From 517a2390cd8d8b55d29b5692410c12c91076fd70 Mon Sep 17 00:00:00 2001 From: Mete Durlu Date: Wed, 6 Mar 2024 12:31:52 +0100 Subject: s390/vtime: fix average steal time calculation [ Upstream commit 367c50f78451d3bd7ad70bc5c89f9ba6dec46ca9 ] Current average steal timer calculation produces volatile and inflated values. The only user of this value is KVM so far and it uses that to decide whether or not to yield the vCPU which is seeing steal time. KVM compares average steal timer to a threshold and if the threshold is past then it does not allow CPU polling and yields it to host, else it keeps the CPU by polling. Since KVM's steal time threshold is very low by default (%10) it most likely is not effected much by the bloated average steal timer values because the operating region is pretty small. However there might be new users in the future who might rely on this number. Fix average steal timer calculation by changing the formula from: avg_steal_timer = avg_steal_timer / 2 + steal_timer; to the following: avg_steal_timer = (avg_steal_timer + steal_timer) / 2; This ensures that avg_steal_timer is actually a naive average of steal timer values. It now closely follows steal timer values but of course in a smoother manner. Fixes: 152e9b8676c6 ("s390/vtime: steal time exponential moving average") Signed-off-by: Mete Durlu Acked-by: Heiko Carstens Acked-by: Christian Borntraeger Signed-off-by: Heiko Carstens Signed-off-by: Sasha Levin --- arch/s390/kernel/vtime.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'arch') diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c index e0a88dcaf5cb..24a18e5ef6e8 100644 --- a/arch/s390/kernel/vtime.c +++ b/arch/s390/kernel/vtime.c @@ -210,13 +210,13 @@ void vtime_flush(struct task_struct *tsk) virt_timer_expire(); steal = S390_lowcore.steal_timer; - avg_steal = S390_lowcore.avg_steal_timer / 2; + avg_steal = S390_lowcore.avg_steal_timer; if ((s64) steal > 0) { S390_lowcore.steal_timer = 0; account_steal_time(cputime_to_nsecs(steal)); avg_steal += steal; } - S390_lowcore.avg_steal_timer = avg_steal; + S390_lowcore.avg_steal_timer = avg_steal / 2; } static u64 vtime_delta(void) -- cgit v1.2.3