Re: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF_NODELAY)

[Darren Hart]

On 05/18/2010 02:52 PM, Brian King wrote:
> Is IRQF_NODELAY something specific to the RT kernel? I don't see it in mainline...

Yes, it basically says "don't make this handler threaded".

--
Darren

> -Brian
>
>
> On 05/18/2010 04:33 PM, dvhltc@xxxxxxxxxxxxxxxxxx wrote:
> > > From ad81664794e33d785f533c5edee37aaba20dd92d Mon Sep 17 00:00:00 2001
> > From: Darren Hart<dvhltc@xxxxxxxxxx>
> > Date: Tue, 18 May 2010 11:07:13 -0700
> > Subject: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF_NODELAY)
> >
> > The underlying hardware is edge triggered but presented by XICS as level
> > triggered. The edge triggered interrupts are not reissued after masking. This
> > is not a problem in mainline which does not mask the interrupt (relying on the
> > EOI mechanism instead). The threaded interrupts in PREEMPT_RT do mask the
> > interrupt, and can lose interrupts that occurred while masked, resulting in a
> > hung ethernet interface.
> >
> > The receive handler simply calls napi_schedule(), as such, there is no
> > significant additional overhead in making this non-threaded, since we either
> > wakeup the threaded irq handler to call napi_schedule(), or just call
> > napi_schedule() directly to wakeup the softirqs.  As the receive handler is
> > lockless, there is no need to convert any of the ehea spinlock_t's to
> > atomic_spinlock_t's.
> >
> > Without this patch, a simple scp file copy loop would fail quickly (usually
> > seconds). We have over two hours of sustained scp activity with the patch
> > applied.
> >
> > Credit goes to Will Schmidt for lots of instrumentation and tracing which
> > clarified the scenario and to Thomas Gleixner for the incredibly simple
> > solution.
> >
> > Signed-off-by: Darren Hart<dvhltc@xxxxxxxxxx>
> > Acked-by: Will Schmidt<will_schmidt@xxxxxxxxxxxx>
> > Cc: Thomas Gleixner<tglx@xxxxxxxxxxxxxx>
> > Cc: Jan-Bernd Themann<themann@xxxxxxxxxx>
> > Cc: Nivedita Singhvi<niv@xxxxxxxxxx>
> > Cc: Brian King<bjking1@xxxxxxxxxx>
> > Cc: Michael Ellerman<ellerman@xxxxxxxxxxx>
> > Cc: Doug Maxey<doug.maxey@xxxxxxxxxx>
> > ---
> >   drivers/net/ehea/ehea_main.c |    2 +-
> >   1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/drivers/net/ehea/ehea_main.c b/drivers/net/ehea/ehea_main.c
> > index 977c3d3..2c53df2 100644
> > --- a/drivers/net/ehea/ehea_main.c
> > +++ b/drivers/net/ehea/ehea_main.c
> > @@ -1263,7 +1263,7 @@ static int ehea_reg_interrupts(struct net_device *dev)
> >   			 "%s-queue%d", dev->name, i);
> >   		ret = ibmebus_request_irq(pr->eq->attr.ist1,
> >   					  ehea_recv_irq_handler,
> > -					  IRQF_DISABLED, pr->int_send_name,
> > +					  IRQF_DISABLED | IRQF_NODELAY, pr->int_send_name,
> >   					  pr);
> >   		if (ret) {
> >   			ehea_error("failed registering irq for ehea_queue "


--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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