* Re:
2022-01-17 12:54 ` 转发: Caine Chen
@ 2022-02-03 11:49 ` Daniel Vacek
0 siblings, 0 replies; 2+ messages in thread
From: Daniel Vacek @ 2022-02-03 11:49 UTC (permalink / raw)
To: Caine Chen; +Cc: linux-rt-users@vger.kernel.org
Hi Caine,
On Tue, Jan 18, 2022 at 4:44 AM Caine Chen <caine.chen@dji.com> wrote:
>
> Hi guys:
> We found that some IRQ threads will block in local_bh_disable( ) for
> long time in some situation and we hope to get your valuable suggestions.
> My kernel version is 5.4 and the irq-delay is caused by the use of
> write_lock_bh().
> It can be described in the following figure:
> (1) Thread_1 which is a SCHED_NORMAL thread runs on CPU1,
> and it uses read_lock_bh() to protect some data.
> (2) Thread_2 which is a SCHED_RR thread runs on CPU1 and it preempts thread_1
> after thread_1 invoked read_lock_bh(). Thread_2 may run 60 ms in my system.
> (3) Thread_3 which is a SCHED_NORMAL thread runs on CPU0. This thread acquires
> writer's lock by invoking write_lock_bh(). This function will disable
> button-half firstly by invoking local_bh_disable( ). But it will block in
> rt_write_lock() , because read lock is held by thread_1.
> (4) At this time, if irq thread without IRQF_NO_THREAD flag on CPU0 trys to
> acquire bh_lock(it has been renamed as softirq_ctrl.lock now), irq
> thread will block because this lock is held by thread_3.
>
> ------------------------------------------------------------------------------------------------------------------------------------
> CPU1 CPU0
> ------------------------------------------------- ---------------------------------------------------------------
> thread_2 thread_1 thread_3 irq_thread
> -------------- ----------- ----------- --------------
> read_lock_bh()
>
> ......
> write_lock_bh()
> /*do work*/ /* irq thread block here*/
> local_bh_disable()
> ......
> read_unlock_bh()
> ......
> /* do work */
> ......
> write_unlock_bh()
> irq_thread_fn()
> ----------------------------------------------------------------------------------------------------------------------------------
>
> In this case, if SCHED_RR thread_2 preempts thread_1 and runs too much time, all
> irq threads on CPU0 will be blocked.
> It looks like a priority reverse problem of real-time thread preempt.
Not really. I guess there's one misunderstanding in your description.
Disabling the bottom half is local to running thread and not to the
CPU which executes that thread. As an effect, preemption practically
enables the bottom half again (as long as the new thread did not have
it already disabled before, of course...).
That said, the irq_thread will _not_ be blocked as bottom half is not
disabled in it's context. From your chart, it's disabled only in
thread_3 context and thread_1 context. But these two are independent
(due to the different thread contexts and not the different CPU
contexts as you misassumed) and they do not block each other either,
it's the rw_lock serializing these threads, right?
You should be able to see this with tracing. There should be no issue
or the issue is different than you think it is and different than you
described here.
Hopefully the above helps you,
Daniel
> How can I avoid this problem? I have a few thoughts:
> (1) The key point, I think, is that write_lock_bh()/read_lock_bh() will disable
> buttom half which will disable some irq threads too. Could I use
> write_lock_irq()/read_lock_irq() instead?
> (2) If my irq handler wants to get better performance, I should request a
> threaded handler for the IRQ as Sebastian suggested in LKML
> <RE: irq thread latency caused by softirq_ctrl.lock contention>.
> Is threaded handler designed for low irq delay?
> (3) Thread_2 takes too long time for running. So it is not suitable to set this
> thread with high rt-priority. Should I reduce this thread's priority to
> solve this problem?
>
> Are there better ways to avoid this problem? We hope to get your valuable
> suggestions. Thanks!
>
> Best regards,
> Caine.chen
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