On Tue, Nov 24, 2020 at 01:06:15AM +0100, Thomas Gleixner wrote:

On Tue, Nov 24 2020 at 00:58, Frederic Weisbecker wrote:

quoted

On Mon, Nov 23, 2020 at 08:27:33PM +0100, Thomas Gleixner wrote:

quoted

On Mon, Nov 23 2020 at 14:44, Frederic Weisbecker wrote:

quoted

On Fri, Nov 13, 2020 at 03:02:21PM +0100, Thomas Gleixner wrote:

quoted

+	/*
+	 * 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

quoted

+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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