On Wed, Feb 25, 2015 at 06:11:10PM +0100, Peter Zijlstra wrote:
>
> #define IPS_BUSY 0x01
> #define IPS_LOOPED 0x02
>
> struct ipi_pull_struct {
> struct irq_work work;
> raw_spinlock_t lock;
> int flags;
> int dst_cpu; /* CPU that issued this search */
> int dst_prio; /* Prio of the originating CPU */
> int src_cpu; /* CPU where the IPI is running */
> int stop_cpu; /* cpu where we can stop iterating */
> ...
> };
>
> Where you can increase seq every time you change it and the IPI will
> continue at least one more rto mask round for every change?
seq got lost.. ignore that bit, look at the code.
> int ips_next(struct ipi_pull_struct *ips)
> {
> int cpu = ips->src_cpu;
> cpu = cpumask_next(cpu, rto_mask);
> if (cpu >= nr_cpu_ids) {
> cpu = 0;
should be -1
> ips->flags |= IPS_LOOPED;
> cpu = cpumask_next(cpu, rto_mask);
> if (cpu >= nr_cpu_ids) /* empty mask *;
> return cpu;
> }
> if (ips->flags & IPS_LOOPED && cpu >= ips->stop_cpu)
> return nr_cpu_ids;
> return cpu;
> }
>
>
>
> struct ipi_pull_struct *ips = __this_cpu_ptr(ips);
>
> raw_spin_lock(&ips->lock);
> if (ips->flags & IPS_BUSY) {
> /* there is an IPI active; update state */
> ips->dst_prio = current->prio;
> ips->stop_cpu = ips->src_cpu;
> ips->flags &= ~IPS_LOOPED;
> } else {
> /* no IPI active, make one go */
> ips->dst_cpu = smp_processor_id();
> ips->dst_prio = current->prio;
> ips->src_cpu = ips->dst_cpu;
> ips->stop_cpu = ips->dst_cpu;
> ips->flags = IPS_BUSY;
>
> cpu = ips_next(ips);
> ips->src_cpu = cpu;
> if (cpu < nr_cpu_ids)
> irq_work_queue_on(&ips->work, cpu);
> }
> raw_spin_unlock(&ips->lock);
There might be a wee race vs the busy looping, seeing how src_cpu might
be checking against the old dst_prio, so we could check one too few rto
mask.
No real problem I think.
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