On Wed, Nov 17, 2021 at 11:37:13AM +0800, yangerkun wrote: > Nowdays we meet the boot regression while enable lots of mtdblock What is your boot regression? Any dmesg log? > compare with 4.4. The main reason was that the blk_mq_freeze_queue in > elevator_init_mq will wait a RCU gap which want to make sure no IO will > happen while blk_mq_init_sched. There isn't RCU grace period implied in the blk_mq_freeze_queue() called from elevator_init_mq(), because the .q_usage_counter works at atomic mode at that time. > > Other module like loop meets this problem too and has been fix with Again, what is the problem? > follow patches: > > 2112f5c1330a loop: Select I/O scheduler 'none' from inside add_disk() > 90b7198001f2 blk-mq: Introduce the BLK_MQ_F_NO_SCHED_BY_DEFAULT flag > > They change the default IO scheduler for loop to 'none'. So no need to > call blk_mq_freeze_queue and blk_mq_init_sched. But it seems not > appropriate for mtdblocks. Mtdblocks can use 'mq-deadline' to help > optimize the random write with the help of mtdblock's cache. Once change > to 'none', we may meet the regression for random write. > > commit 737eb78e82d52d35df166d29af32bf61992de71d > Author: Damien Le Moal <damien.lemoal@xxxxxxx> > Date: Thu Sep 5 18:51:33 2019 +0900 > > block: Delay default elevator initialization > > ... > > Additionally, to make sure that the elevator initialization is never > done while requests are in-flight (there should be none when the device > driver calls device_add_disk()), freeze and quiesce the device request > queue before calling blk_mq_init_sched() in elevator_init_mq(). > ... > > This commit add blk_mq_freeze_queue in elevator_init_mq which try to > make sure no in-flight request while we go through blk_mq_init_sched. > But does there any drivers can leave IO alive while we go through > elevator_init_mq？ And if no, maybe we can just remove this logical to > fix the regression... SCSI should have passthrough requests at that moment. Thanks, Ming
