Messages in this thread | | | Date | Thu, 24 Sep 2009 22:19:43 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/2] itimers: fix racy writes to cpu_itimer fields |
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* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Thu, 2009-09-24 at 19:57 +0200, Stanislaw Gruszka wrote: > > On Thu, 24 Sep 2009 16:48:07 +0200 > > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > > > On Thu, 2009-09-24 at 16:35 +0200, Stanislaw Gruszka wrote: > > > > incr_error and error fields of struct cpu_itimer are used when calculating > > > > next timer tick in check_cpu_itimers() and should not be modified without > > > > tsk->sighand->siglock taken. > > > > > > Won't it be all-round much better to convert these things to hrtimers > > > instead of adding more and more fuzz on top to make them deal with > > > jiffies? > > > > Perhaps it would, but I don't know how to do it :{ . Especially how to > > precisely account user time. The only idea I have is make something like > > microstate accounting (http://lwn.net/Articles/127296/), but this patch > > and whole idea was rejected long time ago. > > That patch does look a little painful indeed. > > I was more thinking about about looking if an itimer was to expire less > than 1 tick away on either sched-in or the tick. > > When we find it is indeed less than 1 tick away, program an hrtimer for > that cpu to expire at the required moment, see hrtick_start(). > > If we happen to de-schedule the task before the timer fires, we clear > the hrtimer again (or let it pend and ignore the fire), see > hrtick_clear(). > > [ there is no reason to rely on the tick though, we can program the > hrtimer on sched in to expire on at the right moment, and do so on > each schedule for as long as an itimer is active - re-setting whatever > pending timer the cpu still had. ]
we should think about the simplest approach: switching itimers to hrtimers.
Ingo
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