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SubjectRe: [PATCH 26/23] io-controller: fix writer preemption with in a group
Vivek Goyal wrote:
> o Found another issue during testing. Consider following hierarchy.
>
> root
> / \
> R1 G1
> /\
> R2 W
>
> Generally in CFQ when readers and writers are running, reader immediately
> preempts writers and hence reader gets the better bandwidth. In case of
> hierarchical setup, it becomes little more tricky. In above diagram, G1
> is a group and R1, R2 are readers and W is writer tasks.
>
> Now assume W runs and then R1 runs and then R2 runs. After R2 has used its
> time slice, if R1 is schedule in, after couple of ms, R1 will get backlogged
> again in group G1, (streaming reader). But it will not preempt R1 as R1 is
> also a reader and also because preemption across group is not allowed for
> isolation reasons. Hence R2 will get backlogged in G1 and will get a
> vdisktime much higher than W. So when G2 gets scheduled again, W will get
> to run its full slice length despite the fact R2 is queue on same service
> tree.
>
> The core issue here is that apart from regular preemptions (preemption
> across classes), CFQ also has this special notion of preemption with-in
> class and that can lead to issues active task is running in a differnt
> group than where new queue gets backlogged.
>
> To solve the issue keep a track of this event (I am calling it late
> preemption). When a group becomes eligible to run again, if late_preemption
> is set, check if there are sync readers backlogged, and if yes, expire the
> writer after one round of dispatch.
>
> This solves the issue of reader not getting enough bandwidth in hierarchical
> setups.
>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Conceptually a nice solution. The code gets a little tricky,
but I guess any code dealing with these situations would end
up that way :)

Acked-by: Rik van Riel <riel@redhat.com>

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