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: Fri, 14 Aug 2020 07:43:57 -0700 [thread overview]
Message-ID: <CAD=FV=XA91CcyGMHKmnMG4LD7HO1d65Fuq3nDWDH_NKPOh+n3Q@mail.gmail.com> (raw)
In-Reply-To: <CAFA6WYPJ_w+R15NRKK5BzZtTxKq8Gh_mGswuYbW0cYZoBYLhxw@mail.gmail.com>
Hi,
On Fri, Aug 14, 2020 at 4:17 AM Sumit Garg <sumit.garg@linaro.org> wrote:
>
> On Thu, 13 Aug 2020 at 20:08, Doug Anderson <dianders@chromium.org> wrote:
> >
> > 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.
>
> 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.
-Doug
next prev parent reply other threads:[~2020-08-14 14:44 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
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 [this message]
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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