi <alex.shi@...aro.org>, Paul Turner <pjt@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [ISSUE] sched/cgroup: Does cpu-cgroup still works fine nowadays?

On 05/14/2014 05:44 PM, Peter Zijlstra wrote:
[snip]
>> and then:
>> 	echo $$ > /sys/fs/cgroup/cpu/A/tasks ; ./my_tool -l
>> 	echo $$ > /sys/fs/cgroup/cpu/B/tasks ; ./my_tool -l
>> 	echo $$ > /sys/fs/cgroup/cpu/C/tasks ; ./my_tool 50
>>
>> the results in top is around:
>>
>> 		A	B	C
>> 	CPU%	550	550	100
> 
> top doesn't do per-cgroup accounting, so how do you get these numbers,
> per the above all instances of the prog are also called the same,
> further making it error prone and difficult to get sane numbers.

Oh, my bad to make it confusing, I myself was checking the PID of my_tool
instant inside top, like:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
24968 root      20   0 55600  720  648 S 558.1  0.0   2:08.76 my_tool           
24984 root      20   0 55600  720  648 S 536.2  0.0   1:10.29 my_tool           
25001 root      20   0 55600  720  648 S 88.6  0.0   0:04.39 my_tool

By 'cat /sys/fs/cgroup/cpu/C/tasks' I got the PID of './my_tool 50' is
25001, and all it's pthread's %CPU was count in, could we check like
that?

> 
> 
[snip]
>> void consume(int spin, int total)
>> {
>> 	unsigned long long begin, now;
>> 	begin = stamp();
>>
>> 	for (;;) {
>> 		pthread_mutex_lock(&my_mutex);
>> 		now = stamp();
>> 		if ((long long)(now - begin) > spin) {
>> 			pthread_mutex_unlock(&my_mutex);
>> 			usleep(total - spin);
>> 			pthread_mutex_lock(&my_mutex);
>> 			begin += total;
>> 		}
>> 		pthread_mutex_unlock(&my_mutex);
>> 	}
>> }
> 
> Uh,.. that's just insane.. what's the point of having a multi-threaded
> program do busy-wait loops if you then serialize the lot on a global
> mutex such that only 1 thread can run at any one time?
> 
> How can one such prog ever consume more than 100% cpu.

That's a good point... however the top show that when only './my_tool 50'
25001 running, it used around 300%, like below:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
25001 root      20   0 55600  720  648 S 284.3  0.0   5:18.00 my_tool           
 2376 root      20   0  950m  85m  29m S  4.4  0.2 163:47.94 python             
 1658 root      20   0 1013m  19m  11m S  3.0  0.1  97:06.11 libvirtd

IMHO, if pthread-mutex was similar like the kernel one's behaviour, then
it may not going to sleep when it's the only one running on CPU.

Oh, I think we got the reason here, when there are other task running,
mutex will going to sleep and the %CPU dropped to serialized case that is
around 100%.

But for the dbench, stress combination, that's not spin-wasted, dbench
throughput do dropped, how could we explain that one?

Regards,
Michael Wang

> 

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