linux-kernel - Re: [PATCH 05/17] writeback: quit throttling when signal pending

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Message-ID: <20100913034808.GA9196@localhost>
Date:	Mon, 13 Sep 2010 11:48:08 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Neil Brown <neilb@...e.de>
Cc:	linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Chris Mason <chris.mason@...cle.com>,
	Christoph Hellwig <hch@....de>,
	"Li, Shaohua" <shaohua.li@...el.com>
Subject: Re: [PATCH 05/17] writeback: quit throttling when signal pending

On Mon, Sep 13, 2010 at 11:21:16AM +0800, Neil Brown wrote:
> On Mon, 13 Sep 2010 09:55:29 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > On Mon, Sep 13, 2010 at 04:46:54AM +0800, Neil Brown wrote:
> > > On Sun, 12 Sep 2010 23:49:50 +0800
> > > Wu Fengguang <fengguang.wu@...el.com> wrote:
> > > 
> > > > This allows quick response to Ctrl-C etc. for impatient users.
> > > > 
> > > > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > > > ---
> > > >  mm/page-writeback.c |    3 +++
> > > >  1 file changed, 3 insertions(+)
> > > > 
> > > > --- linux-next.orig/mm/page-writeback.c	2010-09-09 16:01:14.000000000 +0800
> > > > +++ linux-next/mm/page-writeback.c	2010-09-09 16:02:27.000000000 +0800
> > > > @@ -553,6 +553,9 @@ static void balance_dirty_pages(struct a
> > > >  		__set_current_state(TASK_INTERRUPTIBLE);
> > > >  		io_schedule_timeout(pause);
> > > >  
> > > > +		if (signal_pending(current))
> > > > +			break;
> > > > +
> > > 
> > > Given the patch description,  I think you might want "fatal_signal_pending()"
> > > here ???
> > 
> > __fatal_signal_pending() tests SIGKILL only, while the one often used
> > and need more quick responding is SIGINT..
> >
> 
> I thought that at first too....  but I don't think that is the case.
> 
> In kernel/signal.c, in complete_signal, we have
>   if (sig_fatal() ...)
>            ....
> 		sigaddset(&t->pending.signal, SIGKILL);
> 
> where sig_fatal is
> 
> #define sig_fatal(t, signr) \
> 	(!siginmask(signr, SIG_KERNEL_IGNORE_MASK|SIG_KERNEL_STOP_MASK) && \
> 	 (t)->sighand->action[(signr)-1].sa.sa_handler == SIG_DFL)
> 
> 
> so (if I'm reading the code correctly), if a process receives a signal for
> which the handler is SIG_DFL, then SIGKILL is set in the pending mask, so
> __fatal_signal_pending will be true.
> 
> So it fatal_signal_pending should catch any signal that will cause the
> process to exit.  I assume that it what you want...

Ah yes, it does look so. Thanks for the detailed explanation!
Here is the updated patch.

Thanks,
Fengguang
---
Subject: writeback: quit throttling when fatal signal pending
From: Wu Fengguang <fengguang.wu@...el.com>
Date: Wed Sep 08 17:40:22 CST 2010

This allows quick response to Ctrl-C etc. for impatient users.

It mainly helps the rare bdi/global dirty exceeded cases.
In the normal case of not exceeded, it will quit the loop anyway. 

CC: Neil Brown <neilb@...e.de>
Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
 mm/page-writeback.c |    3 +++
 1 file changed, 3 insertions(+)

--- linux-next.orig/mm/page-writeback.c	2010-09-12 13:25:23.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-09-13 11:39:33.000000000 +0800
@@ -552,6 +552,9 @@ static void balance_dirty_pages(struct a
 		__set_current_state(TASK_INTERRUPTIBLE);
 		io_schedule_timeout(pause);
 
+		if (fatal_signal_pending(current))
+			break;
+
 check_exceeded:
 		/*
 		 * The bdi thresh is somehow "soft" limit derived from the
--
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