Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

[Peter Zijlstra]

So what's wrong with something like:

struct cpu_idletime {
       seqlock_t seqlock;
       unsigned long nr_iowait;
       u64 start;
       u64 idle_time,
       u64 iowait_time,
} __cacheline_aligned_in_smp;

DEFINE_PER_CPU(struct cpu_idletime, cpu_idletime);

void io_schedule(void)
{
	struct cpu_idletime *it = __raw_get_cpu_var(cpu_idletime);

	write_seqlock(&it->seqlock);
	if (!it->nr_iowait++)
		it->start = local_clock();
	write_sequnlock(&it->seqlock);

	current->in_iowait = 1;
	schedule();
	current->in_iowait = 0;

	write_seqlock(&it->seqlock);
	if (!--it->nr_iowait)
		it->iowait_time += local_clock() - it->start;
	write_sequnlock(&it->seqlock);
}

Afaict you don't need the preempt disable and atomic muck at all.

It will all get a little more complicated to deal with overlapping idle
and iowait times, but the idea is the same.
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