From 1c35b07e6d3986474e5635be566e7bc79d97c64d Mon Sep 17 00:00:00 2001 From: Odin Ugedal Date: Thu, 24 Jun 2021 13:18:15 +0200 Subject: [PATCH 1/2] sched/fair: Ensure _sum and _avg values stay consistent The _sum and _avg values are in general sync together with the PELT divider. They are however not always completely in perfect sync, resulting in situations where _sum gets to zero while _avg stays positive. Such situations are undesirable. This comes from the fact that PELT will increase period_contrib, also increasing the PELT divider, without updating _sum and _avg values to stay in perfect sync where (_sum == _avg * divider). However, such PELT change will never lower _sum, making it impossible to end up in a situation where _sum is zero and _avg is not. Therefore, we need to ensure that when subtracting load outside PELT, that when _sum is zero, _avg is also set to zero. This occurs when (_sum < _avg * divider), and the subtracted (_avg * divider) is bigger or equal to the current _sum, while the subtracted _avg is smaller than the current _avg. Reported-by: Sachin Sant Reported-by: Naresh Kamboju Signed-off-by: Odin Ugedal Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Tested-by: Sachin Sant Link: https://lore.kernel.org/r/20210624111815.57937-1-odin@uged.al --- kernel/sched/fair.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 4a3e61a88acc..45edf61eed73 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3657,15 +3657,15 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) r = removed_load; sub_positive(&sa->load_avg, r); - sub_positive(&sa->load_sum, r * divider); + sa->load_sum = sa->load_avg * divider; r = removed_util; sub_positive(&sa->util_avg, r); - sub_positive(&sa->util_sum, r * divider); + sa->util_sum = sa->util_avg * divider; r = removed_runnable; sub_positive(&sa->runnable_avg, r); - sub_positive(&sa->runnable_sum, r * divider); + sa->runnable_sum = sa->runnable_avg * divider; /* * removed_runnable is the unweighted version of removed_load so we From d2343cb8d154fe20c4499711bb3a9af2095b2b4b Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Mon, 28 Jun 2021 21:55:16 +0200 Subject: [PATCH 2/2] sched/core: Disable CONFIG_SCHED_CORE by default This option at minimum adds extra code to the scheduler - even if it's default unused - and most users wouldn't want it. Reported-by: Linus Torvalds Signed-off-by: Ingo Molnar --- kernel/Kconfig.preempt | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt index bd7c4147b9a8..5876e30c5740 100644 --- a/kernel/Kconfig.preempt +++ b/kernel/Kconfig.preempt @@ -102,7 +102,6 @@ config PREEMPT_DYNAMIC config SCHED_CORE bool "Core Scheduling for SMT" - default y depends on SCHED_SMT help This option permits Core Scheduling, a means of coordinated task @@ -115,7 +114,8 @@ config SCHED_CORE - mitigation of some (not all) SMT side channels; - limiting SMT interference to improve determinism and/or performance. - SCHED_CORE is default enabled when SCHED_SMT is enabled -- when - unused there should be no impact on performance. + SCHED_CORE is default disabled. When it is enabled and unused, + which is the likely usage by Linux distributions, there should + be no measurable impact on performance.