On 5/11/21 7:38 PM, Tejun Heo wrote: > When the weight of an active iocg is updated, weight_updated() is called > which in turn calls __propagate_weights() to update the active and inuse > weights so that the effective hierarchical weights are update accordingly. > > The current implementation is incorrect for inner active nodes. For an > active leaf iocg, inuse can be any value between 1 and active and the > difference represents how much the iocg is donating. When weight is updated, > as long as inuse is clamped between 1 and the new weight, we're alright and > this is what __propagate_weights() currently implements. > > However, that's not how an active inner node's inuse is set. An inner node's > inuse is solely determined by the ratio between the sums of inuse's and > active's of its children - ie. they're results of propagating the leaves' > active and inuse weights upwards. __propagate_weights() incorrectly applies > the same clamping as for a leaf when an active inner node's weight is > updated. Consider a hierarchy which looks like the following with saturating > workloads in AA and BB. > > R > / \ > A B > | | > AA BB > > 1. For both A and B, active=100, inuse=100, hwa=0.5, hwi=0.5. > > 2. echo 200 > A/io.weight > > 3. __propagate_weights() update A's active to 200 and leave inuse at 100 as > it's already between 1 and the new active, making A:active=200, > A:inuse=100. As R's active_sum is updated along with A's active, > A:hwa=2/3, B:hwa=1/3. However, because the inuses didn't change, the > hwi's remain unchanged at 0.5. > > 4. The weight of A is now twice that of B but AA and BB still have the same > hwi of 0.5 and thus are doing the same amount of IOs. > > Fix it by making __propgate_weights() always calculate the inuse of an > active inner iocg based on the ratio of child_inuse_sum to child_active_sum. Applied, thanks. -- Jens Axboe
