On Fri 28-08-20 11:07:29, peterz@xxxxxxxxxxxxx wrote: > On Fri, Aug 28, 2020 at 02:07:12PM +0800, Xianting Tian wrote: > > As the normal aio wait path(read_events() -> > > wait_event_interruptible_hrtimeout()) doesn't account iowait time, so use > > this patch to make it to account iowait time, which can truely reflect > > the system io situation when using a tool like 'top'. > > Do be aware though that io_schedule() is potentially far more expensive > than regular schedule() and io-wait accounting as a whole is a > trainwreck. Hum, I didn't know that io_schedule() is that much more expensive. Thanks for info. > When in_iowait is set schedule() and ttwu() will have to do additional > atomic ops, and (much) worse, PSI will take additional locks. > > And all that for a number that, IMO, is mostly useless, see the comment > with nr_iowait(). Well, I understand the limited usefulness of the system or even per CPU percentage spent in IO wait. However whether a particular task is sleeping waiting for IO or not is IMO a useful diagnostic information and there are several places in the kernel that take that into account (PSI, hangcheck timer, cpufreq, ...). So I don't see that properly accounting that a task is waiting for IO is just "expensive random number generator" as you mention below :). But I'm open to being educated... > But, if you don't care about performance, and want to see a shiny random > number generator, by all means, use io_schedule(). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
