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Subject: Re: [PATCH 18/45] writeback: introduce wait queue for
 balance_dirty_pages()
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>,
       Andrew Morton <akpm@linux-foundation.org>, Theodore Tso <tytso@mit.edu>,
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Date: Thu, 08 Oct 2009 10:05:26 +0200
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On Thu, 2009-10-08 at 10:01 +0900, KAMEZAWA Hiroyuki wrote:

> May I ask a question ? (maybe not directly related to this patch itself, sorry)
> 
> Recent works as "writeback: switch to per-bdi threads for flushing data"
> removed congestion_wait() from balance_dirty_pages() and added
> schedule_timeout_interruptible(). 
> 
> And this one replaces it with wake_up+wait_queue.
> 
> IIUC, "iowait" cpustat data was calculated by runqueue->nr_iowait as
> == kernel/schec.c
> void account_idle_time(cputime_t cputime)
> {
>         struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
>         cputime64_t cputime64 = cputime_to_cputime64(cputime);
>         struct rq *rq = this_rq();
> 
>         if (atomic_read(&rq->nr_iowait) > 0)
>                 cpustat->iowait = cputime64_add(cpustat->iowait, cputime64);
>         else
>                 cpustat->idle = cputime64_add(cpustat->idle, cputime64);
> }
> ==
> Then, for showing "cpu is in iowait", runqueue->nr_iowait should be modified
> at some places. In old kernel, congestion_wait() at el did that by calling
> io_schedule_timeout().
> 
> How this runqueue->nr_iowait is handled now ?

Ah, I think you've got a good point, we need a io_schedule() there.