[RFC PATCH v2 3/6] sched: pack small tasks

[Vincent Guittot]

On 14 December 2012 08:45, Mike Galbraith <bitbucket at online.de> wrote:
> On Fri, 2012-12-14 at 14:36 +0800, Alex Shi wrote:
>> On 12/14/2012 12:45 PM, Mike Galbraith wrote:
>> >> > Do you have further ideas for buddy cpu on such example?
>> >>> > >
>> >>> > > Which kind of sched_domain configuration have you for such system ?
>> >>> > > and how many sched_domain level have you ?
>> >> >
>> >> > it is general X86 domain configuration. with 4 levels,
>> >> > sibling/core/cpu/numa.
>> > CPU is a bug that slipped into domain degeneration.  You should have
>> > SIBLING/MC/NUMA (chasing that down is on todo).
>>
>> Maybe.
>> the CPU/NUMA is different on domain flags, CPU has SD_PREFER_SIBLING.
>
> What I noticed during (an unrelated) bisection on a 40 core box was
> domains going from so..
>
> 3.4.0-bisect (virgin)
> [    5.056214] CPU0 attaching sched-domain:
> [    5.065009]  domain 0: span 0,32 level SIBLING
> [    5.075011]   groups: 0 (cpu_power = 589) 32 (cpu_power = 589)
> [    5.088381]   domain 1: span 0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76 level MC
> [    5.107669]    groups: 0,32 (cpu_power = 1178)  4,36 (cpu_power = 1178)  8,40 (cpu_power = 1178) 12,44 (cpu_power = 1178)
>                          16,48 (cpu_power = 1177) 20,52 (cpu_power = 1178) 24,56 (cpu_power = 1177) 28,60 (cpu_power = 1177)
>                          64,72 (cpu_power = 1176) 68,76 (cpu_power = 1176)
> [    5.162115]    domain 2: span 0-79 level NODE
> [    5.171927]     groups: 0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76 (cpu_power = 11773)
>                            1,5,9,13,17,21,25,29,33,37,41,45,49,53,57,61,65,69,73,77 (cpu_power = 11772)
>                            2,6,10,14,18,22,26,30,34,38,42,46,50,54,58,62,66,70,74,78 (cpu_power = 11773)
>                            3,7,11,15,19,23,27,31,35,39,43,47,51,55,59,63,67,71,75,79 (cpu_power = 11770)
>
> ..to so, which looks a little bent.  CPU and MC have identical spans, so
> CPU should have gone away, as it used to do.
>
> 3.6.0-bisect (virgin)
> [    3.978338] CPU0 attaching sched-domain:
> [    3.987125]  domain 0: span 0,32 level SIBLING
> [    3.997125]   groups: 0 (cpu_power = 588) 32 (cpu_power = 589)
> [    4.010477]   domain 1: span 0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76 level MC
> [    4.029748]    groups: 0,32 (cpu_power = 1177)  4,36 (cpu_power = 1177)  8,40 (cpu_power = 1178) 12,44 (cpu_power = 1178)
>                          16,48 (cpu_power = 1178) 20,52 (cpu_power = 1178) 24,56 (cpu_power = 1178) 28,60 (cpu_power = 1178)
>                          64,72 (cpu_power = 1178) 68,76 (cpu_power = 1177)
> [    4.084143]    domain 2: span 0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76 level CPU
> [    4.103796]     groups: 0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76 (cpu_power = 11777)
> [    4.124373]     domain 3: span 0-79 level NUMA
> [    4.134369]      groups: 0,4,8,12,16,20,24,28,32,36,40,44,48,52,56,60,64,68,72,76 (cpu_power = 11777)
>                             1,5,9,13,17,21,25,29,33,37,41,45,49,53,57,61,65,69,73,77 (cpu_power = 11778)
>                             2,6,10,14,18,22,26,30,34,38,42,46,50,54,58,62,66,70,74 ,78 (cpu_power = 11778)
>                             3,7,11,15,19,23,27,31,35,39,43,47,51,55,59,63,67,71,75,79 (cpu_power = 11780)
>

Thanks. that's an interesting example of a numa topology

For your sched_domain difference,
On 3.4, SD_PREFER_SIBLING was set for both MC and CPU level thanks to
sd_balance_for_mc_power and  sd_balance_for_package_power
On 3.6, SD_PREFER_SIBLING is only set for CPU level and this flag
difference with MC level prevents the destruction of CPU sched_domain
during the degeneration

We may need to set SD_PREFER_SIBLING for MC level

Vincent

>         -Mike
>