On Fri, Aug 26, 2011 at 08:51:09PM +0800, Peter Zijlstra wrote:

On Fri, 2011-08-26 at 19:38 +0800, Wu Fengguang wrote:

quoted

+       preempt_disable();
        /*
-        * Check the rate limiting. Also, we do not want to throttle real-time
-        * tasks in balance_dirty_pages(). Period.
+        * This prevents one CPU to accumulate too many dirtied pages without
+        * calling into balance_dirty_pages(), which can happen when there are
+        * 1000+ tasks, all of them start dirtying pages at exactly the same
+        * time, hence all honoured too large initial task->nr_dirtied_pause.
         */
-       preempt_disable();
        p =  &__get_cpu_var(bdp_ratelimits);

	p = &get_cpu_var(bdp_ratelimits);

Ah yeah.. I actually followed your suggestion, and then find we'll
eventually do two __get_cpu_var() calls here, one for bdp_ratelimits
and the other for dirty_leaks in a planned patch. So let's keep the
preempt_disable()/preempt_enable().

quoted

-       *p += nr_pages_dirtied;
-       if (unlikely(*p >= ratelimit)) {
-               ratelimit = sync_writeback_pages(*p);
+       if (unlikely(current->nr_dirtied >= ratelimit))
                *p = 0;
-               preempt_enable();
-               balance_dirty_pages(mapping, ratelimit);
-               return;
+       else {
+               *p += nr_pages_dirtied;
+               if (unlikely(*p >= ratelimit_pages)) {
+                       *p = 0;
+                       ratelimit = 0;
+               }
        }
        preempt_enable(); 

	put_cpu_var(bdp_ratelimits);

ditto.

Thanks,
Fengguang

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