tick/sched: Unify idle cputime accounting

The non-vtime dynticks-idle cputime accounting is a big mess that
accumulates within two concurrent statistics, each having their own
shortcomings:

 * The accounting for online CPUs which is based on the delta between
   tick_nohz_start_idle() and tick_nohz_stop_idle().

   Pros:
       - Works when the tick is off

       - Has nsecs granularity

   Cons:
       - Account idle steal time but doesn't substract it from idle
         cputime.

       - Assumes CONFIG_IRQ_TIME_ACCOUNTING by not accounting IRQs but
         the IRQ time is simply ignored when
         CONFIG_IRQ_TIME_ACCOUNTING=n

       - The windows between 1) idle task scheduling and the first call
         to tick_nohz_start_idle() and 2) idle task between the last
         tick_nohz_stop_idle() and the rest of the idle time are
         blindspots wrt. cputime accounting (though mostly insignificant
         amount)

       - Relies on private fields outside of kernel stats, with specific
         accessors.

 * The accounting for offline CPUs which is based on ticks and the
   jiffies delta during which the tick was stopped.

   Pros:
       - Handles steal time correctly

       - Handle CONFIG_IRQ_TIME_ACCOUNTING=y and
         CONFIG_IRQ_TIME_ACCOUNTING=n correctly.

       - Handles the whole idle task

       - Accounts directly to kernel stats, without midlayer accumulator.

    Cons:
       - Doesn't elapse when the tick is off, which doesn't make it
         suitable for online CPUs.

       - Has TICK_NSEC granularity (jiffies)

       - Needs to track the dyntick-idle ticks that were accounted and
         substract them from the total jiffies time spent while the tick
         was stopped. This is an ugly workaround.

Having two different accounting for a single context is not the only
problem: since those accountings are of different natures, it is
possible to observe the global idle time going backward after a CPU goes
offline.

Clean up the situation with introducing a hybrid approach that stays
coherent and works for both online and offline CPUs:

  * Tick based or native vtime accounting operate before the idle loop
    is entered and resume once the idle loop prepares to exit.

  * When the idle loop starts, switch to dynticks-idle accounting as is
    done currently, except that the statistics accumulate directly to the
    relevant kernel stat fields.

  * Private dyntick cputime accounting fields are removed.

  * Works on both online and offline case.

Further improvement will include:

  * Only switch to dynticks-idle cputime accounting when the tick actually
    goes in dynticks mode.

  * Handle CONFIG_IRQ_TIME_ACCOUNTING=n correctly such that the
    dynticks-idle accounting still elapses while on IRQs.

  * Correctly substract idle steal cputime from idle time

Reported-by: Xin Zhao <jackzxcui1989@163.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Link: https://patch.msgid.link/20260508131647.43868-8-frederic@kernel.org

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