Re: [PATCH V7 4/4] softirq: Allow early break the softirq processing loop

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Sep 25, 2020 at 01:08:11AM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 24, 2020 at 05:37:42PM +0200, Thomas Gleixner wrote:
> > Subject: softirq; Prevent starvation of higher softirq vectors
> > From: Thomas Gleixner <tglx@linutronix.de>
> > Date: Thu, 24 Sep 2020 10:40:24 +0200
> > 
> > From: Thomas Gleixner <tglx@linutronix.de>
> > 
> > The early termination of the softirq processing loop can lead to starvation
> > of the higher numbered soft interrupt vectors because each run starts at
> > the lowest bit. If the loop terminates then the already processed bits can
> > be raised again before the next loop starts. If these lower bits run into
> > the termination again, then a re-raise might starve the higher bits forever.
> > 
> > To prevent this, store the leftovers of the previous run in the upper 16
> > bit of the local softirq_pending storage and ensure that these are
> > processed before any newly raised bits are handled.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > ---
> >  kernel/softirq.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 50 insertions(+), 8 deletions(-)
> > 
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> > @@ -259,11 +259,23 @@ static inline bool __softirq_needs_break
> >  	return need_resched() || __softirq_timeout(tbreak);
> >  }
> >  
> > +/*
> > + * local_softirq_pending() is split into two 16 bit words. The low word
> > + * contains the bits set by raise_softirq(), the high word contains pending
> > + * bits which have not been processed in an early terminated run. This is
> > + * required to prevent starvation of the higher numbered softirqs.
> > + */
> > +#define SIRQ_PREV_SHIFT		16
> 
> Note that in the case of x86, irq_start.__softirq_pending is a u16.

irq_stat even