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

Thomas Gleixner
2020-11-24T00:06:39Z

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.

Thanks,

tglx