Re: [PATCH] cfq: fix starvation of asynchronous writes

[Paolo Valente <paolo.valente@unimore.it>]

> Il giorno 23 set 2016, alle ore 02:59, Glauber Costa =
<glauber@scylladb.com> ha scritto:
>=20
> While debugging timeouts happening in my application workload =
(ScyllaDB), I have
> observed calls to open() taking a long time, ranging everywhere from 2 =
seconds -
> the first ones that are enough to time out my application - to more =
than 30
> seconds.
>=20
> The problem seems to happen because XFS may block on pending metadata =
updates
> under certain circumnstances, and that's confirmed with the following =
backtrace
> taken by the offcputime tool (iovisor/bcc):
>=20
>    ffffffffb90c57b1 finish_task_switch
>    ffffffffb97dffb5 schedule
>    ffffffffb97e310c schedule_timeout
>    ffffffffb97e1f12 __down
>    ffffffffb90ea821 down
>    ffffffffc046a9dc xfs_buf_lock
>    ffffffffc046abfb _xfs_buf_find
>    ffffffffc046ae4a xfs_buf_get_map
>    ffffffffc046babd xfs_buf_read_map
>    ffffffffc0499931 xfs_trans_read_buf_map
>    ffffffffc044a561 xfs_da_read_buf
>    ffffffffc0451390 xfs_dir3_leaf_read.constprop.16
>    ffffffffc0452b90 xfs_dir2_leaf_lookup_int
>    ffffffffc0452e0f xfs_dir2_leaf_lookup
>    ffffffffc044d9d3 xfs_dir_lookup
>    ffffffffc047d1d9 xfs_lookup
>    ffffffffc0479e53 xfs_vn_lookup
>    ffffffffb925347a path_openat
>    ffffffffb9254a71 do_filp_open
>    ffffffffb9242a94 do_sys_open
>    ffffffffb9242b9e sys_open
>    ffffffffb97e42b2 entry_SYSCALL_64_fastpath
>    00007fb0698162ed [unknown]
>=20
> Inspecting my run with blktrace, I can see that the xfsaild kthread =
exhibit very
> high "Dispatch wait" times, on the dozens of seconds range and =
consistent with
> the open() times I have saw in that run.
>=20
> Still from the blktrace output, we can after searching a bit, identify =
the
> request that wasn't dispatched:
>=20
>  8,0   11      152    81.092472813   804  A  WM 141698288 + 8 <- (8,1) =
141696240
>  8,0   11      153    81.092472889   804  Q  WM 141698288 + 8 =
[xfsaild/sda1]
>  8,0   11      154    81.092473207   804  G  WM 141698288 + 8 =
[xfsaild/sda1]
>  8,0   11      206    81.092496118   804  I  WM 141698288 + 8 (   =
22911) [xfsaild/sda1]
>  <=3D=3D=3D=3D 'I' means Inserted (into the IO scheduler) =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D>
>  8,0    0   289372    96.718761435     0  D  WM 141698288 + 8 =
(15626265317) [swapper/0]
>  <=3D=3D=3D=3D Only 15s later the CFQ scheduler dispatches the request =
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D>
>=20
> As we can see above, in this particular example CFQ took 15 seconds to =
dispatch
> this request. Going back to the full trace, we can see that the =
xfsaild queue
> had plenty of opportunity to run, and it was selected as the active =
queue many
> times. It would just always be preempted by something else (example):
>=20
>  8,0    1        0    81.117912979     0  m   N cfq1618SN / =
insert_request
>  8,0    1        0    81.117913419     0  m   N cfq1618SN / add_to_rr
>  8,0    1        0    81.117914044     0  m   N cfq1618SN / preempt
>  8,0    1        0    81.117914398     0  m   N cfq767A  / slice =
expired t=3D1
>  8,0    1        0    81.117914755     0  m   N cfq767A  / resid=3D40
>  8,0    1        0    81.117915340     0  m   N / served: =
vt=3D1948520448 min_vt=3D1948520448
>  8,0    1        0    81.117915858     0  m   N cfq767A  / sl_used=3D1 =
disp=3D0 charge=3D0 iops=3D1 sect=3D0
>=20
> where cfq767 is the xfsaild queue and cfq1618 corresponds to one of =
the ScyllaDB
> IO dispatchers.
>=20
> The requests preempting the xfsaild queue are synchronous requests. =
That's a
> characteristic of ScyllaDB workloads, as we only ever issue O_DIRECT =
requests.
> While it can be argued that preempting ASYNC requests in favor of SYNC =
is part
> of the CFQ logic, I don't believe that doing so for 15+ seconds is =
anyone's
> goal.
>=20
> Moreover, unless I am misunderstanding something, that breaks the =
expectation
> set by the "fifo_expire_async" tunable, which in my system is set to =
the
> default.
>=20
> Looking at the code, it seems to me that the issue is that after we =
make
> an async queue active, there is no guarantee that it will execute any =
request.
>=20
> When the queue itself tests if it cfq_may_dispatch() it can bail if it =
sees SYNC
> requests in flight. An incoming request from another queue can also =
preempt it
> in such situation before we have the chance to execute anything (as =
seen in the
> trace above).
>=20

The BFQ scheduler should not suffer from this issue, because it does
not consider this condition at all for preemption.  Even better, BFQ
performs preemption as a function of just one condition, which is
formally proven not to break any request-delay guarantee.

Would you be willing to give it a try?  It might solve or at least
mitigate the specific latency problem addressed by your patch, and at
the same time provide many other benefits, in terms of low latency and
high throughput.

If you are willing to try it, then in [1] you can find the last BFQ
development branch, rebased against 4.8-rc7.  These commits add BFQ as =
an
extra scheduler.

Otherwise you can find BFQ for 4.7 here [2], and BFQ for 4.6 here [3].

If you prefer patches, then you can find them in [4] for 4.7, or in
[5] for 4.6.

Finally, if you need details about BFQ, have a look at [6], or just ask!

Thanks,
Paolo

[1] https://github.com/linusw/linux-bfq/tree/bfq-v8
[2] https://github.com/linusw/linux-bfq/tree/bfq-v8-v4.7
[3] https://github.com/linusw/linux-bfq/tree/bfq-v8-v4.6
[4] =
http://algogroup.unimore.it/people/paolo/disk_sched/patches/4.7.0-v8r3/
[5] =
http://algogroup.unimore.it/people/paolo/disk_sched/patches/4.6.0-v8r3/
[6] http://algogroup.unimore.it/people/paolo/disk_sched/

> This patch sets the must_dispatch flag if we notice that we have =
requests
> that are already fifo_expired. This flag is always cleared after
> cfq_dispatch_request() returns from cfq_dispatch_requests(), so it =
won't pin
> the queue for subsequent requests (unless they are themselves expired)
>=20
> Care is taken during preempt to still allow rt requests to preempt us
> regardless.
>=20
> Testing my workload with this patch applied produces much better =
results.
> =46rom the application side I see no timeouts, and the open() latency =
histogram
> generated by systemtap looks much better, with the worst outlier at =
131ms:
>=20



> Latency histogram of xfs_buf_lock acquisition (microseconds):
> value |-------------------------------------------------- count
>     0 |                                                     11
>     1 |@@@@                                                161
>     2 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  1966
>     4 |@                                                    54
>     8 |                                                     36
>    16 |                                                      7
>    32 |                                                      0
>    64 |                                                      0
>       ~
>  1024 |                                                      0
>  2048 |                                                      0
>  4096 |                                                      1
>  8192 |                                                      1
> 16384 |                                                      2
> 32768 |                                                      0
> 65536 |                                                      0
> 131072 |                                                      1
> 262144 |                                                      0
> 524288 |                                                      0
>=20
> Signed-off-by: Glauber Costa <glauber@scylladb.com>
> CC: Jens Axboe <axboe@kernel.dk>
> CC: linux-block@vger.kernel.org
> CC: linux-kernel@vger.kernel.org
>=20
> Signed-off-by: Glauber Costa <glauber@scylladb.com>
> ---
> block/cfq-iosched.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>=20
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index cc2f6db..5e24d88 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -3042,7 +3042,6 @@ static struct request *cfq_check_fifo(struct =
cfq_queue *cfqq)
> 	if (ktime_get_ns() < rq->fifo_time)
> 		rq =3D NULL;
>=20
> -	cfq_log_cfqq(cfqq->cfqd, cfqq, "fifo=3D%p", rq);
> 	return rq;
> }
>=20
> @@ -3420,6 +3419,9 @@ static bool cfq_may_dispatch(struct cfq_data =
*cfqd, struct cfq_queue *cfqq)
> {
> 	unsigned int max_dispatch;
>=20
> +	if (cfq_cfqq_must_dispatch(cfqq))
> +		return true;
> +
> 	/*
> 	 * Drain async requests before we start sync IO
> 	 */
> @@ -3511,15 +3513,20 @@ static bool cfq_dispatch_request(struct =
cfq_data *cfqd, struct cfq_queue *cfqq)
>=20
> 	BUG_ON(RB_EMPTY_ROOT(&cfqq->sort_list));
>=20
> +	rq =3D cfq_check_fifo(cfqq);
> +	if (rq)
> +		cfq_mark_cfqq_must_dispatch(cfqq);
> +
> 	if (!cfq_may_dispatch(cfqd, cfqq))
> 		return false;
>=20
> 	/*
> 	 * follow expired path, else get first next available
> 	 */
> -	rq =3D cfq_check_fifo(cfqq);
> 	if (!rq)
> 		rq =3D cfqq->next_rq;
> +	else
> +		cfq_log_cfqq(cfqq->cfqd, cfqq, "fifo=3D%p", rq);
>=20
> 	/*
> 	 * insert request into driver dispatch list
> @@ -3989,7 +3996,7 @@ cfq_should_preempt(struct cfq_data *cfqd, struct =
cfq_queue *new_cfqq,
> 	 * if the new request is sync, but the currently running queue =
is
> 	 * not, let the sync request have priority.
> 	 */
> -	if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
> +	if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq) && =
!cfq_cfqq_must_dispatch(cfqq))
> 		return true;
>=20
> 	/*
> --=20
> 2.5.5
>=20
> --
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--
Paolo Valente
Algogroup
Dipartimento di Scienze Fisiche, Informatiche e Matematiche
Via Campi 213/B
41125 Modena - Italy
http://algogroup.unimore.it/people/paolo/