Re: [PATCH] sched/fair: Only increment deadline once on yield

[Alexander Graf <graf@amazon.com>]

On 01.04.25 14:36, Fernand Sieber wrote:
> If a task yields, the scheduler may decide to pick it again. The task in
> turn may decide to yield immediately or shortly after, leading to a tight
> loop of yields.
>
> If there's another runnable task as this point, the deadline will be
> increased by the slice at each loop. This can cause the deadline to runaway
> pretty quickly, and subsequent elevated run delays later on as the task
> doesn't get picked again. The reason the scheduler can pick the same task
> again and again despite its deadline increasing is because it may be the
> only eligible task at that point.
>
> Fix this by updating the deadline only to one slice ahead.
>
> Note, we might want to consider iterating on the implementation of yield as
> follow up:
> * the yielding task could be forfeiting its remaining slice by
>    incrementing its vruntime correspondingly
> * in case of yield_to the yielding task could be donating its remaining
>    slice to the target task
>
> Signed-off-by: Fernand Sieber <sieberf@amazon.com>


IMHO it's worth noting that this is not a theoretical issue. We have 
seen this in real life: A KVM virtual machine's vCPU which runs into a 
busy guest spin lock calls kvm_vcpu_yield_to() which eventually ends up 
in the yield_task_fair() function. We have seen such spin locks due to 
guest contention rather than host overcommit, which means we go into a 
loop of vCPU execution and spin loop exit, which results in an 
undesirable increase in the vCPU thread's deadline.

Given this impacts real workloads and is a bug present since the 
introduction of EEVDF, I would say it warrants a

Fixes: 147f3efaa24182 ("sched/fair: Implement an EEVDF-like scheduling 
policy")

tag.


Alex