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Date: Fri, 11 Feb 2011 13:54:40 -0500
From: Vivek Goyal <vgoyal@...hat.com>
To: Justin TerAvest <teravest@...gle.com>
Cc: jaxboe@...ionio.com, ctalbott@...gle.com, mrubin@...gle.com,
jmoyer@...hat.com, guijanfeng@...fujitsu.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] Don't update group weights when on service tree.
On Thu, Feb 10, 2011 at 01:08:05PM -0800, Justin TerAvest wrote:
> With some instrumentation, we can observe that the total_weight for a
> service tree can be badly adjusted; particularly when the weight for a
> group is adjusted without taking it off of the tree. This can be
> reproduced on the HEAD of the linux-2.6-block tree.
>
> We have seen this problem in workloads when total_weight becomes 0 and
> we divide by 0 in cfq_group_slice(), crashing the kernel, but it's
> easier to illustrate by adding a BUG_ON and making it signed, like this:
>
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -85,7 +85,7 @@ struct cfq_rb_root {
> struct rb_root rb;
> struct rb_node *left;
> unsigned count;
> - unsigned total_weight;
> + int total_weight;
> u64 min_vdisktime;
> };
> #define CFQ_RB_ROOT (struct cfq_rb_root) { .rb = RB_ROOT, .left = NULL, \
> @@ -903,6 +903,7 @@ cfq_group_service_tree_del(struct cfq_data *cfqd, struct cfq
>
> cfq_log_cfqg(cfqd, cfqg, "del_from_rr group");
> st->total_weight -= cfqg->weight;
> + BUG_ON(st->total_weight < 0);
> if (!RB_EMPTY_NODE(&cfqg->rb_node))
> cfq_rb_erase(&cfqg->rb_node, st);
> cfqg->saved_workload_slice = 0;
>
Thanks for catching this issue Justin. So basically we added a group to
service tree with weight X and then later I update the weight to X+10
and then if it is deleted from service tree, it can very well lead to
negative weights and all sort of problems.
There is a one comment in the patch inline.
Thanks
Vivek
> The "fuzzcon" tool adjusts the weight for a task in a manner that
> triggers this bug:
> http://google3-2.osuosl.org/?p=tests/blkcgroup.git;a=blob;f=scripts/fuzzcon;h=38cfb9e0c847eb92ceccbac67de2a59b43f241ba;hb=eae5e71ad1116b46a9fd84f5d1b1e4c45aa6bd58
>
>
> Here's the BUG_ON output when the case is encountered:
> [ 1711.376954] ------------[ cut here ]------------
> [ 1711.377789] kernel BUG at block/cfq-iosched.c:906!
> [ 1711.377789] invalid opcode: 0000 [#1] SMP·
> [ 1711.377789] last sysfs file: /sys/devices/system/node/node0/distance
> [ 1711.377789] CPU 0·
> [ 1711.377789] Modules linked in: tg3 msr cpuid ipv6 genrtc
> [ 1711.377789]·
> [ 1711.377789] Pid: 13702, comm: dd Tainted: G W 2.6.38-smp-detect
> [ 1711.377789] RIP: 0010:[<ffffffff81210387>] [<ffffffff81210387>] cfq_group_service_tree_del+0x5b/0x99
> [ 1711.377789] RSP: 0018:ffff8800189c1b68 EFLAGS: 00010086
> [ 1711.377789] RAX: 00000000ffffff9c RBX: ffff880011a6d400 RCX: 000000000000b690
> [ 1711.377789] RDX: 0000000000000000 RSI: ffff880011a6d400 RDI: 0000000000000000
> [ 1711.377789] RBP: ffff8800189c1b78 R08: 0000000000000000 R09: 0000000000000001
> [ 1711.377789] R10: 0000000000000000 R11: 000000000000001b R12: ffff8800065e8000
> [ 1711.377789] R13: ffff880011a6d528 R14: 000000000000001b R15: 000000000000001b
> [ 1711.377789] FS: 0000000000000000(0000) GS:ffff880009c00000(0000) knlGS:0000000000000000
> [ 1711.377789] CS: 0010 DS: 002b ES: 002b CR0: 000000008005003b
> [ 1711.377789] CR2: 00000000f74db000 CR3: 0000000001803000 CR4: 00000000000006f0
> [ 1711.377789] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 1711.377789] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [ 1711.377789] Process dd (pid: 13702, threadinfo ffff8800189c0000, task ffff8800067dafa0)
> [ 1711.377789] Stack:
> [ 1711.377789] ffff880006596b40 ffff8800065e8000 ffff8800189c1be8 ffffffff81210d2d
> [ 1711.377789] ffff8800067dafa0 ffff8800060c0418 ffff8800189c1bc8 ffff8800060c0410
> [ 1711.377789] 0000000000000082 ffff8800065e8008 ffff8800189c1bc8 ffff880006596b40
> [ 1711.377789] Call Trace:
> [ 1711.377789] [<ffffffff81210d2d>] __cfq_slice_expired+0x3e0/0x45d
> [ 1711.377789] [<ffffffff812111c3>] cfq_exit_cfqq+0x22/0x3f
> [ 1711.377789] [<ffffffff81211257>] __cfq_exit_single_io_context+0x77/0x84
> [ 1711.377789] [<ffffffff812112ab>] cfq_exit_single_io_context+0x47/0x62
> [ 1711.377789] [<ffffffff81211264>] ? cfq_exit_single_io_context+0x0/0x62
> [ 1711.377789] [<ffffffff8120fcc1>] call_for_each_cic+0x31/0x3e
> [ 1711.377789] [<ffffffff8120fce3>] cfq_exit_io_context+0x15/0x17
> [ 1711.377789] [<ffffffff81207211>] exit_io_context+0x5a/0x6d
> [ 1711.377789] [<ffffffff810723a2>] do_exit+0x72a/0x756
> [ 1711.377789] [<ffffffff81072444>] do_group_exit+0x76/0x9e
> [ 1711.377789] [<ffffffff8107fa9f>] get_signal_to_deliver+0x33c/0x35d
> [ 1711.377789] [<ffffffff81035f45>] do_signal+0x72/0x699
> [ 1711.377789] [<ffffffff8111ad8f>] ? fsnotify_modify+0x62/0x6a
> [ 1711.377789] [<ffffffff81036598>] do_notify_resume+0x2c/0x72
> [ 1711.377789] [<ffffffff81036d88>] int_signal+0x12/0x17
> [ 1711.377789] Code: 48 85 ff 74 15 48 8d 96 42 01 00 00 31 c0 48 c7 c6 0d 13 72 81 e8 d2 cd eb ff 41 8b 44 24 1c 2b 43 20 85 c0 41 89 44 24 1c 79 04 <0f> 0b eb fe 48 8b 03 48 83 e0 fc 48 39 c3 74 0d 49 8d 74 24 08·
> [ 1711.377789] RIP [<ffffffff81210387>] cfq_group_service_tree_del+0x5b/0x99
> [ 1711.377789] RSP <ffff8800189c1b68>
> [ 1711.377789] ---[ end trace 5e95326ab7063969 ]---
>
>
> Justin TerAvest (1):
> Don't update group weights when on service tree.
>
> block/cfq-iosched.c | 57 ++++++++++++++++++++++++++++++++++++++------------
> 1 files changed, 43 insertions(+), 14 deletions(-)
>
> --
> 1.7.3.1
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