Messages in this thread |  | | | Date | Tue, 21 Dec 2010 15:06:40 +0100 | | From | Frederic Weisbecker <> | | Subject | Re: [RFC PATCH 10/15] nohz_task: Enter in extended quiescent state when in userspace |
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On Tue, Dec 21, 2010 at 09:04:54AM +0100, Peter Zijlstra wrote: > On Tue, 2010-12-21 at 02:27 +0100, Frederic Weisbecker wrote: > > On Mon, Dec 20, 2010 at 05:18:40PM +0100, Peter Zijlstra wrote: > > > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote: > > > > (we check if the local cpu is in nohz mode, which means > > > > no other task compete on that CPU) > > > > > > You keep repeating that definition, but its not true.. It means there is > > > not work for the tick to do, the tick does _tons_ more besides > > > preemption, so nr_running==1 is necessary but not sufficient. > > > > Sure but the point is that if the tick is not running, it means > > that the nohz task is the only task running on that CPU. > > No, that too isn't true, the cpu could be idle. > > > Now indeed there are other reasons for the tick to restart like RCU > > or the effective nohz mode to physically happen or to be delayed, > > which is decided by tick_nohz_stop_sched_tick(). > > You really really really badly need to read through the whole tick path > and look at all the things it does, put them in a list, then look at the > current nohz code, mark those it deals with, then go through the nohz > code again and find the nr_running==0 assumptions, then make sure you've > covered everything. > > I'm very confident you'll find a number of things to fix. At the very > least your current patch set totally forgets about task runtime > accounting (account_process_tick()), the existing NO_HZ doesn't need to > worry about that because the system is idle so all it needs to do is add > idle ticks when it wakes up (and possibly steal time for the virt muck). > > You also miss the profile_tick(), and you need to go through the load > accounting muck (both of them) to see if there are any nr_running==0 > assumptions there. > > You also need to fix the perf counter list rotation stuff, and again, > check if no_hz load-balancing can deal with the nr_running==1 situation. > > Now, there might be more, this is just a quick one-minute scan through > the tick code, but the fact that nowhere in this whole patch-set is even > a mention of these things makes me worry about your whole approach.
Yeah it's right I missed a lot of important things. I'll try to handle them for the next take.
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