xen: credit2: don't miss accounting while doing a credit reset.

xen: credit2: don't miss accounting while doing a credit reset.

A credit reset basically means going through all the
vCPUs of a runqueue and altering their credits, as a
consequence of a 'scheduling epoch' having come to an
end.

Blocked or runnable vCPUs are fine, all the credits
they've spent running so far have been accounted to
them when they were scheduled out.

But if a vCPU is running on a pCPU, when a reset event
occurs (on another pCPU), that does not get properly
accounted. Let's therefore begin to do so, for better
accuracy and fairness.

In fact, after this patch, we see this in a trace:

 csched2:schedule cpu 10, rq# 1, busy, not tickled
 csched2:burn_credits d1v5, credit = 9998353, delta = 202996
 runstate_continue d1v5 running->running
 ...
 csched2:schedule cpu 12, rq# 1, busy, not tickled
 csched2:burn_credits d1v6, credit = -1327, delta = 9999544
 csched2:reset_credits d0v13, credit_start = 10500000, credit_end = 10500000, mult = 1
 csched2:reset_credits d0v14, credit_start = 10500000, credit_end = 10500000, mult = 1
 csched2:reset_credits d0v7, credit_start = 10500000, credit_end = 10500000, mult = 1
 csched2:burn_credits d1v5, credit = 201805, delta = 9796548
 csched2:reset_credits d1v5, credit_start = 201805, credit_end = 10201805, mult = 1
 csched2:burn_credits d1v6, credit = -1327, delta = 0
 csched2:reset_credits d1v6, credit_start = -1327, credit_end = 9998673, mult = 1

Which shows how d1v5 actually executed for ~9.796 ms,
on pCPU 10, when reset_credit() is executed, on pCPU
12, because of d1v6's credits going below 0.

Without this patch, this 9.796ms are not accounted
to anyone. With this patch, d1v5 is charged for that,
and its credits drop down from 9796548 to 201805.

And this is important, as it means that it will
begin the new epoch with 10201805 credits, instead
of 10500000 (which he would have, before this patch).

Basically, we were forgetting one round of accounting
in epoch x, for the vCPUs that are running at the time
the epoch ends. And this meant favouring a little bit
these same vCPUs, in epoch x+1, providing them with
the chance of execute longer than their fair share.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>