Re: [PATCH v4] bus: mhi: host: Disable preemption while processing data events

[Qiang Yu <quic_qianyu@quicinc.com>]

On 12/29/2022 12:35 AM, Manivannan Sadhasivam wrote:
> On Mon, Nov 21, 2022 at 10:48:54PM -0700, Jeffrey Hugo wrote:
>> On 11/21/2022 2:34 AM, Qiang Yu wrote:
>>> If data processing of an event is scheduled out because core
>>> is busy handling multiple irqs, this can starve the processing
>>> of MHI M0 state change event on another core. Fix this issue by
>>> disabling irq on the core processing data events.
>>>
>>> Signed-off-by: Qiang Yu <quic_qianyu@quicinc.com>
>> I've been pondering this off and on since it's been proposed.
>>
>> This solution will break the described deadlock, but I don't like it.
>>
>> What I really don't like is that this is selfish.  We already preempt
>> anything else on the CPU that isn't a hard IRQ because we are using a
>> tasklet (which is deprecated, see include/linux/interrupt.h).  Now we are
>> going to essentially preempt IRQs as well by preventing them from being
>> serviced.  So, now the CPU is essentially dedicated to processing MHI
>> events.  It seems selfish to say that MHI is the most important thing on a
>> particular CPU.
>>
>> This can have a huge effect on system behavior.  If say the ssh IRQ is
>> assigned to the same CPU, and we block that CPU long enough, then it will
>> appear to the user as if the ssh connection has frozen.  I've witnessed this
>> occur with other drivers.
>>
>> How long can we block the CPU?  According to the code, pretty much for an
>> unlimited amount of time.  If the tasklet is processing
>> mhi_process_data_event_ring(), then we can process U32_MAX events before
>> throttling (which might as well be unlimited).  If the tasklet is processing
>> mhi_process_ctrl_ev_ring() then there is no throttling.
>>
>> I'm thinking it would be better of the IRQ handling was refactored to use
>> threaded interrupts.  The thread is an actual process, so it could move to
>> another CPU.  It is also FIFO priority, so it basically will preempt
>> everything but hard IRQs and soft IRQs (eg tasklets).  The downside of a
>> tasklet is that it is bound to the scheduling CPU, which in our case is the
>> CPU servicing the IRQ, and more than a few systems tend to load the majority
>> of the IRQs to CPU0.
>>
> This sounds like a plausible solution.
>
>> I'm not going to go refactor the IRQ code at this time.  This looks like an
>> issue that is actually observed based on how it was reported, so it likely
>> should be addressed.  I'm not happy with this solution, but I don't have an
>> alternative at this time.
>>
>> Mani, up to you if you want to pick this up.  I'm not nack'ing it.
>> Technically I've reviewed it, but I'd say I'm "on the fence" about if this
>> really should be accepted.  I can't say there is a flaw in the logic, but I
>> don't feel good about this.
>>
> I do agree with you.
>
> Qiang, can you please look into Jeff's suggestion on fixing this performance
> issue?
>
> Thanks,
> Mani

Jeff's suggestion is reasonable. I have no reasons to insist that the 
patch should be accepted.

Thanks,

Qiang

>>> ---
>>> v3->v4: modify the comment
>>> v2->v3: modify the comment
>>> v1->v2: add comments about why we disable local irq
>>>
>>>    drivers/bus/mhi/host/main.c | 10 ++++++++--
>>>    1 file changed, 8 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
>>> index f3aef77a..6c804c3 100644
>>> --- a/drivers/bus/mhi/host/main.c
>>> +++ b/drivers/bus/mhi/host/main.c
>>> @@ -1029,11 +1029,17 @@ void mhi_ev_task(unsigned long data)
>>>    {
>>>    	struct mhi_event *mhi_event = (struct mhi_event *)data;
>>>    	struct mhi_controller *mhi_cntrl = mhi_event->mhi_cntrl;
>>> +	unsigned long flags;
>>> +	/*
>>> +	 * When multiple IRQs arrive, the tasklet will be scheduled out with event ring lock
>>> +	 * acquired, causing other high priority events like M0 state transition getting stuck
>>> +	 * while trying to acquire the same event ring lock. Thus, let's disable local IRQs here.
>>> +	 */
>>> +	spin_lock_irqsave(&mhi_event->lock, flags);
>>>    	/* process all pending events */
>>> -	spin_lock_bh(&mhi_event->lock);
>>>    	mhi_event->process_event(mhi_cntrl, mhi_event, U32_MAX);
>>> -	spin_unlock_bh(&mhi_event->lock);
>>> +	spin_unlock_irqrestore(&mhi_event->lock, flags);
>>>    }
>>>    void mhi_ctrl_ev_task(unsigned long data)