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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
