sched/uclamp: Fix relationship between uclamp and migration margin

sched/uclamp: Fix relationship between uclamp and migration margin

[ Upstream commit 48d5e9daa8b767e75ed9421665b037a49ce4bc04 ]

fits_capacity() verifies that a util is within 20% margin of the
capacity of a CPU, which is an attempt to speed up upmigration.

But when uclamp is used, this 20% margin is problematic because for
example if a task is boosted to 1024, then it will not fit on any CPU
according to fits_capacity() logic.

Or if a task is boosted to capacity_orig_of(medium_cpu). The task will
end up on big instead on the desired medium CPU.

Similar corner cases exist for uclamp and usage of capacity_of().
Slightest irq pressure on biggest CPU for example will make a 1024
boosted task look like it can't fit.

What we really want is for uclamp comparisons to ignore the migration
margin and capacity pressure, yet retain them for when checking the
_actual_ util signal.

For example, task p:

	p->util_avg = 300
	p->uclamp[UCLAMP_MIN] = 1024

Will fit a big CPU. But

	p->util_avg = 900
	p->uclamp[UCLAMP_MIN] = 1024

will not, this should trigger overutilized state because the big CPU is
now *actually* being saturated.

Similar reasoning applies to capping tasks with UCLAMP_MAX. For example:

	p->util_avg = 1024
	p->uclamp[UCLAMP_MAX] = capacity_orig_of(medium_cpu)

Should fit the task on medium cpus without triggering overutilized
state.

Inlined comments expand more on desired behavior in more scenarios.

Introduce new util_fits_cpu() function which encapsulates the new logic.
The new function is not used anywhere yet, but will be used to update
various users of fits_capacity() in later patches.

Fixes: af24bde8df202 ("sched/uclamp: Add uclamp support to energy_compute()")
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220804143609.515789-2-qais.yousef@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>