From: Mete Durlu meted@linux.ibm.com
[ 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 meted@linux.ibm.com Acked-by: Heiko Carstens hca@linux.ibm.com Acked-by: Christian Borntraeger borntraeger@linux.ibm.com Signed-off-by: Heiko Carstens hca@linux.ibm.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/s390/kernel/vtime.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c index f216a1b2f8257..86137029235ac 100644 --- a/arch/s390/kernel/vtime.c +++ b/arch/s390/kernel/vtime.c @@ -211,13 +211,13 @@ 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)