On Thu 09-02-23 11:09:33, Bart Van Assche wrote: > On 2/9/23 00:56, Jan Kara wrote: > > On Wed 08-02-23 09:53:41, Bart Van Assche wrote: > > > The test results I shared some time ago show that IOPRIO_CLASS_NONE was the > > > default I/O priority two years ago (see also https://lore.kernel.org/linux-block/20210927220328.1410161-5-bvanassche@xxxxxxx/). > > > The none-to-rt policy increases the priority of bio's that have not been > > > assigned an I/O priority to RT. Does this answer your question? > > > > Not quite. I know that historically we didn't set bio I/O priority in some > > paths (but we did set it in other paths such as some (but not all) direct > > IO implementations). But that was exactly a mess because how none-to-rt > > actually behaved depended on the exact details of the kernel internal IO > > path. So my question is: Was none-to-rt actually just a misnomer and the > > intended behavior was "always override to RT"? Or what was exactly the > > expectation around when IO priority is not set and should be overridden? > > > > How should it interact with AIO submissions with IOCB_FLAG_IOPRIO? How > > should it interact with task having its IO priority modified with > > ioprio_set(2)? And what if task has its normal scheduling priority modified > > but that translates into different IO priority (which happens in > > __get_task_ioprio())? > > > > So I think that none-to-rt is just poorly defined and if we can just get > > rid of it (or redefine to promote-to-rt), that would be good. But maybe I'm > > missing some intended usecase... > > Hi Jan, > > We have no plans to use the ioprio_set() system call since it only affects > foreground I/O and not page cache writeback. > > While Android supports io_uring, there are no plans to support libaio in the > Android C library (Bionic). > > Regarding __get_task_ioprio(), I haven't found any code in that function > that derives an I/O priority from the scheduling priority. Did I perhaps > overlook something? This condition in __get_task_ioprio(): if (IOPRIO_PRIO_CLASS(prio) == IOPRIO_CLASS_NONE) prio = IOPRIO_PRIO_VALUE(task_nice_ioclass(p), task_nice_ioprio(p)); sets task's IO priority based on scheduling priority. > Until recently "none-to-rt" meant "if no I/O priority has been assigned to a > task, use IOPRIO_CLASS_RT". Promoting the I/O priority to IOPRIO_CLASS_RT > works for us. I'm fine with changing the meaning of "none-to-rt" into > promoting the I/O priority class to RT. Introducing "promote-to-rt" as a > synonym of "none-to-rt" is also fine with me. OK, so it seems we are all in agreement here that "none-to-rt" behavior is not really needed. Hou, can you perhaps update your patches and the documentation to make "none-to-rt" just an alias for "promote-to-rt"? Thanks! Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
