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Message-ID: <20131107221904.GB2054@quack.suse.cz>
Date: Thu, 7 Nov 2013 23:19:04 +0100
From: Jan Kara <jack@...e.cz>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Michal Hocko <mhocko@...e.cz>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/4] irq_work: Provide a irq work that can be processed
on any cpu
On Thu 07-11-13 23:13:39, Frederic Weisbecker wrote:
> 2013/11/7 Jan Kara <jack@...e.cz>:
> > Provide new irq work flag - IRQ_WORK_UNBOUND - meaning that can be
> > processed on any cpu. This flag implies IRQ_WORK_LAZY so that things are
> > simple and we don't have to pick any particular cpu to do the work. We
> > just do the work from a timer tick on whichever cpu it happens first.
> > This is useful as a lightweight and simple code path without locking or
> > other dependencies to offload work to other cpu if possible.
> >
> > We will use this type of irq work to make a guarantee of forward
> > progress of printing to a (serial) console when printing on one cpu
> > would cause interrupts to be disabled for too long.
> >
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > ---
> > include/linux/irq_work.h | 2 ++
> > kernel/irq_work.c | 41 +++++++++++++++++++++++++----------------
> > 2 files changed, 27 insertions(+), 16 deletions(-)
> >
> > diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
> > index 66017028dcb3..ca07a16355ed 100644
> > --- a/include/linux/irq_work.h
> > +++ b/include/linux/irq_work.h
> > @@ -16,6 +16,8 @@
> > #define IRQ_WORK_BUSY 2UL
> > #define IRQ_WORK_FLAGS 3UL
> > #define IRQ_WORK_LAZY 4UL /* Doesn't want IPI, wait for tick */
> > +#define __IRQ_WORK_UNBOUND 8UL /* Use IRQ_WORK_UNBOUND instead! */
> > +#define IRQ_WORK_UNBOUND (__IRQ_WORK_UNBOUND | IRQ_WORK_LAZY) /* Any cpu can process this work */
> >
> > struct irq_work {
> > unsigned long flags;
> > diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> > index 55fcce6065cf..b06350b63c67 100644
> > --- a/kernel/irq_work.c
> > +++ b/kernel/irq_work.c
> > @@ -22,6 +22,9 @@
> > static DEFINE_PER_CPU(struct llist_head, irq_work_list);
> > static DEFINE_PER_CPU(int, irq_work_raised);
> >
> > +/* List of irq-work any CPU can pick up */
> > +static LLIST_HEAD(unbound_irq_work_list);
> > +
> > /*
> > * Claim the entry so that no one else will poke at it.
> > */
> > @@ -70,12 +73,16 @@ void irq_work_queue(struct irq_work *work)
> > /* Queue the entry and raise the IPI if needed. */
> > preempt_disable();
> >
> > - llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
> > + if (work->flags & __IRQ_WORK_UNBOUND)
> > + llist_add(&work->llnode, &unbound_irq_work_list);
> > + else
> > + llist_add(&work->llnode, &__get_cpu_var(irq_work_list));
> >
> > /*
> > * If the work is not "lazy" or the tick is stopped, raise the irq
> > * work interrupt (if supported by the arch), otherwise, just wait
> > - * for the next tick.
> > + * for the next tick. We do this even for unbound work to make sure
> > + * *some* CPU will be doing the work.
> > */
> > if (!(work->flags & IRQ_WORK_LAZY) || tick_nohz_tick_stopped()) {
> > if (!this_cpu_cmpxchg(irq_work_raised, 0, 1))
> > @@ -100,28 +107,17 @@ bool irq_work_needs_cpu(void)
> > return true;
> > }
> >
> > -static void __irq_work_run(void)
> > +static void process_irq_work_list(struct llist_head *llhead)
> > {
> > unsigned long flags;
> > struct irq_work *work;
> > - struct llist_head *this_list;
> > struct llist_node *llnode;
> >
> > -
> > - /*
> > - * Reset the "raised" state right before we check the list because
> > - * an NMI may enqueue after we find the list empty from the runner.
> > - */
> > - __this_cpu_write(irq_work_raised, 0);
> > - barrier();
> > -
> > - this_list = &__get_cpu_var(irq_work_list);
> > - if (llist_empty(this_list))
> > + if (llist_empty(llhead))
> > return;
> >
> > BUG_ON(!irqs_disabled());
> > -
> > - llnode = llist_del_all(this_list);
> > + llnode = llist_del_all(llhead);
> > while (llnode != NULL) {
> > work = llist_entry(llnode, struct irq_work, llnode);
> >
> > @@ -146,6 +142,19 @@ static void __irq_work_run(void)
> > }
> > }
> >
> > +static void __irq_work_run(void)
> > +{
> > + /*
> > + * Reset the "raised" state right before we check the list because
> > + * an NMI may enqueue after we find the list empty from the runner.
> > + */
> > + __this_cpu_write(irq_work_raised, 0);
> > + barrier();
> > +
> > + process_irq_work_list(&__get_cpu_var(irq_work_list));
> > + process_irq_work_list(&unbound_irq_work_list);
> > +}
> > +
>
> But then, who's going to process that work if every CPUs is idle?
Have a look into irq_work_queue(). There is:
/*
* If the work is not "lazy" or the tick is stopped, raise the irq
* work interrupt (if supported by the arch), otherwise, just wait
* for the next tick. We do this even for unbound work to make sure
* *some* CPU will be doing the work.
*/
if (!(work->flags & IRQ_WORK_LAZY) || tick_nohz_tick_stopped()) {
if (!this_cpu_cmpxchg(irq_work_raised, 0, 1))
arch_irq_work_raise();
}
So we raise an interrupt if there would be no timer ticking (which is
what I suppose you mean by "CPU is idle"). That is nothing changed by my
patches...
Honza
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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