linux-kernel - Re: [PATCH v7 2/3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

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Date:   Wed, 14 Sep 2022 13:04:26 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Aaron Tomlin <atomlin@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>, cl@...ux.com,
        tglx@...utronix.de, mingo@...nel.org, peterz@...radead.org,
        pauld@...hat.com, neelx@...hat.com, oleksandr@...alenko.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v7 2/3] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too

On Mon, Sep 12, 2022 at 03:38:23PM +0100, Aaron Tomlin wrote:
> On Fri 2022-09-09 16:35 -0300, Marcelo Tosatti wrote:
> Hi Frederic,
> 
> Sorry about that. How about the following:
> 
>  - Note: CPU X is part of 'tick_nohz_full_mask'
> 
>     1.      CPU Y migrated running task A to CPU X that
> 	    was in an idle state i.e. waiting for an IRQ;
> 	    marked the current task on CPU X to need/or
> 	    require a reschedule i.e., set TIF_NEED_RESCHED
> 	    and invoked a reschedule IPI to CPU X
> 	    (see sched_move_task())
> 
>     2.      CPU X acknowledged the reschedule IPI. Generic
> 	    idle loop code noticed the TIF_NEED_RESCHED flag
> 	    against the idle task and attempts to exit of the
> 	    loop and calls the main scheduler function i.e.
> 	    __schedule().
> 
> 	    Since the idle tick was previously stopped no
> 	    scheduling-clock tick would occur.
> 	    So, no deferred timers would be handled
> 
>     3.      Post transition to kernel execution Task A
> 	    running on CPU X, indirectly released a few pages
> 	    (e.g. see __free_one_page()); CPU X's
> 	    'vm_stat_diff[NR_FREE_PAGES]' was updated and zone
> 	    specific 'vm_stat[]' update was deferred as per the
> 	    CPU-specific stat threshold
> 
>     4.      Task A does invoke exit(2) and the kernel does
> 	    remove the task from the run-queue; the idle task
> 	    was selected to execute next since there are no
> 	    other runnable tasks assigned to the given CPU
> 	    (see pick_next_task() and pick_next_task_idle())
> 
>     5.      On return to the idle loop since the idle tick
> 	    was already stopped and can remain so (see [1]
> 	    below) e.g. no pending soft IRQs, no attempt is
> 	    made to zero and fold CPU X's vmstat counters
> 	    since reprogramming of the scheduling-clock tick
> 	    is not required/or needed (see [2])

Much better thanks.

Please cut the patch in two patches: one that fixes the stuff in
the idle path and another one that fixes the return to user path.

The first one is definetly a fix, the second one is rather a feature
that is definetly wanted as well but I need to think it through further.

> 
> 
> 
> Kind regards,
> 
> -- 
> Aaron Tomlin
>