Patch "s390/vtime: fix average steal time calculation" has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    s390/vtime: fix average steal time calculation

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     s390-vtime-fix-average-steal-time-calculation.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 60b4b1916ee3b4167f903a218efbe6f82ef415fc
Author: Mete Durlu <meted@xxxxxxxxxxxxx>
Date:   Wed Mar 6 12:31:52 2024 +0100

    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 <meted@xxxxxxxxxxxxx>
    Acked-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
    Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
    Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c
index 9436f3053b88c..003c926a0f4de 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)




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