Date:   Tue, 25 May 2021 19:37:03 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Siddharth Gupta <sidgup@...eaurora.org>
Cc:     ohad@...ery.com, linux-remoteproc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, psodagud@...eaurora.org
Subject: Re: [PATCH] remoteproc: core: Invoke subdev callbacks in list order

On Tue 25 May 14:48 CDT 2021, Siddharth Gupta wrote:

> 
> On 5/24/2021 8:03 PM, Bjorn Andersson wrote:
> > On Mon 17 May 18:08 CDT 2021, Siddharth Gupta wrote:
> > 
> > > Subdevices at the beginning of the subdev list should have
> > > higher priority than those at the end of the list. Reverse
> > > traversal of the list causes priority inversion, which can
> > > impact the performance of the device.
> > > 
> > The subdev lists layers of the communication onion, we bring them up
> > inside out and we take them down outside in.
> > 
> > This stems from the primary idea that we want to be able to shut things
> > down cleanly (in the case of a stop) and we pass the "crashed" flag to
> > indicate to each recipient during "stop" that it may not rely on the
> > response of a lower layer.
> > 
> > As such, I don't think it's right to say that we have a priority
> > inversion.
> My understanding of the topic was that each subdevice should be
> independent of the other. In our case unfortunately the sysmon
> subdevice depends on the glink endpoint.
> 

We need to care for the ordering if sysmon is to be able to use smd or
glink to send the shutdown request.

> However the priority inversion doesn't happen in these
> subdevices, it happens due to the SSR notifications that we send
> to kernel clients. In this case kernel clients also can have QMI
> sockets that in turn depend on the glink endpoint, which means
> when they go to release the QMI socket a broadcast will be sent
> out to all connected clients about the closure of the connection
> which in this case happens to be the remoteproc which died. So
> if we peel the onion, we will be unnecessarily be waiting for a
> dead remoteproc.

I see, that is indeed a problem.

> > 
> > > For example a device adds the glink, sysmon and ssr subdevs
> > > to its list. During a crash the ssr notification would go
> > > before the glink and sysmon notifications. This can cause a
> > > degraded response when a client driver waits for a response
> > > from the crashed rproc.
> > > 
> > In general the design is such that components are not expected to
> > communicate with the crashed remote when "crashed" is set, this avoids
> > the single-remote crash.
> Here the glink device on the rpmsg bus won't know about the
> crashed remoteproc till we send glink notification first, right?
> Since we send out sysmon and SSR notifications first, the glink
> device will still be "alive" on the rpmsg bus.

Yes, and this all stems from the design that everything communicating
over glink is a child of glink, which isn't the case when you have a SSR
event that will end up blocking the sequence in qrtr.

For sysmon this is not a problem, because sysmon is implemented to not
attempt to communicate with the parent remoteproc upon a crash. And
all rpmsg devices will be torn down as a result of glink being torn
down, so glink can fail early based on this (not sure if this was
implemented downstream though).

> > 
> > The case where this isn't holding up is when two remote processors
> > crashes simultaneously, in which case e.g. sysmon has been seen hitting
> > its timeout waiting for an ack from a dead remoteproc - but I was under
> > the impression that this window shrunk dramatically as a side effect of
> > us fixing the notification ordering.
> You are right, the window would become smaller in the case of two
> remoteprocs, but this issue can come up with even a single
> remoteproc unless prioritize certain subdevices.

The problem that you describe where an SSR notification will directly or
indirectly attempt to communicate over QRTR will certainly cause issues
in the single-rproc case as well.


But is there any reason why these listeners has to do the wrong thing at
stop(crashed=true)?

> > 
> > > Signed-off-by: Siddharth Gupta <sidgup@...eaurora.org>
> > > ---
> > >   drivers/remoteproc/remoteproc_core.c | 24 ++++++++++++++----------
> > >   1 file changed, 14 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/drivers/remoteproc/remoteproc_core.c b/drivers/remoteproc/remoteproc_core.c
> > > index 626a6b90f..ac8fc42 100644
> > > --- a/drivers/remoteproc/remoteproc_core.c
> > > +++ b/drivers/remoteproc/remoteproc_core.c
> > > @@ -1167,7 +1167,7 @@ static int rproc_handle_resources(struct rproc *rproc,
> > >   static int rproc_prepare_subdevices(struct rproc *rproc)
> > >   {
> > > -	struct rproc_subdev *subdev;
> > > +	struct rproc_subdev *subdev, *itr;
> > >   	int ret;
> > >   	list_for_each_entry(subdev, &rproc->subdevs, node) {
> > > @@ -1181,9 +1181,11 @@ static int rproc_prepare_subdevices(struct rproc *rproc)
> > >   	return 0;
> > >   unroll_preparation:
> > > -	list_for_each_entry_continue_reverse(subdev, &rproc->subdevs, node) {
> > > -		if (subdev->unprepare)
> > > -			subdev->unprepare(subdev);
> > > +	list_for_each_entry(itr, &rproc->subdevs, node) {
> > > +		if (itr == subdev)
> > > +			break;
> > > +		if (itr->unprepare)
> > > +			itr->unprepare(subdev);
> > >   	}
> > >   	return ret;
> > > @@ -1191,7 +1193,7 @@ static int rproc_prepare_subdevices(struct rproc *rproc)
> > >   static int rproc_start_subdevices(struct rproc *rproc)
> > >   {
> > > -	struct rproc_subdev *subdev;
> > > +	struct rproc_subdev *subdev, *itr;
> > >   	int ret;
> > >   	list_for_each_entry(subdev, &rproc->subdevs, node) {
> > > @@ -1205,9 +1207,11 @@ static int rproc_start_subdevices(struct rproc *rproc)
> > >   	return 0;
> > >   unroll_registration:
> > > -	list_for_each_entry_continue_reverse(subdev, &rproc->subdevs, node) {
> > > -		if (subdev->stop)
> > > -			subdev->stop(subdev, true);
> > > +	list_for_each_entry(itr, &rproc->subdevs, node) {
> > > +		if (itr == subdev)
> > > +			break;
> > > +		if (itr->stop)
> > > +			itr->stop(itr, true);
> > >   	}
> > >   	return ret;
> > > @@ -1217,7 +1221,7 @@ static void rproc_stop_subdevices(struct rproc *rproc, bool crashed)
> > >   {
> > >   	struct rproc_subdev *subdev;
> > > -	list_for_each_entry_reverse(subdev, &rproc->subdevs, node) {
> > > +	list_for_each_entry(subdev, &rproc->subdevs, node) {
> > I presume this is the case you actually care about, can you help me
> > understand if you changed the others for consistence or if there's some
> > flow of events where that might be necessary.
> Yes you are right, I only changed the others for consistence.
> However, I will give this more thought and see if unprepare in
> the reverse order can make a difference.
> 

Per above argument I don't think things depend on the unrolling on error
happening in reverse order. But it's idiomatic.

Regards,
Bjorn

> Thanks,
> Sid
> > 
> > Regards,
> > Bjorn
> > 
> > >   		if (subdev->stop)
> > >   			subdev->stop(subdev, crashed);
> > >   	}
> > > @@ -1227,7 +1231,7 @@ static void rproc_unprepare_subdevices(struct rproc *rproc)
> > >   {
> > >   	struct rproc_subdev *subdev;
> > > -	list_for_each_entry_reverse(subdev, &rproc->subdevs, node) {
> > > +	list_for_each_entry(subdev, &rproc->subdevs, node) {
> > >   		if (subdev->unprepare)
> > >   			subdev->unprepare(subdev);
> > >   	}
> > > -- 
> > > Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> > > a Linux Foundation Collaborative Project
> > > 

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