Messages in this thread | | | Date | Mon, 13 Dec 2004 20:29:39 -0800 | From | Andrew Morton <> | Subject | Re: dynamic-hz |
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Nish Aravamudan <nish.aravamudan@gmail.com> wrote: > > On Mon, 13 Dec 2004 03:25:21 -0800, Andrew Morton <akpm@osdl.org> wrote: > > Andrea Arcangeli <andrea@suse.de> wrote: > > > > > > The patch only does HZ at dynamic time. But of course it's absolutely > > > trivial to define it at compile time, it's probably a 3 liner on top of > > > my current patch ;). However personally I don't think the three liner > > > will worth the few seconds more spent configuring the kernel ;). > > > > We still have 1000-odd places which do things like > > > > schedule_timeout(HZ/10); > > Yes, yes, we do :) I replaced far more than I ever thought I could... > There are a few issues I have with the remaining schedule_timeout() > calls which I think fit ok with this thread... I'd especially like > your input, Andrew, as you end up getting most of my patches from KJ. > > Many drivers use > > set_current_state(TASK_{UN,}INTERRUPTIBLE); > schedule_timeout(1); // or some other small value < 10 > > This may or may not hide a dependency on a particular HZ value. If the > code is somewhat old, perhaps the author intended the task to sleep > for 1 jiffy when HZ was equal to 100. That meants that they ended up > sleeping for 10 ms. If the code is new, the author intends that the > task sleeps for 1 ms (HZ==1000). The question is, what should the > replacement be?
Presumably they meant 10 milliseconds. Or at least, that is the delay which the developer did his testing with.
> If they really meant to use schedule_timeout(1) in the sense of > highest resolution delay possible (the latter above), then they > probably should just call schedule() directly.
argh. Never do that. It's basically a busywait and can cause lockups if the calling task has realtime scheduling policy.
> schedule_timeout(1) > simply sets up a timer to fire off after 1 jiffy & then calls > schedule() itself. The overhead of setting up a timer and the > execution of schedule() itself probably means that the timer will go > off in the middle of the schedule() call or very shortly thereafter (I > think). In which case, it makes more sense to use schedule() > directly... > > If they meant to schedule a delay of 10ms, then msleep() should be > used in those cases. msleep() will also resolve the issues with 0-time > timeouts because of rounding, as it adds 1 to the converted parameter. > > Obviously, changing more and more sleeps to msecs & secs will really > help make the changing of HZ more transparent. And specifying the time > in real time units just seems so much clearer to me. > > What do people think?
I'd say that replacing them with msleep(10) is the safest approach. Depending on what the surronding code is actually doing, of course.
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