Re: [PATCH v14 04/14] task_isolation: add initial support

[Chris Metcalf]

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.

For vmstat, you're right that it's somewhat heavyweight to do the
quiesce, and if we don't need it, it's wasted time on the return path.
So I will add a guard call to the new vmstat_idle() before invoking
quiet_vmstat_sync().  This slows down the path where it turns out we
do need to quieten vmstat, but not by too much.

The LRU quiesce is quite light-weight.  We just check pagevec_count()
on a handful of pagevec's, confirm they are all zero, and return
without further work.  So for that one, adding a separate
lru_add_drain_needed() guard test would just be wasted effort.

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".

> > +	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.

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com