Re: [PATCH] platform/chrome: cros_ec_spi: Transfer messages at high priority

[Doug Anderson]

Hi,

On Tue, Apr 2, 2019 at 4:19 PM Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote:
>
> Hi Doug,
>
> On Tue, Apr 02, 2019 at 03:44:44PM -0700, Douglas Anderson wrote:
> > The software running on the Chrome OS Embedded Controller (cros_ec)
> > handles SPI transfers in a bit of a wonky way.  Specifically if the EC
> > sees too long of a delay in a SPI transfer it will give up and the
> > transfer will be counted as failed.  Unfortunately the timeout is
> > fairly short, though the actual number may be different for different
> > EC codebases.
> >
> > We can end up tripping the timeout pretty easily if we happen to
> > preempt the task running the SPI transfer and don't get back to it for
> > a little while.
> >
> > Historically this hasn't been a _huge_ deal because:
> > 1. On old devices Chrome OS used to run PREEMPT_VOLUNTARY.  That meant
> >    we were pretty unlikely to take a big break from the transfer.
> > 2. On recent devices we had faster / more processors.
> > 3. Recent devices didn't use "cros-ec-spi-pre-delay".  Using that
> >    delay makes us more likely to trip this use case.
> > 4. For whatever reasons (I didn't dig) old kernels seem to be less
> >    likely to trip this.
> > 5. For the most part it's kinda OK if a few transfers to the EC fail.
> >    Mostly we're just polling the battery or doing some other task
> >    where we'll try again.
> >
> > Even with the above things, this issue has reared its ugly head
> > periodically.  We could solve this in a nice way by adding reliable
> > retries to the EC protocol [1] or by re-designing the code in the EC
> > codebase to allow it to wait longer, but that code doesn't ever seem
> > to get changed.  ...and even if it did, it wouldn't help old devices.
> >
> > It's now time to finally take a crack at making this a little better.
> > This patch isn't guaranteed to make every cros_ec SPI transfer
> > perfect, but it should improve things by a few orders of magnitude.
> > Specifically you can try this on a rk3288-veyron Chromebook (which is
> > slower and also _does_ need "cros-ec-spi-pre-delay"):
> >   md5sum /dev/zero &
> >   md5sum /dev/zero &
> >   md5sum /dev/zero &
> >   md5sum /dev/zero &
> >   while true; do
> >     cat /sys/class/power_supply/sbs-20-000b/charge_now > /dev/null;
> >   done
> > ...before this patch you'll see boatloads of errors.  After this patch I
> > don't see any in the testing I did.
> >
> > The way this patch works is by effectively boosting the priority of
> > the cros_ec transfers.  As far as I know there is no simple way to
> > just boost the priority of the current process temporarily so the way
> > we accomplish this is by creating a "WQ_HIGHPRI" workqueue and doing
> > the transfers there.
> >
> > NOTE: this patch relies on the fact that the SPI framework attempts to
> > push the messages out on the calling context (which is the one that is
> > boosted to high priority).  As I understand from earlier (long ago)
> > discussions with Mark Brown this should be a fine assumption.  Even if
> > it isn't true sometimes this patch will still not make things worse.
> >
> > [1] https://crbug.com/678675
> >
> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > ---
> >
> >  drivers/platform/chrome/cros_ec_spi.c | 107 ++++++++++++++++++++++++--
> >  1 file changed, 101 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
> > index ffc38f9d4829..101f2deb7d3c 100644
> > --- a/drivers/platform/chrome/cros_ec_spi.c
> > +++ b/drivers/platform/chrome/cros_ec_spi.c
> >
> > ...
> >
> > +static int cros_ec_pkt_xfer_spi(struct cros_ec_device *ec_dev,
> > +                             struct cros_ec_command *ec_msg)
> > +{
> > +     struct cros_ec_spi *ec_spi = ec_dev->priv;
> > +     struct cros_ec_xfer_work_params params;
> > +
> > +     INIT_WORK(&params.work, cros_ec_pkt_xfer_spi_work);
> > +     params.ec_dev = ec_dev;
> > +     params.ec_msg = ec_msg;
> > +
> > +     queue_work(ec_spi->high_pri_wq, &params.work);
> > +     flush_workqueue(ec_spi->high_pri_wq);
>
> IIRC dedicated workqueues should be avoided unless they are needed. In
> this case it seems you could use system_highpri_wq + a
> completion. This would add a few extra lines to deal with the
> completion, in exchange the code to create the workqueue could be
> removed.

I'm not convinced using the "system_highpri_wq" is a great idea here.
Using flush_workqueue() on the "system_highpri_wq" seems like a recipe
for deadlock but I need to flush to get the result back.  See the
comments in flush_scheduled_work() for some discussion here.

I guess you're suggesting using a completion instead of the flush but
I think the deadlock potentials are the same.  If we're currently
running on the "system_highpri_wq" (because one of our callers
happened to be on it) or there are some shared resources between
another user of the "system_highpri_wq" and us then we'll just sitting
waiting for the completion, won't we?

I would bet that currently nobody actually ends up in this situation
because there aren't lots of users of the "system_highpri_wq", but it
still doesn't seem like a good design.  Is it really that expensive to
have our own workqueue?


> > +     return params.ret;
> > +}
> > +
> > +static void cros_ec_cmd_xfer_spi_work(struct work_struct *work)
> > +{
> > +     struct cros_ec_xfer_work_params *params;
> > +
> > +     params = container_of(work, struct cros_ec_xfer_work_params, work);
> > +     params->ret = do_cros_ec_cmd_xfer_spi(params->ec_dev, params->ec_msg);
> > +}
> > +
> > +static int cros_ec_cmd_xfer_spi(struct cros_ec_device *ec_dev,
> > +                             struct cros_ec_command *ec_msg)
> > +{
> > +     struct cros_ec_spi *ec_spi = ec_dev->priv;
> > +     struct cros_ec_xfer_work_params params;
> > +
> > +     INIT_WORK(&params.work, cros_ec_cmd_xfer_spi_work);
> > +     params.ec_dev = ec_dev;
> > +     params.ec_msg = ec_msg;
> > +
> > +     queue_work(ec_spi->high_pri_wq, &params.work);
> > +     flush_workqueue(ec_spi->high_pri_wq);
> > +
> > +     return params.ret;
> > +}
>
> This is essentially a copy of cros_ec_pkt_xfer_spi() above. You
> could add a wrapper that receives the work function to avoid the
> duplicate code.

Good point.  I'll wait a day or two for more feedback and then post a
new version with that change.

-Doug