On Mon, Aug 15, 2016 at 10:59:55AM -0400, Chris Metcalf wrote:
On 8/11/2016 2:50 PM, Christoph Lameter wrote:Yes but we need to sort out what needs to be called only once on prctl().
On Thu, 11 Aug 2016, Frederic Weisbecker wrote:It's true that task_isolation_enter() is called every time before
Do we need to quiesce vmstat everytime before entering userspace?Once is sufficient after disabling the tick.
I thought that vmstat only need to be offlined once and for all?
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.
Once vmstat is quiesced, it's not going to need quiescing again even if we
restart the tick.
The thing to remember is that this is only relevant if the user hasThat's right. Although NOSIG is the only thing we can afford as long as
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".
we drag around the 1Hz.
I don't think it helps either way. If reschedule is pending, the currentIt won't be better than spinning in a loop if there aren't any other+ if (!tick_nohz_tick_stopped())
+ set_tsk_need_resched(current);
Again, that won't help
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.
task already has TIF_RESCHED set.