Messages in this thread | | | Subject | Re: [PATCH v14 04/14] task_isolation: add initial support | From | Chris Metcalf <> | Date | Tue, 30 Aug 2016 11:41:36 -0400 |
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On 8/29/2016 8:55 PM, Frederic Weisbecker wrote: > On Mon, Aug 15, 2016 at 10:59:55AM -0400, Chris Metcalf wrote: >> On 8/11/2016 2:50 PM, Christoph Lameter wrote: >>> On Thu, 11 Aug 2016, Frederic Weisbecker wrote: >>> >>>> Do we need to quiesce vmstat everytime before entering userspace? >>>> I thought that vmstat only need to be offlined once and for all? >>> Once is sufficient after disabling the tick. >> It's true that task_isolation_enter() is called every time before >> returning to user space while task isolation is enabled. >> >> But once we enter the kernel again after returning from the initial >> prctl() -- assuming we are in NOSIG mode so doing so is legal in the >> first place -- almost anything can happen, certainly including >> restarting the tick. Thus, we have to make sure that normal quiescing >> happens again before we return to userspace. > Yes but we need to sort out what needs to be called only once on prctl(). > > Once vmstat is quiesced, it's not going to need quiescing again even if we > restart the tick.
That's true, but I really do like the idea of having a clean structure where we verify all our prerequisites in task_isolation_ready(), and have code to try to get things fixed up in task_isolation_enter(). If we start moving some things here and some things there, it gets harder to manage. I think by testing "!vmstat_idle()" in task_isolation_enter() we are avoiding any substantial overhead.
I think it would be clearer to rename task_isolation_enter() to task_isolation_prepare(); it might be less confusing.
Remember too that in general, we really don't need to think about return-to-userspace as a hot path for task isolation, unlike how we think about it all the rest of the time. So it makes sense to prioritize keeping things clean from a software development perspective first, and high-performance only secondarily.
>> The thing to remember is that this is only relevant if the user has >> explicitly requested the NOSIG behavior from task isolation, which we >> don't really expect to be the default - we are implicitly encouraging >> use of the default semantics of "you can't enter the kernel again >> until you turn off isolation". > That's right. Although NOSIG is the only thing we can afford as long as > we drag around the 1Hz.
True enough. Hopefully we'll finish sorting that out soon enough.
>>>> + if (!tick_nohz_tick_stopped()) >>>> + set_tsk_need_resched(current); >>>> Again, that won't help >> It won't be better than spinning in a loop if there aren't any other >> schedulable processes, but it won't be worse either. If there is >> another schedulable process, we at least will schedule it sooner than >> if we just sat in a busy loop and waited for the scheduler to kick >> us. But there's nothing else we can do anyway if we want to maintain >> the guarantee that the dyn tick is stopped before return to userspace. > I don't think it helps either way. If reschedule is pending, the current > task already has TIF_RESCHED set.
See the other thread with Peter Z for the longer discussion of this. At this point I'm leaning towards replacing the set_tsk_need_resched() with
set_current_state(TASK_INTERRUPTIBLE); schedule(); __set_current_state(TASK_RUNNING);
-- Chris Metcalf, Mellanox Technologies http://www.mellanox.com
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