* [PATCH -next v2 0/2] block, bfq: make bfq_has_work() more accurate
@ 2022-05-13 2:35 Yu Kuai
2022-05-13 2:35 ` [PATCH -next v2 1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock' Yu Kuai
` (2 more replies)
0 siblings, 3 replies; 10+ messages in thread
From: Yu Kuai @ 2022-05-13 2:35 UTC (permalink / raw)
To: jack, paolo.valente, axboe; +Cc: linux-block, linux-kernel, yukuai3, yi.zhang
Changes in v2:
- add reviewed-by tag for patch 1
- use WRITE_ONCE() for updating of 'bfqd->queued' in patch 2
This patchset try to make bfq_has_work() more accurate, patch 1 is a
small problem found by code review.
BTW, I not sure why blk_mq_run_hw_queues() is called with 'bfqd->lock'
held, I think this is not necessary. And bfq_has_work() can be more
accurate by reading 'bfqd->queued' with 'bfqd->lock' held after patch 2.
Previous versions:
v1: https://lore.kernel.org/all/20220510131629.1964415-1-yukuai3@huawei.com/
Yu Kuai (2):
block, bfq: protect 'bfqd->queued' by 'bfqd->lock'
block, bfq: make bfq_has_work() more accurate
block/bfq-iosched.c | 20 +++++++++++++++-----
1 file changed, 15 insertions(+), 5 deletions(-)
--
2.31.1
^ permalink raw reply [flat|nested] 10+ messages in thread
* [PATCH -next v2 1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock'
2022-05-13 2:35 [PATCH -next v2 0/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
@ 2022-05-13 2:35 ` Yu Kuai
2022-05-13 6:13 ` Chaitanya Kulkarni
2022-05-13 2:35 ` [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
2022-05-16 17:39 ` [PATCH -next v2 0/2] " Jens Axboe
2 siblings, 1 reply; 10+ messages in thread
From: Yu Kuai @ 2022-05-13 2:35 UTC (permalink / raw)
To: jack, paolo.valente, axboe; +Cc: linux-block, linux-kernel, yukuai3, yi.zhang
If bfq_schedule_dispatch() is called from bfq_idle_slice_timer_body(),
then 'bfqd->queued' is read without holding 'bfqd->lock'. This is
wrong since it can be wrote concurrently.
Fix the problem by holding 'bfqd->lock' in such case.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
block/bfq-iosched.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 272d48d8f326..61750696e87f 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -456,6 +456,8 @@ static struct bfq_io_cq *bfq_bic_lookup(struct request_queue *q)
*/
void bfq_schedule_dispatch(struct bfq_data *bfqd)
{
+ lockdep_assert_held(&bfqd->lock);
+
if (bfqd->queued != 0) {
bfq_log(bfqd, "schedule dispatch");
blk_mq_run_hw_queues(bfqd->queue, true);
@@ -6898,8 +6900,8 @@ bfq_idle_slice_timer_body(struct bfq_data *bfqd, struct bfq_queue *bfqq)
bfq_bfqq_expire(bfqd, bfqq, true, reason);
schedule_dispatch:
- spin_unlock_irqrestore(&bfqd->lock, flags);
bfq_schedule_dispatch(bfqd);
+ spin_unlock_irqrestore(&bfqd->lock, flags);
}
/*
--
2.31.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate
2022-05-13 2:35 [PATCH -next v2 0/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
2022-05-13 2:35 ` [PATCH -next v2 1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock' Yu Kuai
@ 2022-05-13 2:35 ` Yu Kuai
2022-05-16 9:56 ` Jan Kara
2022-05-16 17:39 ` [PATCH -next v2 0/2] " Jens Axboe
2 siblings, 1 reply; 10+ messages in thread
From: Yu Kuai @ 2022-05-13 2:35 UTC (permalink / raw)
To: jack, paolo.valente, axboe; +Cc: linux-block, linux-kernel, yukuai3, yi.zhang
bfq_has_work() is using busy_queues currently, which is not accurate
because bfq_queue is busy doesn't represent that it has requests. Since
bfqd aready has a counter 'queued' to record how many requests are in
bfq, use it instead of busy_queues.
Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
block/bfq-iosched.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 61750696e87f..740dd83853a6 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2210,7 +2210,11 @@ static void bfq_add_request(struct request *rq)
bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
bfqq->queued[rq_is_sync(rq)]++;
- bfqd->queued++;
+ /*
+ * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
+ * may be read without holding the lock in bfq_has_work().
+ */
+ WRITE_ONCE(bfqd->queued, bfqd->queued + 1);
if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
bfq_check_waker(bfqd, bfqq, now_ns);
@@ -2402,7 +2406,11 @@ static void bfq_remove_request(struct request_queue *q,
if (rq->queuelist.prev != &rq->queuelist)
list_del_init(&rq->queuelist);
bfqq->queued[sync]--;
- bfqd->queued--;
+ /*
+ * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
+ * may be read without holding the lock in bfq_has_work().
+ */
+ WRITE_ONCE(bfqd->queued, bfqd->queued - 1);
elv_rb_del(&bfqq->sort_list, rq);
elv_rqhash_del(q, rq);
@@ -5063,11 +5071,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
/*
- * Avoiding lock: a race on bfqd->busy_queues should cause at
+ * Avoiding lock: a race on bfqd->queued should cause at
* most a call to dispatch for nothing
*/
return !list_empty_careful(&bfqd->dispatch) ||
- bfq_tot_busy_queues(bfqd) > 0;
+ READ_ONCE(bfqd->queued);
}
static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
--
2.31.1
^ permalink raw reply related [flat|nested] 10+ messages in thread
* Re: [PATCH -next v2 1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock'
2022-05-13 2:35 ` [PATCH -next v2 1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock' Yu Kuai
@ 2022-05-13 6:13 ` Chaitanya Kulkarni
0 siblings, 0 replies; 10+ messages in thread
From: Chaitanya Kulkarni @ 2022-05-13 6:13 UTC (permalink / raw)
To: Yu Kuai, jack, paolo.valente, axboe; +Cc: linux-block, linux-kernel, yi.zhang
On 5/12/2022 7:35 PM, Yu Kuai wrote:
> If bfq_schedule_dispatch() is called from bfq_idle_slice_timer_body(),
> then 'bfqd->queued' is read without holding 'bfqd->lock'. This is
> wrong since it can be wrote concurrently.
>
> Fix the problem by holding 'bfqd->lock' in such case.
>
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> ---
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
-ck
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate
2022-05-13 2:35 ` [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
@ 2022-05-16 9:56 ` Jan Kara
2022-05-17 14:21 ` Paolo Valente
0 siblings, 1 reply; 10+ messages in thread
From: Jan Kara @ 2022-05-16 9:56 UTC (permalink / raw)
To: Yu Kuai; +Cc: jack, paolo.valente, axboe, linux-block, linux-kernel, yi.zhang
On Fri 13-05-22 10:35:07, Yu Kuai wrote:
> bfq_has_work() is using busy_queues currently, which is not accurate
> because bfq_queue is busy doesn't represent that it has requests. Since
> bfqd aready has a counter 'queued' to record how many requests are in
> bfq, use it instead of busy_queues.
>
> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
> lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
>
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@suse.cz>
Honza
> ---
> block/bfq-iosched.c | 16 ++++++++++++----
> 1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 61750696e87f..740dd83853a6 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -2210,7 +2210,11 @@ static void bfq_add_request(struct request *rq)
>
> bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
> bfqq->queued[rq_is_sync(rq)]++;
> - bfqd->queued++;
> + /*
> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
> + * may be read without holding the lock in bfq_has_work().
> + */
> + WRITE_ONCE(bfqd->queued, bfqd->queued + 1);
>
> if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
> bfq_check_waker(bfqd, bfqq, now_ns);
> @@ -2402,7 +2406,11 @@ static void bfq_remove_request(struct request_queue *q,
> if (rq->queuelist.prev != &rq->queuelist)
> list_del_init(&rq->queuelist);
> bfqq->queued[sync]--;
> - bfqd->queued--;
> + /*
> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
> + * may be read without holding the lock in bfq_has_work().
> + */
> + WRITE_ONCE(bfqd->queued, bfqd->queued - 1);
> elv_rb_del(&bfqq->sort_list, rq);
>
> elv_rqhash_del(q, rq);
> @@ -5063,11 +5071,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
> struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
>
> /*
> - * Avoiding lock: a race on bfqd->busy_queues should cause at
> + * Avoiding lock: a race on bfqd->queued should cause at
> * most a call to dispatch for nothing
> */
> return !list_empty_careful(&bfqd->dispatch) ||
> - bfq_tot_busy_queues(bfqd) > 0;
> + READ_ONCE(bfqd->queued);
> }
>
> static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
> --
> 2.31.1
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH -next v2 0/2] block, bfq: make bfq_has_work() more accurate
2022-05-13 2:35 [PATCH -next v2 0/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
2022-05-13 2:35 ` [PATCH -next v2 1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock' Yu Kuai
2022-05-13 2:35 ` [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
@ 2022-05-16 17:39 ` Jens Axboe
2 siblings, 0 replies; 10+ messages in thread
From: Jens Axboe @ 2022-05-16 17:39 UTC (permalink / raw)
To: yukuai3, jack, paolo.valente; +Cc: linux-kernel, linux-block, yi.zhang
On Fri, 13 May 2022 10:35:05 +0800, Yu Kuai wrote:
> Changes in v2:
> - add reviewed-by tag for patch 1
> - use WRITE_ONCE() for updating of 'bfqd->queued' in patch 2
>
> This patchset try to make bfq_has_work() more accurate, patch 1 is a
> small problem found by code review.
>
> [...]
Applied, thanks!
[1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock'
commit: 181490d5321806e537dc5386db5ea640b826bf78
[2/2] block, bfq: make bfq_has_work() more accurate
commit: ddc25c86b466d2359b57bc7798f167baa1735a44
Best regards,
--
Jens Axboe
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate
2022-05-16 9:56 ` Jan Kara
@ 2022-05-17 14:21 ` Paolo Valente
2022-05-17 15:06 ` Paolo Valente
0 siblings, 1 reply; 10+ messages in thread
From: Paolo Valente @ 2022-05-17 14:21 UTC (permalink / raw)
To: Jan Kara; +Cc: Yu Kuai, Jens Axboe, linux-block, linux-kernel, yi.zhang
> Il giorno 16 mag 2022, alle ore 11:56, Jan Kara <jack@suse.cz> ha scritto:
>
> On Fri 13-05-22 10:35:07, Yu Kuai wrote:
>> bfq_has_work() is using busy_queues currently, which is not accurate
>> because bfq_queue is busy doesn't represent that it has requests. Since
>> bfqd aready has a counter 'queued' to record how many requests are in
>> bfq, use it instead of busy_queues.
>>
The number of requests queued is not equal to the number of busy
queues (it is >=). If this patch is based on this assumption then
unfortunately it is wrong :(
Paolo
>> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
>> lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
>>
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>
> Looks good. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@suse.cz>
>
> Honza
>
>> ---
>> block/bfq-iosched.c | 16 ++++++++++++----
>> 1 file changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>> index 61750696e87f..740dd83853a6 100644
>> --- a/block/bfq-iosched.c
>> +++ b/block/bfq-iosched.c
>> @@ -2210,7 +2210,11 @@ static void bfq_add_request(struct request *rq)
>>
>> bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
>> bfqq->queued[rq_is_sync(rq)]++;
>> - bfqd->queued++;
>> + /*
>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>> + * may be read without holding the lock in bfq_has_work().
>> + */
>> + WRITE_ONCE(bfqd->queued, bfqd->queued + 1);
>>
>> if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
>> bfq_check_waker(bfqd, bfqq, now_ns);
>> @@ -2402,7 +2406,11 @@ static void bfq_remove_request(struct request_queue *q,
>> if (rq->queuelist.prev != &rq->queuelist)
>> list_del_init(&rq->queuelist);
>> bfqq->queued[sync]--;
>> - bfqd->queued--;
>> + /*
>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>> + * may be read without holding the lock in bfq_has_work().
>> + */
>> + WRITE_ONCE(bfqd->queued, bfqd->queued - 1);
>> elv_rb_del(&bfqq->sort_list, rq);
>>
>> elv_rqhash_del(q, rq);
>> @@ -5063,11 +5071,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
>> struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
>>
>> /*
>> - * Avoiding lock: a race on bfqd->busy_queues should cause at
>> + * Avoiding lock: a race on bfqd->queued should cause at
>> * most a call to dispatch for nothing
>> */
>> return !list_empty_careful(&bfqd->dispatch) ||
>> - bfq_tot_busy_queues(bfqd) > 0;
>> + READ_ONCE(bfqd->queued);
>> }
>>
>> static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
>> --
>> 2.31.1
>>
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate
2022-05-17 14:21 ` Paolo Valente
@ 2022-05-17 15:06 ` Paolo Valente
2022-05-18 1:17 ` yukuai (C)
0 siblings, 1 reply; 10+ messages in thread
From: Paolo Valente @ 2022-05-17 15:06 UTC (permalink / raw)
To: Jan Kara; +Cc: Yu Kuai, Jens Axboe, linux-block, linux-kernel, yi.zhang
> Il giorno 17 mag 2022, alle ore 16:21, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>
>
>
>> Il giorno 16 mag 2022, alle ore 11:56, Jan Kara <jack@suse.cz> ha scritto:
>>
>> On Fri 13-05-22 10:35:07, Yu Kuai wrote:
>>> bfq_has_work() is using busy_queues currently, which is not accurate
>>> because bfq_queue is busy doesn't represent that it has requests. Since
>>> bfqd aready has a counter 'queued' to record how many requests are in
>>> bfq, use it instead of busy_queues.
>>>
>
> The number of requests queued is not equal to the number of busy
> queues (it is >=).
No, sorry. It is actually != in general.
In particular, if queued == 0 but there are busy queues (although
still waiting for I/O to arrive), then responding that there is no
work caused blk-mq to stop asking, and hence an I/O freeze. IOW I/O
eventually arrives for a busy queue, but blk-mq does not ask for a new
request any longer. But maybe things have changed around bfq since
then.
Paolo
> If this patch is based on this assumption then
> unfortunately it is wrong :(
>
> Paolo
>
>>> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
>>> lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
>>>
>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>>
>> Looks good. Feel free to add:
>>
>> Reviewed-by: Jan Kara <jack@suse.cz>
>>
>> Honza
>>
>>> ---
>>> block/bfq-iosched.c | 16 ++++++++++++----
>>> 1 file changed, 12 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>>> index 61750696e87f..740dd83853a6 100644
>>> --- a/block/bfq-iosched.c
>>> +++ b/block/bfq-iosched.c
>>> @@ -2210,7 +2210,11 @@ static void bfq_add_request(struct request *rq)
>>>
>>> bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
>>> bfqq->queued[rq_is_sync(rq)]++;
>>> - bfqd->queued++;
>>> + /*
>>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>>> + * may be read without holding the lock in bfq_has_work().
>>> + */
>>> + WRITE_ONCE(bfqd->queued, bfqd->queued + 1);
>>>
>>> if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
>>> bfq_check_waker(bfqd, bfqq, now_ns);
>>> @@ -2402,7 +2406,11 @@ static void bfq_remove_request(struct request_queue *q,
>>> if (rq->queuelist.prev != &rq->queuelist)
>>> list_del_init(&rq->queuelist);
>>> bfqq->queued[sync]--;
>>> - bfqd->queued--;
>>> + /*
>>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>>> + * may be read without holding the lock in bfq_has_work().
>>> + */
>>> + WRITE_ONCE(bfqd->queued, bfqd->queued - 1);
>>> elv_rb_del(&bfqq->sort_list, rq);
>>>
>>> elv_rqhash_del(q, rq);
>>> @@ -5063,11 +5071,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
>>> struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
>>>
>>> /*
>>> - * Avoiding lock: a race on bfqd->busy_queues should cause at
>>> + * Avoiding lock: a race on bfqd->queued should cause at
>>> * most a call to dispatch for nothing
>>> */
>>> return !list_empty_careful(&bfqd->dispatch) ||
>>> - bfq_tot_busy_queues(bfqd) > 0;
>>> + READ_ONCE(bfqd->queued);
>>> }
>>>
>>> static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
>>> --
>>> 2.31.1
>>>
>> --
>> Jan Kara <jack@suse.com>
>> SUSE Labs, CR
>
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate
2022-05-17 15:06 ` Paolo Valente
@ 2022-05-18 1:17 ` yukuai (C)
2022-05-18 13:40 ` Paolo VALENTE
0 siblings, 1 reply; 10+ messages in thread
From: yukuai (C) @ 2022-05-18 1:17 UTC (permalink / raw)
To: Paolo Valente, Jan Kara; +Cc: Jens Axboe, linux-block, linux-kernel, yi.zhang
在 2022/05/17 23:06, Paolo Valente 写道:
>
>
>> Il giorno 17 mag 2022, alle ore 16:21, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>
>>
>>
>>> Il giorno 16 mag 2022, alle ore 11:56, Jan Kara <jack@suse.cz> ha scritto:
>>>
>>> On Fri 13-05-22 10:35:07, Yu Kuai wrote:
>>>> bfq_has_work() is using busy_queues currently, which is not accurate
>>>> because bfq_queue is busy doesn't represent that it has requests. Since
>>>> bfqd aready has a counter 'queued' to record how many requests are in
>>>> bfq, use it instead of busy_queues.
>>>>
>>
>> The number of requests queued is not equal to the number of busy
>> queues (it is >=).
>
> No, sorry. It is actually != in general.
Hi, Paolo
I'm aware that number of requests queued is not equal to the number of
busy queues, and that is the motivation of this patch.
>
> In particular, if queued == 0 but there are busy queues (although
> still waiting for I/O to arrive), then responding that there is no
> work caused blk-mq to stop asking, and hence an I/O freeze. IOW I/O
> eventually arrives for a busy queue, but blk-mq does not ask for a new
> request any longer. But maybe things have changed around bfq since
> then.
The problem is that if queued == 0 while there are busy queues, is there
any point to return true in bfq_has_work() ? IMO, it will only cause
unecessary run queue. And if new request arrives,
blk_mq_sched_insert_request() will trigger a run queue.
Thanks,
Kuai
>
> Paolo
>
>> If this patch is based on this assumption then
>> unfortunately it is wrong :(
>>
>> Paolo
>>
>>>> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
>>>> lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
>>>>
>>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>>>
>>> Looks good. Feel free to add:
>>>
>>> Reviewed-by: Jan Kara <jack@suse.cz>
>>>
>>> Honza
>>>
>>>> ---
>>>> block/bfq-iosched.c | 16 ++++++++++++----
>>>> 1 file changed, 12 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>>>> index 61750696e87f..740dd83853a6 100644
>>>> --- a/block/bfq-iosched.c
>>>> +++ b/block/bfq-iosched.c
>>>> @@ -2210,7 +2210,11 @@ static void bfq_add_request(struct request *rq)
>>>>
>>>> bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
>>>> bfqq->queued[rq_is_sync(rq)]++;
>>>> - bfqd->queued++;
>>>> + /*
>>>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>>>> + * may be read without holding the lock in bfq_has_work().
>>>> + */
>>>> + WRITE_ONCE(bfqd->queued, bfqd->queued + 1);
>>>>
>>>> if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
>>>> bfq_check_waker(bfqd, bfqq, now_ns);
>>>> @@ -2402,7 +2406,11 @@ static void bfq_remove_request(struct request_queue *q,
>>>> if (rq->queuelist.prev != &rq->queuelist)
>>>> list_del_init(&rq->queuelist);
>>>> bfqq->queued[sync]--;
>>>> - bfqd->queued--;
>>>> + /*
>>>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>>>> + * may be read without holding the lock in bfq_has_work().
>>>> + */
>>>> + WRITE_ONCE(bfqd->queued, bfqd->queued - 1);
>>>> elv_rb_del(&bfqq->sort_list, rq);
>>>>
>>>> elv_rqhash_del(q, rq);
>>>> @@ -5063,11 +5071,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
>>>> struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
>>>>
>>>> /*
>>>> - * Avoiding lock: a race on bfqd->busy_queues should cause at
>>>> + * Avoiding lock: a race on bfqd->queued should cause at
>>>> * most a call to dispatch for nothing
>>>> */
>>>> return !list_empty_careful(&bfqd->dispatch) ||
>>>> - bfq_tot_busy_queues(bfqd) > 0;
>>>> + READ_ONCE(bfqd->queued);
>>>> }
>>>>
>>>> static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
>>>> --
>>>> 2.31.1
>>>>
>>> --
>>> Jan Kara <jack@suse.com>
>>> SUSE Labs, CR
>>
>
> .
>
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate
2022-05-18 1:17 ` yukuai (C)
@ 2022-05-18 13:40 ` Paolo VALENTE
0 siblings, 0 replies; 10+ messages in thread
From: Paolo VALENTE @ 2022-05-18 13:40 UTC (permalink / raw)
To: yukuai (C); +Cc: Jan Kara, Jens Axboe, linux-block, linux-kernel, yi.zhang
> Il giorno 18 mag 2022, alle ore 03:17, yukuai (C) <yukuai3@huawei.com> ha scritto:
>
> 在 2022/05/17 23:06, Paolo Valente 写道:
>>> Il giorno 17 mag 2022, alle ore 16:21, Paolo Valente <paolo.valente@linaro.org> ha scritto:
>>>
>>>
>>>
>>>> Il giorno 16 mag 2022, alle ore 11:56, Jan Kara <jack@suse.cz> ha scritto:
>>>>
>>>> On Fri 13-05-22 10:35:07, Yu Kuai wrote:
>>>>> bfq_has_work() is using busy_queues currently, which is not accurate
>>>>> because bfq_queue is busy doesn't represent that it has requests. Since
>>>>> bfqd aready has a counter 'queued' to record how many requests are in
>>>>> bfq, use it instead of busy_queues.
>>>>>
>>>
>>> The number of requests queued is not equal to the number of busy
>>> queues (it is >=).
>> No, sorry. It is actually != in general.
> Hi, Paolo
>
> I'm aware that number of requests queued is not equal to the number of
> busy queues, and that is the motivation of this patch.
>
>> In particular, if queued == 0 but there are busy queues (although
>> still waiting for I/O to arrive), then responding that there is no
>> work caused blk-mq to stop asking, and hence an I/O freeze. IOW I/O
>> eventually arrives for a busy queue, but blk-mq does not ask for a new
>> request any longer. But maybe things have changed around bfq since
>> then.
>
> The problem is that if queued == 0 while there are busy queues, is there
> any point to return true in bfq_has_work() ? IMO, it will only cause
> unecessary run queue. And if new request arrives,
> blk_mq_sched_insert_request() will trigger a run queue.
Great, if this is the scheme now, then the patch is correct and optimizing.
Thanks,
Paolo
>
> Thanks,
> Kuai
>> Paolo
>>> If this patch is based on this assumption then
>>> unfortunately it is wrong :(
>>>
>>> Paolo
>>>
>>>>> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the
>>>>> lock can't be held in bfq_has_work() to protect 'bfqd->queued'.
>>>>>
>>>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>>>>
>>>> Looks good. Feel free to add:
>>>>
>>>> Reviewed-by: Jan Kara <jack@suse.cz>
>>>>
>>>> Honza
>>>>
>>>>> ---
>>>>> block/bfq-iosched.c | 16 ++++++++++++----
>>>>> 1 file changed, 12 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
>>>>> index 61750696e87f..740dd83853a6 100644
>>>>> --- a/block/bfq-iosched.c
>>>>> +++ b/block/bfq-iosched.c
>>>>> @@ -2210,7 +2210,11 @@ static void bfq_add_request(struct request *rq)
>>>>>
>>>>> bfq_log_bfqq(bfqd, bfqq, "add_request %d", rq_is_sync(rq));
>>>>> bfqq->queued[rq_is_sync(rq)]++;
>>>>> - bfqd->queued++;
>>>>> + /*
>>>>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>>>>> + * may be read without holding the lock in bfq_has_work().
>>>>> + */
>>>>> + WRITE_ONCE(bfqd->queued, bfqd->queued + 1);
>>>>>
>>>>> if (RB_EMPTY_ROOT(&bfqq->sort_list) && bfq_bfqq_sync(bfqq)) {
>>>>> bfq_check_waker(bfqd, bfqq, now_ns);
>>>>> @@ -2402,7 +2406,11 @@ static void bfq_remove_request(struct request_queue *q,
>>>>> if (rq->queuelist.prev != &rq->queuelist)
>>>>> list_del_init(&rq->queuelist);
>>>>> bfqq->queued[sync]--;
>>>>> - bfqd->queued--;
>>>>> + /*
>>>>> + * Updating of 'bfqd->queued' is protected by 'bfqd->lock', however, it
>>>>> + * may be read without holding the lock in bfq_has_work().
>>>>> + */
>>>>> + WRITE_ONCE(bfqd->queued, bfqd->queued - 1);
>>>>> elv_rb_del(&bfqq->sort_list, rq);
>>>>>
>>>>> elv_rqhash_del(q, rq);
>>>>> @@ -5063,11 +5071,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
>>>>> struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
>>>>>
>>>>> /*
>>>>> - * Avoiding lock: a race on bfqd->busy_queues should cause at
>>>>> + * Avoiding lock: a race on bfqd->queued should cause at
>>>>> * most a call to dispatch for nothing
>>>>> */
>>>>> return !list_empty_careful(&bfqd->dispatch) ||
>>>>> - bfq_tot_busy_queues(bfqd) > 0;
>>>>> + READ_ONCE(bfqd->queued);
>>>>> }
>>>>>
>>>>> static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
>>>>> --
>>>>> 2.31.1
>>>>>
>>>> --
>>>> Jan Kara <jack@suse.com>
>>>> SUSE Labs, CR
>>>
>> .
^ permalink raw reply [flat|nested] 10+ messages in thread
end of thread, other threads:[~2022-05-18 13:41 UTC | newest]
Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-05-13 2:35 [PATCH -next v2 0/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
2022-05-13 2:35 ` [PATCH -next v2 1/2] block, bfq: protect 'bfqd->queued' by 'bfqd->lock' Yu Kuai
2022-05-13 6:13 ` Chaitanya Kulkarni
2022-05-13 2:35 ` [PATCH -next v2 2/2] block, bfq: make bfq_has_work() more accurate Yu Kuai
2022-05-16 9:56 ` Jan Kara
2022-05-17 14:21 ` Paolo Valente
2022-05-17 15:06 ` Paolo Valente
2022-05-18 1:17 ` yukuai (C)
2022-05-18 13:40 ` Paolo VALENTE
2022-05-16 17:39 ` [PATCH -next v2 0/2] " Jens Axboe
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