sched_ext: Place dsq_vtime next to dsq_priq

scx_dispatch_enqueue() calls rb_add() for a vtime-ordered DSQ while
holding dsq->lock. At each level of the descent, scx_dsq_priq_less()
loads the visited task's dsq_vtime, and that comparison selects rb_left
or rb_right from the same task's dsq_priq. In the x86-64 benchmark
configuration, these accesses fell on different 64-byte cache lines
before this change, so a cold visited task could require a second cache
line fill on the dependent descent path.

Move dsq_vtime immediately before dsq_priq, keeping dsq_seq and
dsq_flags next to dsq_list. In the two x86-64 layouts inspected, where
scx starts at offsets 776 and 840 in task_struct, the fields from
dsq_list through dsq_priq occupy one cache line. The exact cache line
placement depends on the containing task_struct layout and is not
guaranteed for every configuration or architecture. Where they share a
line, avoiding the dependent cache line fill helps reduce dsq->lock hold
time during vtime insertion and therefore contention on the lock.

In a 16-vCPU KVM guest, the instrumented enqueue interval was 4-6%
shorter with scx_lavd and scx_layered. scx_mitosis and end-to-end
workload time showed no consistent change.

This only reorders fields; no scheduling behavior change is intended.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>

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