On 04/21/2018 10:10 PM, Jens Axboe wrote:
On 4/21/18 7:34 AM, jianchao.wang wrote:
Hi Bart
Thanks for your kindly response.
On 04/20/2018 10:11 PM, Bart Van Assche wrote:
On Fri, 2018-04-20 at 14:55 +0800, jianchao.wang wrote:
Hi Bart
On 04/20/2018 12:43 AM, Bart Van Assche wrote:
Use the deadline instead of the request generation to detect whether or not a request timer fired after reinitialization of a request
Maybe use deadline to do this is not suitable.
Let's think of the following scenario.
T1/T2 times in nano seconds J1/J2 times in jiffies
rq is started at T1,J1 blk_mq_timeout_work -> blk_mq_check_expired -> save rq->__deadline (J1 + MQ_RQ_IN_FLIGHT)
rq is completed and freed rq is allocated and started again on T2 rq->__deadline = J2 + MQ_RQ_IN_FLIGHT-> synchronize srcu/rcu -> blk_mq_terminate_expired -> rq->__deadline (J2 + MQ_RQ_IN_FLIGHT)
deadline = rq->__deadline & ~RQ_STATE_MASK (0x3) if J2-J1 < 4 jiffies, we will get the same deadline value.
even if we do some bit shift when save and get deadline if T2 - T1 < time of 1 jiffies, we sill get the same deadline.
Hello Jianchao,
How about using the upper 16 or 32 bits of rq->__deadline to store a generation number? I don't know any block driver for which the product of (deadline in seconds) and HZ exceeds (1 << 32).
yes, we don't need so long timeout value. However, req->__deadline is an absolute time, not a relative one, its type is unsigned long variable which is same with jiffies. If we reserve some bits (not just 1 or 2 bits) of req->__deadline for generation number, how to handle it when mod_timer ?
We don't need to support timeouts to the full granularity of jiffies. Most normal commands are in the 30-60s range, and some lower level commands may take minutes. We're well within the range of chopping off some bits and not having to worry about that at all.
Yes. So we have a __deadline as below:
deadline gen state |______________|____|_| ___ __/ V granularity
and even we usually have a 30~60s timeout, but we should support a 1s granularity at least. How to decide the granularity ?
Thanks Jianchao