From d76343c6b2b79f5e89c392bc9ce9dabc4c9e90cb Mon Sep 17 00:00:00 2001 From: Valentin Schneider Date: Mon, 30 Mar 2020 10:01:27 +0100 Subject: [PATCH] sched/fair: Align rq->avg_idle and rq->avg_scan_cost sched/core.c uses update_avg() for rq->avg_idle and sched/fair.c uses an open-coded version (with the exact same decay factor) for rq->avg_scan_cost. On top of that, select_idle_cpu() expects to be able to compare these two fields. The only difference between the two is that rq->avg_scan_cost is computed using a pure division rather than a shift. Turns out it actually matters, first of all because the shifted value can be negative, and the standard has this to say about it: """ The result of E1 >> E2 is E1 right-shifted E2 bit positions. [...] If E1 has a signed type and a negative value, the resulting value is implementation-defined. """ Not only this, but (arithmetic) right shifting a negative value (using 2's complement) is *not* equivalent to dividing it by the corresponding power of 2. Let's look at a few examples: -4 -> 0xF..FC -4 >> 3 -> 0xF..FF == -1 != -4 / 8 -8 -> 0xF..F8 -8 >> 3 -> 0xF..FF == -1 == -8 / 8 -9 -> 0xF..F7 -9 >> 3 -> 0xF..FE == -2 != -9 / 8 Make update_avg() use a division, and export it to the private scheduler header to reuse it where relevant. Note that this still lets compilers use a shift here, but should prevent any unwanted surprise. The disassembly of select_idle_cpu() remains unchanged on arm64, and ttwu_do_wakeup() gains 2 instructions; the diff sort of looks like this: - sub x1, x1, x0 + subs x1, x1, x0 // set condition codes + add x0, x1, #0x7 + csel x0, x0, x1, mi // x0 = x1 < 0 ? x0 : x1 add x0, x3, x0, asr #3 which does the right thing (i.e. gives us the expected result while still using an arithmetic shift) Signed-off-by: Valentin Schneider Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Link: https://lkml.kernel.org/r/20200330090127.16294-1-valentin.schneider@arm.com --- kernel/sched/core.c | 6 ------ kernel/sched/fair.c | 7 ++----- kernel/sched/sched.h | 6 ++++++ 3 files changed, 8 insertions(+), 11 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a2694ba82874..f6b329bca0c6 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2119,12 +2119,6 @@ int select_task_rq(struct task_struct *p, int cpu, int sd_flags, int wake_flags) return cpu; } -static void update_avg(u64 *avg, u64 sample) -{ - s64 diff = sample - *avg; - *avg += diff >> 3; -} - void sched_set_stop_task(int cpu, struct task_struct *stop) { struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 }; diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 1ea3dddafe69..fb025e946f83 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6080,8 +6080,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask); struct sched_domain *this_sd; u64 avg_cost, avg_idle; - u64 time, cost; - s64 delta; + u64 time; int this = smp_processor_id(); int cpu, nr = INT_MAX; @@ -6119,9 +6118,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t } time = cpu_clock(this) - time; - cost = this_sd->avg_scan_cost; - delta = (s64)(time - cost) / 8; - this_sd->avg_scan_cost += delta; + update_avg(&this_sd->avg_scan_cost, time); return cpu; } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 0f616bf7bce3..cd008147eccb 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -195,6 +195,12 @@ static inline int task_has_dl_policy(struct task_struct *p) #define cap_scale(v, s) ((v)*(s) >> SCHED_CAPACITY_SHIFT) +static inline void update_avg(u64 *avg, u64 sample) +{ + s64 diff = sample - *avg; + *avg += diff / 8; +} + /* * !! For sched_setattr_nocheck() (kernel) only !! * -- 2.34.1