On Tue, Oct 29, 2013 at 09:00:52AM +0530, Preeti U Murthy wrote:

quoted

Oh nice, that gets rid of the multiple atomics, and it nicely splits
this nohz logic into per topology groups -- now if only we could split
the rest too :-)

I am sorry, I don't get you here. By the 'rest', do you refer to
nohz_kick_needed() as below? Or am I missing something?

Nah, the rest of the NOHZ infrastructure. Currently its global state;
there were some patches a few years ago that attempted to make that
per-node state, but that work stalled due to people switching jobs.

quoted

quoted

+	sd = highest_flag_domain(cpu, SD_ASYM_PACKING);
+
+	if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
+				  sched_domain_span(sd)) < cpu))
+		goto need_kick_unlock;
+
 	rcu_read_unlock();
 	return 0;

This again is a bit sad; most archs will not have SD_ASYM_PACKING set at
all; this means that they all will do a complete (and pointless) sched
domain tree walk here.

There will not be a 'complete' sched domain tree walk right? The
iteration will break at the first level of the sched domain for those
archs which do not have SD_ASYM_PACKING set at all.

Ah indeed; I think I got confused due to me modifying
highest_flag_domain() earlier to assume a flag is carried from the
lowest domain upwards.

But it is true that doing a sched domain tree walk regularly is a bad
idea, might as well update the domain with SD_ASYM_PACKING flag set once
and query this domain when required.

I will send out the patch with sd_asym domain introduced rather than the
above.

Thanks