Re: [patch 14/19] softirq: Make softirq control and processing RT aware

Frederic Weisbecker
2020-11-23T23:58:26Z

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.