From: Doug Anderson <dianders@chromium.org>
To: Sumit Garg <sumit.garg@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Daniel Thompson <daniel.thompson@linaro.org>,
linux-serial@vger.kernel.org,
kgdb-bugreport@lists.sourceforge.net,
Jiri Slaby <jslaby@suse.com>,
Russell King - ARM Linux <linux@armlinux.org.uk>,
Jason Wessel <jason.wessel@windriver.com>,
LKML <linux-kernel@vger.kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC 2/5] serial: core: Add framework to allow NMI aware serial drivers
Date: Thu, 13 Aug 2020 07:37:57 -0700 [thread overview]
Message-ID: <CAD=FV=Vu3PGSUzargD-6e2XOw=Eh7CZaQ_+a09dr8SR1T8eE2g@mail.gmail.com> (raw)
In-Reply-To: <CAFA6WYNq-Z5WD=AqJn2_DEg0F6G1CYte2y5Snc964vsgCnr0Bw@mail.gmail.com>
Hi,
On Thu, Aug 13, 2020 at 7:19 AM Sumit Garg <sumit.garg@linaro.org> wrote:
>
> On Thu, 13 Aug 2020 at 05:29, Doug Anderson <dianders@chromium.org> wrote:
> >
> > Hi,
> >
> > On Tue, Jul 21, 2020 at 5:11 AM Sumit Garg <sumit.garg@linaro.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@linaro.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.
-Doug
next prev parent reply other threads:[~2020-08-13 14:38 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-21 12:10 [RFC 0/5] Introduce NMI aware serial drivers Sumit Garg
2020-07-21 12:10 ` [RFC 1/5] tty/sysrq: Make sysrq handler NMI aware Sumit Garg
2020-08-12 23:59 ` Doug Anderson
2020-08-14 7:24 ` Sumit Garg
2020-08-14 14:34 ` peterz
2020-08-14 14:57 ` Doug Anderson
2020-08-17 14:08 ` Sumit Garg
2020-08-17 17:19 ` Doug Anderson
2020-08-18 13:30 ` Sumit Garg
2020-07-21 12:10 ` [RFC 2/5] serial: core: Add framework to allow NMI aware serial drivers Sumit Garg
2020-08-12 23:59 ` Doug Anderson
2020-08-13 14:19 ` Sumit Garg
2020-08-13 14:37 ` Doug Anderson [this message]
2020-08-14 11:17 ` Sumit Garg
2020-08-14 14:13 ` Daniel Thompson
2020-08-17 12:27 ` Sumit Garg
2020-08-17 13:57 ` Doug Anderson
2020-08-17 14:23 ` Sumit Garg
2020-08-17 14:32 ` Daniel Thompson
2020-08-18 13:18 ` Sumit Garg
2020-08-17 14:28 ` Daniel Thompson
2020-08-18 13:06 ` Sumit Garg
2020-08-14 14:43 ` Doug Anderson
2020-08-17 12:29 ` Sumit Garg
2020-07-21 12:10 ` [RFC 3/5] serial: amba-pl011: Re-order APIs definition Sumit Garg
2020-07-21 12:10 ` [RFC 4/5] serial: amba-pl011: Enable NMI aware uart port Sumit Garg
2020-08-12 23:59 ` Doug Anderson
2020-08-13 10:34 ` Sumit Garg
2020-07-21 12:10 ` [RFC 5/5] serial: Remove KGDB NMI serial driver Sumit Garg
2020-08-11 13:50 ` [RFC 0/5] Introduce NMI aware serial drivers Sumit Garg
2020-08-11 13:58 ` Greg Kroah-Hartman
2020-08-11 14:29 ` Sumit Garg
2020-08-11 14:58 ` Greg Kroah-Hartman
2020-08-11 17:15 ` Doug Anderson
2020-08-12 14:52 ` Sumit Garg
2020-08-12 15:27 ` Doug Anderson
2020-08-13 0:08 ` Doug Anderson
2020-08-13 9:25 ` Sumit Garg
2020-08-13 10:17 ` Daniel Thompson
2020-08-14 12:06 ` Sumit Garg
2020-08-14 14:18 ` Daniel Thompson
2020-08-17 5:12 ` Sumit Garg
2020-08-17 9:28 ` Daniel Thompson
2020-08-17 14:12 ` Sumit Garg
2020-08-13 15:26 ` Doug Anderson
2020-08-14 12:50 ` Sumit Garg
2020-08-12 5:48 ` Sumit Garg
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