Re: [PATCH 0/11] Per-bdi writeback flusher threads #4

From: Jens Axboe
Date: Wed May 20 2009 - 07:24:06 EST

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On Wed, May 20 2009, Jens Axboe wrote:
> On Wed, May 20 2009, Zhang, Yanmin wrote:
> > On Wed, 2009-05-20 at 10:54 +0200, Jens Axboe wrote:
> > > On Wed, May 20 2009, Jens Axboe wrote:
> > > > On Wed, May 20 2009, Zhang, Yanmin wrote:
> > > > > On Tue, 2009-05-19 at 08:20 +0200, Jens Axboe wrote:
> > > > > > On Tue, May 19 2009, Zhang, Yanmin wrote:
> > > > > > > On Mon, 2009-05-18 at 14:19 +0200, Jens Axboe wrote:
> > > > > > > > Hi,
> > > > > > > >
> > > > > > > > This is the fourth version of this patchset. Chances since v3:
> > > > > > > >
> > > > > > > > - Dropped a prep patch, it has been included in mainline since.
> > > > > > > >
> > > > > > > > - Add a work-to-do list to the bdi. This is struct bdi_work. Each
> > > > > > > > wb thread will notice and execute work on bdi->work_list. The arguments
> > > > > > > > are which sb (or NULL for all) to flush and how many pages to flush.
> > > > > > > >
> > > > > > > > - Fix a bug where not all bdi's would end up on the bdi_list, so potentially
> > > > > > > > some data would not be flushed.
> > > > > > > >
> > > > > > > > - Make wb_kupdated() pass on wbc->older_than_this so we maintain the same
> > > > > > > > behaviour for kupdated flushes.
> > > > > > > >
> > > > > > > > - Have the wb thread flush first before sleeping, to avoid losing the
> > > > > > > > first flush on lazy register.
> > > > > > > >
> > > > > > > > - Rebase to newer kernels.
> > > > >
> > > > > > I'm attaching two patches - apply #1 to -rc6, and then #2 is a roll-up
> > > > > > of the patch series that you can apply next.
> > > > > Jens,
> > > > >
> > > > > I run into 2 issues with kernel 2.6.30-rc6+BDI_Flusher_V4. Below is one.
> > > > >
> > > > > Tue May 19 00:00:00 CST 2009
> > > > > BUG: unable to handle kernel NULL pointer dereference at 00000000000001d8
> > > > > IP: [<ffffffff803f3c4c>] generic_make_request+0x10a/0x384
> > > > > PGD 0
> > > > > Oops: 0000 [#1] SMP
> > > > > last sysfs file: /sys/block/sdb/stat
> > > > > CPU 0
> > > > > Modules linked in: igb
> > > > > Pid: 1445, comm: bdi-8:16 Not tainted 2.6.30-rc6-bdiflusherv4 #1 X8DTN
> > > > > RIP: 0010:[<ffffffff803f3c4c>] [<ffffffff803f3c4c>] generic_make_request+0x10a/0x384
> > > > > RSP: 0018:ffff8800bd04da60 EFLAGS: 00010206
> > > > > RAX: 0000000000000000 RBX: ffff8801be45d500 RCX: 00000000038a0df8
> > > > > RDX: 0000000000000008 RSI: 0000000000000576 RDI: ffff8801bf408680
> > > > > RBP: ffff8801be45d500 R08: ffffe20001ee8140 R09: ffff8800bd04da98
> > > > > R10: 0000000000000000 R11: ffff8800bd72eb40 R12: ffff8801be45d500
> > > > > R13: ffff88005f51f310 R14: 0000000000000008 R15: ffff8800b15a5458
> > > > > FS: 0000000000000000(0000) GS:ffffc20000000000(0000) knlGS:0000000000000000
> > > > > CS: 0010 DS: 0018 ES: 0018 CR0: 000000008005003b
> > > > > CR2: 00000000000001d8 CR3: 0000000000201000 CR4: 00000000000006e0
> > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> > > > > Process bdi-8:16 (pid: 1445, threadinfo ffff8800bd04c000, task ffff8800bd1b75f0)
> > > > > Stack:
> > > > > 0000000000000008 ffffffff8027a613 00000000848dc000 ffffffffffffffff
> > > > > ffff8800a8190f50 ffffffff00000012 ffff8800a81938e0 ffffc2000000001b
> > > > > 0000000000000000 0000000000000000 ffffe200026f9c30 0000000000000000
> > > > > Call Trace:
> > > > > [<ffffffff8027a613>] ? mempool_alloc+0x59/0x10f
> > > > > [<ffffffff803f3f70>] ? submit_bio+0xaa/0xb1
> > > > > [<ffffffff802c6a3f>] ? submit_bh+0xe3/0x103
> > > > > [<ffffffff802c92ea>] ? __block_write_full_page+0x1fb/0x2f2
> > > > > [<ffffffff802c7d6a>] ? end_buffer_async_write+0x0/0xfb
> > > > > [<ffffffff8027e8d2>] ? __writepage+0xa/0x25
> > > > > [<ffffffff8027f036>] ? write_cache_pages+0x21c/0x338
> > > > > [<ffffffff8027e8c8>] ? __writepage+0x0/0x25
> > > > > [<ffffffff8027f195>] ? do_writepages+0x27/0x2d
> > > > > [<ffffffff802c22c1>] ? __writeback_single_inode+0x159/0x2b3
> > > > > [<ffffffff8071e52a>] ? thread_return+0x3e/0xaa
> > > > > [<ffffffff8027f267>] ? determine_dirtyable_memory+0xd/0x1d
> > > > > [<ffffffff8027f2dd>] ? get_dirty_limits+0x1d/0x255
> > > > > [<ffffffff802c27bc>] ? generic_sync_wb_inodes+0x1b4/0x220
> > > > > [<ffffffff802c3130>] ? wb_do_writeback+0x16c/0x215
> > > > > [<ffffffff802c323e>] ? bdi_writeback_task+0x65/0x10d
> > > > > [<ffffffff8024cc06>] ? autoremove_wake_function+0x0/0x2e
> > > > > [<ffffffff8024cb27>] ? bit_waitqueue+0x10/0xa0
> > > > > [<ffffffff80289257>] ? bdi_start_fn+0x0/0xba
> > > > > [<ffffffff802892c6>] ? bdi_start_fn+0x6f/0xba
> > > > > [<ffffffff8024c860>] ? kthread+0x54/0x80
> > > > > [<ffffffff8020c97a>] ? child_rip+0xa/0x20
> > > > > [<ffffffff8024c80c>] ? kthread+0x0/0x80
> > > > > [<ffffffff8020c970>] ? child_rip+0x0/0x20
> > > > >
> >
> >
> > >
> > > I found one issue yesterday and one today that could cause issues, not
> > > sure it would explain this one. But at least it's worth a try, if it's
> > > reproducible.
> > I just reproduced it a moment ago manually.
> >
> > [global]
> > direct=0
> > ioengine=mmap
> > iodepth=256
> > iodepth_batch=32
> > size=4G
> > bs=4k
> > pre_read=1
> > overwrite=1
> > numjobs=1
> > loops=5
> > runtime=600
> > group_reporting
> > directory=/mnt/stp/fiodata
> > [job_group0_sub0]
> > startdelay=0
> > rw=randwrite
> > filename=data0/f1:data0/f2
> >
> >
> > The fio includes my preread patch to flush files to memory.
> >
> > Before starting the second testing, I did a cache dropping by:
> > #echo "3">/proc/sys/vm/drop_caches.
> >
> > I suspect the drop_caches trigger it.
>
> Thanks, will try this. What filesystem and mount options did you use?

No luck reproducing so far. In other news, I have finally merged your
fio pre_read patch :-)

I've run it here many times, works fine with the current writeback
branch. Since I did the runs anyway, I did comparisons between mainline
and writeback for this test. Each test was run 10 times, averages below.
The throughput deviated less than 1MB/sec, so results are very stable.
CPU usage percentages were always within 0.5%.

Kernel Throughput usr sys disk util
-----------------------------------------------------------------
writeback 175MB/sec 17.55% 43.04% 97.80%
vanilla 147MB/sec 13.44% 47.33% 85.98%

The results for this test is particularly interesting, since it's very
heavy on the writeback side. pdflush/bdi threads were pretty busy. User
time is up (even if corrected for higher throughput), but system time is
down a lot. Vanilla isn't close to keeping the disk busy, with the
writeback patches we are basically there (100% would be pretty much
impossible to reach).

Please try with the patches I sent. If you still see problems, we need
to look more closely into that.

--
Jens Axboe

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Next message: Avi Kivity: "[PATCH 46/46] KVM: Remove exception_injected() callback."
Previous message: Avi Kivity: "[PATCH 42/46] KVM: VMX: Consolidate userspace and kernel interrupt injection for VMX"
In reply to: Jens Axboe: "Re: [PATCH 0/11] Per-bdi writeback flusher threads #4"
Next in thread: Zhang, Yanmin: "Re: [PATCH 0/11] Per-bdi writeback flusher threads #4"
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