Re: [PATCH v2] sched/uclamp: Fix uclamp_tg_restrict()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 17, 2021 at 05:51:55PM +0100, Qais Yousef wrote:
> Now cpu.uclamp.min acts as a protection, we need to make sure that the
> uclamp request of the task is within the allowed range of the cgroup,
> that is it is clamp()'ed correctly by tg->uclamp[UCLAMP_MIN] and
> tg->uclamp[UCLAMP_MAX].
> 
> As reported by Xuewen [1] we can have some corner cases where there's
> inversion between uclamp requested by task (p) and the uclamp values of
> the taskgroup it's attached to (tg). Following table demonstrates
> 2 corner cases:
> 
> 	           |  p  |  tg  |  effective
> 	-----------+-----+------+-----------
> 	CASE 1
> 	-----------+-----+------+-----------
> 	uclamp_min | 60% | 0%   |  60%
> 	-----------+-----+------+-----------
> 	uclamp_max | 80% | 50%  |  50%
> 	-----------+-----+------+-----------
> 	CASE 2
> 	-----------+-----+------+-----------
> 	uclamp_min | 0%  | 30%  |  30%
> 	-----------+-----+------+-----------
> 	uclamp_max | 20% | 50%  |  20%
> 	-----------+-----+------+-----------
> 
> With this fix we get:
> 
> 	           |  p  |  tg  |  effective
> 	-----------+-----+------+-----------
> 	CASE 1
> 	-----------+-----+------+-----------
> 	uclamp_min | 60% | 0%   |  50%
> 	-----------+-----+------+-----------
> 	uclamp_max | 80% | 50%  |  50%
> 	-----------+-----+------+-----------
> 	CASE 2
> 	-----------+-----+------+-----------
> 	uclamp_min | 0%  | 30%  |  30%
> 	-----------+-----+------+-----------
> 	uclamp_max | 20% | 50%  |  30%
> 	-----------+-----+------+-----------
> 
> Additionally uclamp_update_active_tasks() must now unconditionally
> update both UCLAMP_MIN/MAX because changing the tg's UCLAMP_MAX for
> instance could have an impact on the effective UCLAMP_MIN of the tasks.
> 
> 	           |  p  |  tg  |  effective
> 	-----------+-----+------+-----------
> 	old
> 	-----------+-----+------+-----------
> 	uclamp_min | 60% | 0%   |  50%
> 	-----------+-----+------+-----------
> 	uclamp_max | 80% | 50%  |  50%
> 	-----------+-----+------+-----------
> 	*new*
> 	-----------+-----+------+-----------
> 	uclamp_min | 60% | 0%   | *60%*
> 	-----------+-----+------+-----------
> 	uclamp_max | 80% |*70%* | *70%*
> 	-----------+-----+------+-----------
> 
> [1] https://lore.kernel.org/lkml/CAB8ipk_a6VFNjiEnHRHkUMBKbA+qzPQvhtNjJ_YNzQhqV_o8Zw@mail.gmail.com/
> 
> Reported-by: Xuewen Yan <xuewen.yan94@gmail.com>
> Fixes: 0c18f2ecfcc2 ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min")
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>

Thanks!