Re: [PATCH] Give kjournald a IOPRIO_CLASS_RT io priority

[Arjan van de Ven <arjan@infradead.org>]

> [  517.067572]  [<c042ee64>] warn_on_slowpath+0x41/0x65
> [  517.067652]  [<c070ec83>] io_schedule+0x77/0xb0
> [  517.067659]  [<c04abc72>] sync_buffer+0x33/0x37
> [  517.067666]  [<c070f010>] __wait_on_bit_lock+0x34/0x5e
> [  517.067682]  [<c070f0e5>] out_of_line_wait_on_bit_lock+0xab/0xb3
> [  517.067707]  [<c04abfa1>] __lock_buffer+0x24/0x2a
> [  517.067715]  [<c04dd7fc>] do_get_write_access+0x64/0x3b1
> [  517.067743]  [<c04ddb64>] journal_get_write_access+0x1b/0x2a
> [  517.067752]  [<c04da374>] __ext3_journal_get_write_access+0x19/0x3c
> [  517.067761]  [<c04cf672>] ext3_reserve_inode_write+0x34/0x68
> [  517.067769]  [<c04cf6d5>] ext3_mark_inode_dirty+0x2f/0x46
> [  517.067777]  [<c04cf7f7>] ext3_dirty_inode+0x53/0x67
> [  517.067784]  [<c04a7bed>] __mark_inode_dirty+0x29/0x144
> [  517.067794]  [<c049e60f>] file_update_time+0x80/0xa9
> [  517.067803]  [<c046b66c>] __generic_file_aio_write_nolock+0x2f0/0x41b
> [  517.067842]  [<c046bf0d>] generic_file_aio_write+0x5a/0xb7
> [  517.067850]  [<c04cdc65>] ext3_file_write+0x1a/0x89
> [  517.067858]  [<c048da41>] do_sync_write+0xab/0xe9
> [  517.067896]  [<c048e302>] vfs_write+0x8a/0x12e
> [  517.067903]  [<c048e43f>] sys_write+0x3b/0x60
> [  517.067910]  [<c0403b0b>] sysenter_do_call+0x12/0x2f
> [  517.067919]  =======================
> [  517.067923] ---[ end trace de523043f88bd9a7 ]---  

> That's the one - the lock_buffer() in do_get_write_access().  It's a
> major contention site and it'd be a major win if we could fix it.
> Even if we resorted to some nasty thing like taking a temp copy of the
> buffer's contents.

I also notice it's part of "file_update_time". Do we really need to go all the way 
down to this level of synchronicity for that?
(I also randomly wonder if we, in the write path, dirty the inode twice, once for size once for item, and
if we then also reserve two slots in the journal for that..... but I'm showing
my total ignorance of JBD internals here)

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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