On Fri, Sep 23, 2005 at 12:02:15AM +0530, Srivatsa Vaddagiri wrote:
> I feel this is a bit tricky on non-comparator based interrupt sources like
> a decrementer on PPC64 or the local APIC timer.
[snip]
> We could consider passing absolute value to 'reprogram' (say 105), like below:
>
> unsigned int dyn_tick_reprogram_timer(void)
> {
> int cpu = smp_processor_id();
> unsigned long next, delta, seq;
>
> cpu_set(cpu, nohz_cpu_mask);
>
> smp_wmb();
>
> if (rcu_pending(cpu) || local_softirq_pending()) {
> cpu_clear(cpu, nohz_cpu_mask);
> return 0;
> }
>
> do {
> read_seqbegin(&xtime_lock);
>
> next = next_timer_interrupt();
> delta = next - jiffies;
>
> if (delta < dyn_tick->min_skip) {
> cpu_clear(cpu, nohz_cpu_mask);
> return 0;
> }
>
> if (delta > dyn_tick->max_skip)
> next = jiffies + dyn_tick->max_skip;
>
> } while (read_seqretry(&xtime_lock, seq));
>
> dyn_tick->reprogram(next);
>
> return delta;
> }
>
>
> Since reprogram has to convert it back to some relative number, it will need
> to reference jiffy, which makes it racy and require the read_seqbegin/retry
> based conversion to relative number. I feel it is lot cleaner in such
> a case to just take a write_lock(&xtime_lock) for the whole of
> dyn_tick_reprogram_timer.
OTOH, write_seqlock is probably more heavier compared to read_seqlock. So
I am OK if we want to call 'reprogram' w/o any xtime_lock held and that
routine internally uses a read_seqlock if it wants.
Let me know what you guys think about this and the rest of the interface.
If it seems Ok, I can post modified i386 patch based on this interface and
would request Martin/Tony to do the S390/ARM ports.
--
Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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