On 04/08/2016 12:47 PM, Tejun Heo wrote:
Hello, Waiman.
On Fri, Apr 08, 2016 at 12:16:20PM -0400, Waiman Long wrote:
+/**
+ * __percpu_stats_add - add given count to percpu value
+ * @pcs : Pointer to percpu_stats structure
+ * @stat: The statistics count that needs to be updated
+ * @cnt: The value to be added to the statistics count
+ */
+void __percpu_stats_add(struct percpu_stats *pcs, int stat, int cnt)
+{
+ /*
+ * u64_stats_update_begin/u64_stats_update_end alone are not safe
+ * against recursive add on the same CPU caused by interrupt.
+ * So we need to set the PCPU_STAT_INTSAFE flag if this is required.
+ */
+ if (IS_STATS64(pcs)) {
+ uint64_t *pstats64;
+ unsigned long flags;
+
+ pstats64 = get_cpu_ptr(pcs->stats64);
+ if (pcs->flags& PCPU_STAT_INTSAFE)
+ local_irq_save(flags);
+
+ u64_stats_update_begin(&pcs->sync);
+ pstats64[stat] += cnt;
+ u64_stats_update_end(&pcs->sync);
+
+ if (pcs->flags& PCPU_STAT_INTSAFE)
+ local_irq_restore(flags);
+
+ put_cpu_ptr(pcs->stats64);
+ }
+}
Heh, that's a handful, and, right, u64_stats needs separate irq
protection. I'm not sure. If we have to do the above, it's likely
that it'll perform worse than percpu_counter on 32bits. On 64bits,
percpu_counter would incur extra preempt_disable/enable() operations
but that comes from it not using this_cpu_add_return(). I wonder
whether it'd be better to either use percpu_counter instead or if
necessary extend it to handle multiple counters. What do you think?
Thanks.
Yes, I think it will be more efficient to use percpu_counter in this
case. The preempt_disable/enable() calls are pretty cheap. Once in a
while, you need to take the lock and update the global count. How about
I change the 2nd patch to use percpu_counter internally when 64-bit
counts are needed in 32-bit archs, but use the regular percpu counts on
64-bit archs? If you are OK with that, I can update the patch accordingly.
Cheers,
Longman
