Re: [PATCH v2 02/21] sched: Refuse SCHED_FLAG_KEEP_PARAMS that would change the policy

[Juri Lelli]

Hello,

On 04/11/26 16:11, Mathieu Desnoyers wrote:
> SCHED_FLAG_KEEP_PARAMS keeps the parameters of a task: it skips
> __setscheduler_params(), which is what sets p->policy. A request that
> asks for another policy is then carried out by half, and succeeds:
> SCHED_OTHER to SCHED_BATCH with SCHED_FLAG_KEEP_PARAMS returns 0 and
> leaves the task where it was.
> 
> For SCHED_DEADLINE it costs bandwidth. A task that priority inheritance
> boosted into the deadline class is of that class already, so
> prev_class == next_class and the class check of
> commit 8a70415351ba ("sched: Reject SCHED_FLAG_KEEP_PARAMS when it would
> change scheduling class") lets the request through: sched_dl_overflow()
> admits it and adds
> its bandwidth, which the skipped __setscheduler_params() never gives to
> the task. It keeps SCHED_NORMAL and a dl_bw of 0, so nothing releases
> that bandwidth, and total_bw of the root domain holds it for good.
> 
> Refuse a policy change as well, where the class change was refused
> alone.
> 
> A SCHED_OTHER owner of a PTHREAD_PRIO_INHERIT mutex with a slice of
> 40ms, boosted by a deadline waiter blocked on it, leaves 0.4 of a CPU
> behind at every such call: total_bw 419424 -> 838854 and there to stay,
> in 8-CPU VMs of v7.4-rc1 with the four deadline fixes of sched/urgent,
> and of this series up to here. With this change the call is refused and
> total_bw is as it was. The selftest added later in this series has the
> case, params_keep_boosted.
> 
> Found by the Sashiko AI review of the first posting of this series.
> 
> Fixes: a509a7cd7974 ("sched/uclamp: Extend sched_setattr() to support utilization clamping")
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---

Makes sense to me.

Reviewed-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

Thanks,
Juri