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    SubjectRe: [patch 14/19] softirq: Make softirq control and processing RT aware
    On Tue, Nov 24, 2020 at 01:06:15AM +0100, Thomas Gleixner wrote:
    > On Tue, Nov 24 2020 at 00:58, Frederic Weisbecker wrote:
    > > On Mon, Nov 23, 2020 at 08:27:33PM +0100, Thomas Gleixner wrote:
    > >> On Mon, Nov 23 2020 at 14:44, Frederic Weisbecker wrote:
    > >> > On Fri, Nov 13, 2020 at 03:02:21PM +0100, Thomas Gleixner wrote:
    > >> >> + /*
    > >> >> + * Adjust softirq count to SOFTIRQ_OFFSET which makes
    > >> >> + * in_serving_softirq() become true.
    > >> >> + */
    > >> >> + cnt = SOFTIRQ_OFFSET;
    > >> >> + __local_bh_enable(cnt, false);
    > >> >
    > >> > But then you enter __do_softirq() with softirq_count() == SOFTIRQ_OFFSET.
    > >> > __do_softirq() calls softirq_handle_begin() which then sets it back to
    > >> > SOFTIRQ_DISABLE_OFFSET...
    > >>
    > >> The RT variant of it added in this very same patch
    > >> > +static inline void softirq_handle_begin(void) { }
    > >> > +static inline void softirq_handle_end(void) { }
    > >
    > > Ah but then account_irq_enter_time() is called with SOFTIRQ_OFFSET (it's
    > > currently called with softirq_count == 0 at this point) and that may mess
    > > up irqtime accounting which relies on it. It could spuriously account all
    > > the time between the last (soft-)IRQ exit until now as softirq time.
    >
    > Good point. Haven't thought about that. Let me have a look again.

    But I'm cooking a patchset which moves account_irq_enter_time() after
    HARDIRQ_OFFSET or SOFTIRQ_OFFSET is incremented. This will allow us to move
    tick_irq_enter() under this layout:

    preempt_count_add(HARDIRQ_OFFSET)
    lockdep_hardirq_enter()
    tick_irq_enter()
    account_irq_enter_time()

    This way tick_irq_enter() can be correctly handled by lockdep and we can remove
    the nasty hack which temporarily disables softirqs around it.

    And as a side effect it should also fix your issue.

    I should have that ready soonish.

    Thanks.

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