Re: [PATCH 1/9] s390/idle: Fix cpu idle exit cpu time accounting

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, Feb 18, 2026 at 03:20:04PM +0100, Heiko Carstens a écrit :
> With the conversion to generic entry [1] cpu idle exit cpu time accounting
> was converted from assembly to C. This introduced an reversed order of cpu
> time accounting.
> 
> On cpu idle exit the current accounting happens with the following call
> chain:
> 
> -> do_io_irq()/do_ext_irq()
>  -> irq_enter_rcu()
>   -> account_hardirq_enter()
>    -> vtime_account_irq()
>     -> vtime_account_kernel()
> 
> vtime_account_kernel() accounts the passed cpu time since last_update_timer
> as system time, and updates last_update_timer to the current cpu timer
> value.
> 
> However the subsequent call of
> 
>  -> account_idle_time_irq()
> 
> will incorrectly subtract passed cpu time from timer_idle_enter to the
> updated last_update_timer value from system_timer. Then last_update_timer
> is updated to a sys_enter_timer, which means that last_update_timer goes
> back in time.
> 
> Subsequently account_hardirq_exit() will account too much cpu time as
> hardirq time. The sum of all accounted cpu times is still correct, however
> some cpu time which was previously accounted as system time is now
> accounted as hardirq time, plus there is the oddity that last_update_timer
> goes back in time.
> 
> Restore previous behavior by extracting cpu time accounting code from
> account_idle_time_irq() into a new update_timer_idle() function and call it
> before irq_enter_rcu().
> 
> Fixes: 56e62a737028 ("s390: convert to generic entry") [1]
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

-- 
Frederic Weisbecker
SUSE Labs