Date:   Mon, 17 Aug 2020 17:59:03 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-serial@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
        Jiri Slaby <jslaby@...e.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Jason Wessel <jason.wessel@...driver.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 2/5] serial: core: Add framework to allow NMI aware serial drivers

On Fri, 14 Aug 2020 at 20:14, Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Fri, Aug 14, 2020 at 4:17 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> >
> > On Thu, 13 Aug 2020 at 20:08, Doug Anderson <dianders@...omium.org> wrote:
> > >
> > > Hi,
> > >
> > > On Thu, Aug 13, 2020 at 7:19 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> > > >
> > > > On Thu, 13 Aug 2020 at 05:29, Doug Anderson <dianders@...omium.org> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > On Tue, Jul 21, 2020 at 5:11 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> > > > > >
> > > > > > Add NMI framework APIs in serial core which can be leveraged by serial
> > > > > > drivers to have NMI driven serial transfers. These APIs are kept under
> > > > > > CONFIG_CONSOLE_POLL as currently kgdb initializing uart in polling mode
> > > > > > is the only known user to enable NMI driven serial port.
> > > > > >
> > > > > > The general idea is to intercept RX characters in NMI context, if those
> > > > > > are specific to magic sysrq then allow corresponding handler to run in
> > > > > > NMI context. Otherwise defer all other RX and TX operations to IRQ work
> > > > > > queue in order to run those in normal interrupt context.
> > > > > >
> > > > > > Also, since magic sysrq entry APIs will need to be invoked from NMI
> > > > > > context, so make those APIs NMI safe via deferring NMI unsafe work to
> > > > > > IRQ work queue.
> > > > > >
> > > > > > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > > > > > ---
> > > > > >  drivers/tty/serial/serial_core.c | 120 ++++++++++++++++++++++++++++++++++++++-
> > > > > >  include/linux/serial_core.h      |  67 ++++++++++++++++++++++
> > > > > >  2 files changed, 185 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> > > > > > index 57840cf..6342e90 100644
> > > > > > --- a/drivers/tty/serial/serial_core.c
> > > > > > +++ b/drivers/tty/serial/serial_core.c
> > > > > > @@ -3181,8 +3181,14 @@ static bool uart_try_toggle_sysrq(struct uart_port *port, unsigned int ch)
> > > > > >                 return true;
> > > > > >         }
> > > > > >
> > > > > > +#ifdef CONFIG_CONSOLE_POLL
> > > > > > +       if (in_nmi())
> > > > > > +               irq_work_queue(&port->nmi_state.sysrq_toggle_work);
> > > > > > +       else
> > > > > > +               schedule_work(&sysrq_enable_work);
> > > > > > +#else
> > > > > >         schedule_work(&sysrq_enable_work);
> > > > > > -
> > > > > > +#endif
> > > > >
> > > > > It should be a very high bar to have #ifdefs inside functions.  I
> > > > > don't think this meets it.  Instead maybe something like this
> > > > > (untested and maybe slightly wrong syntax, but hopefully makes
> > > > > sense?):
> > > > >
> > > > > Outside the function:
> > > > >
> > > > > #ifdef CONFIG_CONSOLE_POLL
> > > > > #define queue_port_nmi_work(port, work_type)
> > > > > irq_work_queue(&port->nmi_state.work_type)
> > > > > #else
> > > > > #define queue_port_nmi_work(port, work_type)
> > > > > #endif
> > > > >
> > > > > ...and then:
> > > > >
> > > > > if (IS_ENABLED(CONFIG_CONSOLE_POLL) && in_nmi())
> > > > >   queue_port_nmi_work(port, sysrq_toggle_work);
> > > > > else
> > > > >   schedule_work(&sysrq_enable_work);
> > > > >
> > > > > ---
> > > > >
> > > > > The whole double-hopping is really quite annoying.  I guess
> > > > > schedule_work() can't be called from NMI context but can be called
> > > > > from IRQ context?  So you need to first transition from NMI context to
> > > > > IRQ context and then go and schedule the work?  Almost feels like we
> > > > > should just fix schedule_work() to do this double-hop for you if
> > > > > called from NMI context.  Seems like you could even re-use the list
> > > > > pointers in the work_struct to keep the queue of people who need to be
> > > > > scheduled from the next irq_work?  Worst case it seems like you could
> > > > > add a schedule_work_nmi() that would do all the hoops for you.  ...but
> > > > > I also know very little about NMI so maybe I'm being naive.
> > > > >
> > > >
> > > > Thanks for this suggestion and yes indeed we could make
> > > > schedule_work() NMI safe and in turn get rid of all this #ifdefs. Have
> > > > a look at below changes:
> > > >
> > > > diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> > > > index 26de0ca..1daf1b4 100644
> > > > --- a/include/linux/workqueue.h
> > > > +++ b/include/linux/workqueue.h
> > > > @@ -14,6 +14,7 @@
> > > >  #include <linux/atomic.h>
> > > >  #include <linux/cpumask.h>
> > > >  #include <linux/rcupdate.h>
> > > > +#include <linux/irq_work.h>
> > > >
> > > >  struct workqueue_struct;
> > > >
> > > > @@ -106,6 +107,7 @@ struct work_struct {
> > > >  #ifdef CONFIG_LOCKDEP
> > > >         struct lockdep_map lockdep_map;
> > > >  #endif
> > > > +       struct irq_work iw;
> > >
> > > Hrm, I was thinking you could just have a single queue per CPU then
> > > you don't need to add all this extra data to every single "struct
> > > work_struct".  I was thinking you could use the existing list node in
> > > the "struct work_struct" to keep track of the list of things.  ...but
> > > maybe my idea this isn't actually valid because the linked list might
> > > be in use if we're scheduling work that's already pending / running?
> > >
> > > In any case, I worry that people won't be happy with the extra
> > > overhead per "struct work_struct".  Can we reduce it at all?  It still
> > > does feel like you could get by with a single global queue and thus
> > > you wouldn't need to store the function pointer and flags with every
> > > "struct work_struct", right?  So all you'd need is a single pointer
> > > for the linked list?  I haven't actually tried implementing this,
> > > though, so I could certainly be wrong.
> >
> > Let me try to elaborate here:
> >
> > Here we are dealing with 2 different layers of deferring work, one is
> > irq_work (NMI safe) using "struct irq_work" and other is normal
> > workqueue (NMI unsafe) using "struct work_struct".
> >
> > So when we are in NMI context, the only option is to use irq_work to
> > defer work and need to pass reference to "struct irq_work". Now in
> > following irq_work function:
> >
> > +void queue_work_nmi(struct irq_work *iw)
> > +{
> > +       struct work_struct *work = container_of(iw, struct work_struct, iw);
> > +
> > +       queue_work(system_wq, work);
> > +}
> > +EXPORT_SYMBOL(queue_work_nmi);
> >
> > we can't find a reference to "struct work_struct" until there is 1:1
> > mapping with "struct irq_work". So we require a way to establish this
> > mapping and having "struct irq_work" as part of "struct work_struct"
> > tries to achieve that. If you have any better way to achieve this, I
> > can use that instead.
>
> So I guess the two options to avoid the overhead are:
>
> 1. Create a new struct:
>
> struct nmi_queuable_work_struct {
>   struct work_struct work;
>   struct irq_work iw;
> };
>
> Then the overhead is only needed for those that want this
> functionality.  Those people would need to use a variant
> nmi_schedule_work() which, depending on in_nmi(), would either
> schedule it directly or use the extra work.
>
> Looks like Daniel already responded and suggested this.
>
>
> 2. Something that duplicates the code of at least part of irq_work and
> therefore saves the need to store the function pointer.  Think of it
> this way: if you made a whole copy of irq_work that was hardcoded to
> just call the function you wanted then you wouldn't need to store a
> function pointer.  This is, of course, excessive.  I was trying to
> figure out if you could do less by only copying the NMI-safe
> linked-list manipulation, but this is probably impossible and not
> worth it anyway.
>

Thanks for your suggestions. I came up with an approach without any
overhead (see my reply to Daniel).

-Sumit

> -Doug

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