Messages in this thread Patch in this message | | | From | Venkatesh Pallipadi <> | Subject | [PATCH] sched: Resolve sd_idle and first_idle_cpu Catch-22 | Date | Fri, 4 Feb 2011 12:51:28 -0800 |
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Consider a system with { [ (A B) (C D) ] [ (E F) (G H) ] }, () denoting SMT siblings, [] cores on same socket and {} system wide Further, A, C and D are idle, B is busy and one of EFGH has excess load.
With sd_idle logic, a check in rebalance_domains() converts tick based load balance requests from CPU A to busy load balance for core and above domains (lower rate of balance and higher load_idx).
With first_idle_cpu logic, when CPU C or D tries to balance across domains the logic finds CPU A as first idle CPU in the group and nominates CPU A to idle balance across sockets.
But, sd_idle above would not allow CPU A to do cross socket idle balance as CPU A switches its higher level balancing to busy balance.
So, this can result is no cross socket balancing for extended periods.
The fix here adds additional check to detect sd_idle logic in first_idle_cpu code path. We will now nominate (in order or preference): * First fully idle CPU * First semi-idle CPU * First CPU
Note that this solution works fine for 2 SMT siblings case and won't be perfect in picking proper semi-idle in case of more than 2 SMT threads.
The problem was found by looking at the code and schedstat output. I don't yet have any data to show impact of this on any workload.
Signed-off-by: Venkatesh Pallipadi <venki@google.com> --- kernel/sched_fair.c | 41 +++++++++++++++++++++++++++++++++++++++-- 1 files changed, 39 insertions(+), 2 deletions(-)
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 62723a4..1790cc2 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -2603,6 +2603,37 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group) return 0; } +/* + * Find if there is any busy CPUs in SD_SHARE_CPUPOWER domain of + * requested CPU. + * Bypass the check in case of SD_POWERSAVINGS_BALANCE on + * parent domain. In that case requested CPU can still be nominated as + * balancer for higher domains. + */ +static int is_cpupower_sharing_domain_idle(int cpu) +{ + struct sched_domain *sd; + int i; + + if (!(sysctl_sched_compat_yield & 0x4)) + return 1; + + for_each_domain(cpu, sd) { + if (!(sd->flags & SD_SHARE_CPUPOWER)) + break; + + if (test_sd_parent(sd, SD_POWERSAVINGS_BALANCE)) + return 1; + + for_each_cpu(i, sched_domain_span(sd)) { + if (!idle_cpu(i)) + return 0; + } + } + + return 1; +} + /** * update_sg_lb_stats - Update sched_group's statistics for load balancing. * @sd: The sched_domain whose statistics are to be updated. @@ -2625,6 +2656,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd, unsigned long load, max_cpu_load, min_cpu_load, max_nr_running; int i; unsigned int balance_cpu = -1, first_idle_cpu = 0; + unsigned int first_semiidle_cpu = 0; unsigned long avg_load_per_task = 0; if (local_group) @@ -2644,8 +2676,13 @@ static inline void update_sg_lb_stats(struct sched_domain *sd, /* Bias balancing toward cpus of our domain */ if (local_group) { if (idle_cpu(i) && !first_idle_cpu) { - first_idle_cpu = 1; - balance_cpu = i; + if (is_cpupower_sharing_domain_idle(i)) { + first_idle_cpu = 1; + balance_cpu = i; + } else if (!first_semiidle_cpu) { + first_semiidle_cpu = 1; + balance_cpu = i; + } } load = target_load(i, load_idx); -- 1.7.3.1
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