linux-kernel - Re: Excessive stall times on ext4 in 3.9-rc2

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Date:	Sat, 20 Apr 2013 20:05:22 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>
Subject: Re: Excessive stall times on ext4 in 3.9-rc2

As an update to this thread, we brought up this issue at LSF/MM, and
there is a thought that we should be able to solve this problem by
having lock_buffer() check to see if the buffer is locked due to a
write being queued, to have the priority of the write bumped up in the
write queues to resolve the priority inversion.  I believe Jeff Moyer
was going to look into this, if I remember correctly.

An alternate solution which I've been playing around adds buffer_head
flags so we can indicate that a buffer contains metadata and/or should
have I/O submitted with the REQ_PRIO flag set.

Adding a buffer_head flag for at least BH_Meta is probably a good
thing, since that way the blktrace will be properly annotated.
Whether we should keep the BH_Prio flag or rely on lock_buffer()
automatically raising the priority is, my feeling is that if
lock_buffer() can do the right thing, we should probably do it via
lock_buffer().  I have a feeling this might be decidedly non-trivial,
though, so perhaps we should just doing via BH flags?

	   	      	     	  	- Ted
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