On Fri, 2018-08-10 at 09:39 +-0800, jianchao.wang wrote: +AD4- On 08/10/2018 03:41 AM, Bart Van Assche wrote: +AD4- +AD4- +-/+ACo- +AD4- +AD4- +- +ACo- Whether or not blk+AF8-queue+AF8-enter() should proceed. RQF+AF8-PM requests are always +AD4- +AD4- +- +ACo- allowed. RQF+AF8-DV requests are allowed if the PM+AF8-ONLY queue flag has not been +AD4- +AD4- +- +ACo- set. Other requests are only allowed if neither PM+AF8-ONLY nor DV+AF8-ONLY has been +AD4- +AD4- +- +ACo- set. +AD4- +AD4- +- +ACo-/ +AD4- +AD4- +-static inline bool blk+AF8-enter+AF8-allowed(struct request+AF8-queue +ACo-q, +AD4- +AD4- +- blk+AF8-mq+AF8-req+AF8-flags+AF8-t flags) +AD4- +AD4- +-+AHs- +AD4- +AD4- +- return flags +ACY- BLK+AF8-MQ+AF8-REQ+AF8-PM +AHwAfA- +AD4- +AD4- +- (+ACE-blk+AF8-queue+AF8-pm+AF8-only(q) +ACYAJg- +AD4- +AD4- +- (flags +ACY- BLK+AF8-MQ+AF8-REQ+AF8-DV +AHwAfA- +ACE-blk+AF8-queue+AF8-dv+AF8-only(q)))+ADs- +AD4- +AD4- +-+AH0- +AD4- +AD4- If a new state is indeed necessary, I think this kind of checking in hot path is inefficient. +AD4- How about introduce a new state into request+AF8-queue, such as request+AF8-queue-+AD4-gate+AF8-state. +AD4- Set the PM+AF8-ONLY and DV+AF8-ONLY into this state, then we could just check request+AF8-queue-+AD4-gate+AF8-state +AD4- 0 +AD4- before do further checking. Hello Jianchao, I agree with you that the hot path should be as efficient as possible. But I don't think that the above code adds significantly more overhead to the hot path. The requests for which we care about performance are the requests that read and write data. BLK+AF8-MQ+AF8-REQ+AF8-PM is not set for any of these requests. For the high performance case we care about the pm-only flag won't be set. If that flag is not set the rest of the boolean expression does not have to be evaluated (flags +ACY- BLK+AF8-MQ+AF8-REQ+AF8-DV +AHwAfA- +ACE-blk+AF8-queue+AF8-dv+AF8-only(q)). So I think the above change only adds one additional boolean test to the hot path. That shouldn't have a measurable performance impact. Bart.
