Messages in this thread | | | Date | Wed, 30 Oct 2013 15:33:32 +0530 | From | Preeti U Murthy <> | Subject | Re: [PATCH V2 2/2] sched: Remove un-necessary iteration over sched domains to update nr_busy_cpus |
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Hi Kamalesh,
On 10/30/2013 02:53 PM, Kamalesh Babulal wrote: > Hi Preeti, > >> nr_busy_cpus parameter is used by nohz_kick_needed() to find out the number >> of busy cpus in a sched domain which has SD_SHARE_PKG_RESOURCES flag set. >> Therefore instead of updating nr_busy_cpus at every level of sched domain, >> since it is irrelevant, we can update this parameter only at the parent >> domain of the sd which has this flag set. Introduce a per-cpu parameter >> sd_busy which represents this parent domain. >> >> In nohz_kick_needed() we directly query the nr_busy_cpus parameter >> associated with the groups of sd_busy. >> >> By associating sd_busy with the highest domain which has >> SD_SHARE_PKG_RESOURCES flag set, we cover all lower level domains which could >> have this flag set and trigger nohz_idle_balancing if any of the levels have >> more than one busy cpu. >> >> sd_busy is irrelevant for asymmetric load balancing. However sd_asym has been >> introduced to represent the highest sched domain which has SD_ASYM_PACKING flag set >> so that it can be queried directly when required. >> >> While we are at it, we might as well change the nohz_idle parameter to be >> updated at the sd_busy domain level alone and not the base domain level of a CPU. >> This will unify the concept of busy cpus at just one level of sched domain >> where it is currently used. >> >> Signed-off-by: Preeti U Murthy<preeti@linux.vnet.ibm.com> >> --- >> kernel/sched/core.c | 6 ++++++ >> kernel/sched/fair.c | 38 ++++++++++++++++++++------------------ >> kernel/sched/sched.h | 2 ++ >> 3 files changed, 28 insertions(+), 18 deletions(-) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index c06b8d3..e6a6244 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -5271,6 +5271,8 @@ DEFINE_PER_CPU(struct sched_domain *, sd_llc); >> DEFINE_PER_CPU(int, sd_llc_size); >> DEFINE_PER_CPU(int, sd_llc_id); >> DEFINE_PER_CPU(struct sched_domain *, sd_numa); >> +DEFINE_PER_CPU(struct sched_domain *, sd_busy); >> +DEFINE_PER_CPU(struct sched_domain *, sd_asym); >> >> static void update_top_cache_domain(int cpu) >> { >> @@ -5282,6 +5284,7 @@ static void update_top_cache_domain(int cpu) >> if (sd) { >> id = cpumask_first(sched_domain_span(sd)); >> size = cpumask_weight(sched_domain_span(sd)); >> + rcu_assign_pointer(per_cpu(sd_busy, cpu), sd->parent); >> } > > > consider a machine with single socket, dual core with HT enabled. The top most > domain is also the highest domain with SD_SHARE_PKG_RESOURCES flag set, > i.e MC domain (the machine toplogy consist of SIBLING and MC domain). > > # lstopo-no-graphics --no-bridges --no-io > Machine (7869MB) + Socket L#0 + L3 L#0 (3072KB) > L2 L#0 (256KB) + L1d L#0 (32KB) + L1i L#0 (32KB) + Core L#0 > PU L#0 (P#0) > PU L#1 (P#1) > L2 L#1 (256KB) + L1d L#1 (32KB) + L1i L#1 (32KB) + Core L#1 > PU L#2 (P#2) > PU L#3 (P#3) > > With this approach parent of MC domain is NULL and given that sd_busy is NULL, > nr_busy_cpus of sched domain sd_busy will never be incremented/decremented. > Resulting is nohz_kick_needed returning 0.
Right and it *should* return 0. There is no sibling domain that can offload tasks from it. Therefore there is no point kicking nohz idle balance.
Regards Preeti U Murthy > > Thanks, > Kamalesh. >
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