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List:       linux-rt-users
Subject:    =?gb2312?B?16q3ojog?=
From:       Caine Chen <caine.chen () dji ! com>
Date:       2022-01-17 12:54:16
Message-ID: dd9673737c354eb898fc22ee466efbda () dji ! com
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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.
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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