linux-kernel - Re: [PATCH 18/45] writeback: introduce wait queue for balance_dirty

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Message-Id: <20091008114037.56f78b0e.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 8 Oct 2009 11:40:37 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Myklebust Trond <Trond.Myklebust@...app.com>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	Jan Kara <jack@...e.cz>, Nick Piggin <npiggin@...e.de>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 18/45] writeback: introduce wait queue for
 balance_dirty_pages()

On Thu, 8 Oct 2009 09:58:22 +0800
Wu Fengguang <fengguang.wu@...el.com> wrote:
> On Thu, Oct 08, 2009 at 09:01:59AM +0800, KAMEZAWA Hiroyuki wrote:
> > 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 ?
> 
> Good question. io_schedule() has an old comment for throttling IO wait:
> 
>          * But don't do that if it is a deliberate, throttling IO wait (this task
>          * has set its backing_dev_info: the queue against which it should throttle)
>          */
>         void __sched io_schedule(void)
> 
> So it looks both Jens' and this patch behaves right in ignoring the
> iowait accounting for balance_dirty_pages() :)
> 
Thank you for clarification.
Then, hmm, %iotwait (which 'top' shows) didn't work as desgined and we need
to update throttle_vm_writeout() and some in vmscan.c. Thanks for input.

BTW, I'm glad if I can know "how many threads/ios are throttoled now" per bdi.

Regards,
-Kame






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