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

[Manivannan Sadhasivam <mani@kernel.org>]

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

> > ---
> > 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)
> 

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