On 9/23/19 5:19 AM, Matthew Wilcox wrote: > > Ping Jens? > > On Wed, Sep 18, 2019 at 08:49:49PM -0700, Matthew Wilcox wrote: >> On Thu, Sep 19, 2019 at 10:33:10AM +0800, Lin Feng wrote: >>> On 9/18/19 20:33, Michal Hocko wrote: >>>> I absolutely agree here. From you changelog it is also not clear what is >>>> the underlying problem. Both congestion_wait and wait_iff_congested >>>> should wake up early if the congestion is handled. Is this not the case? >>> >>> For now I don't know why, codes seem should work as you said, maybe I need to >>> trace more of the internals. >>> But weird thing is that once I set the people-disliked-tunable iowait >>> drop down instantly, this is contradictory to the code design. >> >> Yes, this is quite strange. If setting a smaller timeout makes a >> difference, that indicates we're not waking up soon enough. I see >> two possibilities; one is that a wakeup is missing somewhere -- ie the >> conditions under which we call clear_wb_congested() are wrong. Or we >> need to wake up sooner. >> >> Umm. We have clear_wb_congested() called from exactly one spot -- >> clear_bdi_congested(). That is only called from: >> >> drivers/block/pktcdvd.c >> fs/ceph/addr.c >> fs/fuse/control.c >> fs/fuse/dev.c >> fs/nfs/write.c >> >> Jens, is something supposed to be calling clear_bdi_congested() in the >> block layer? blk_clear_congested() used to exist until October 29th >> last year. Or is something else supposed to be waking up tasks that >> are sleeping on congestion? Congestion isn't there anymore. It was always broken as a concept imho, since it was inherently racy. We used the old batching mechanism in the legacy stack to signal it, and it only worked for some devices. -- Jens Axboe
