On Thu, Jul 12, 2018 at 10:00 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 7/12/18 6:28 AM, Ming Lei wrote:
>> On Thu, Jul 12, 2018 at 05:58:28PM +0800, Ming Lei wrote:
>>> On Thu, Jul 12, 2018 at 12:29:05AM +0800, Ming Lei wrote:
>>>> This patch introduces blk_mq_pm_add_request() which is called after
>>>> allocating one request. Also blk_mq_pm_put_request() is introduced
>>>> and called after one request is freed.
>>>>
>>>> For blk-mq, it can be quite expensive to accounting in-flight IOs,
>>>> so this patch calls pm_runtime_mark_last_busy() simply after each IO
>>>> is done, instead of doing that only after the last in-flight IO is done.
>>>> This way is still workable, since the active non-PM IO will be checked
>>>> in blk_pre_runtime_suspend(), and runtime suspend will be prevented
>>>> if there is any active non-PM IO.
>>>>
>>>> Also makes blk_post_runtime_resume() to cover blk-mq.
>>>>
>>>> Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
>>>> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
>>>> Cc: linux-pm@xxxxxxxxxxxxxxx
>>>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>>>> Cc: Christoph Hellwig <hch@xxxxxx>
>>>> Cc: Bart Van Assche <bart.vanassche@xxxxxxx>
>>>> Cc: "James E.J. Bottomley" <jejb@xxxxxxxxxxxxxxxxxx>
>>>> Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
>>>> Cc: linux-scsi@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
>>>> ---
>>>> block/blk-core.c | 12 ++++++++++--
>>>> block/blk-mq.c | 24 ++++++++++++++++++++++++
>>>> 2 files changed, 34 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>>> index c4b57d8806fe..bf66d561980d 100644
>>>> --- a/block/blk-core.c
>>>> +++ b/block/blk-core.c
>>>> @@ -3804,12 +3804,17 @@ EXPORT_SYMBOL(blk_pm_runtime_init);
>>>> int blk_pre_runtime_suspend(struct request_queue *q)
>>>> {
>>>> int ret = 0;
>>>> + bool active;
>>>>
>>>> if (!q->dev)
>>>> return ret;
>>>>
>>>> spin_lock_irq(q->queue_lock);
>>>> - if (q->nr_pending) {
>>>> + if (!q->mq_ops)
>>>> + active = !!q->nr_pending;
>>>> + else
>>>> + active = !blk_mq_pm_queue_idle(q);
>>>> + if (active) {
>>>> ret = -EBUSY;
>>>> pm_runtime_mark_last_busy(q->dev);
>>>> } else {
>>>
>>> Looks there is one big issue, one new IO may come just after reading
>>> 'active' and before writing RPM_SUSPENDING to q->rpm_status, and both
>>> the suspending and the new IO may be in-progress at the same time.
>>
>> One idea I thought of is to use seqlock to sync changing & reading q->rpm_status,
>> and looks read lock(read_seqcount_begin/read_seqcount_retry) shouldn't introduce
>> big cost in fast path.
>
> Let's please keep in mind that this is runtime pm stuff. Better to
> make the rules relaxed around it, instead of adding synchronization.
But the race has to be avoided, otherwise IO may be failed. I don't
find any simple solution yet for avoiding the race without adding sync.
Any idea for avoiding the race without using sync like seqlock or others?
Thanks,
Ming Lei
