On Thu, Apr 16, 2020 at 11:18:38PM +0200, Uladzislau Rezki wrote:
> On Thu, Apr 16, 2020 at 01:25:30PM -0700, Paul E. McKenney wrote:
> > On Thu, Apr 16, 2020 at 10:05:57PM +0200, Uladzislau Rezki wrote:
> > > On Thu, Apr 16, 2020 at 12:53:27PM -0700, Paul E. McKenney wrote:
> > > > On Thu, Apr 16, 2020 at 03:26:23PM -0400, Steven Rostedt wrote:
> > > > > On Thu, 16 Apr 2020 14:59:34 -0400
> > > > > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > > But, then will it be safe for kfree_rcu() callers from hard IRQ context to
> > > > > > call this in PREEMPT_RT? That could would just break then as you cannot sleep
> > > > > > in hard IRQ context even on PREEMPT_RT.
> > > > >
> > > > > But where in PREEMPT_RT would it be called in hard IRQ context?
> > > >
> > > > I believe that call_rcu() is invoked with raw spinlocks held, so we should
> > > > allow kfree_rcu() to be invoked from similar contexts. It obviously
> > > > cannot allocate memory in such contexts, so perhaps the rule is that
> > > > single-argument kfree_rcu() cannot be invoked within hard IRQ contexts
> > > > or with raw spinlocks held. In those contexts, you would instead need
> > > > to invoke two-argument kfree_rcu(), which never needs to allocate memory.
> > > >
> > > > Seem reasonable?
> > > >
> > > Paul, just to make it more clear, even invoking two arguments fkree_rcu()
> > > currently does an allocation. We maintain an array that contains pointers
> > > for "bulk logic".
> >
> > True, but that is an optimization rather than an absolute necessity.
> > In addition, most two-argument kfree_rcu() callers will take another slot
> > from the array that has already been allocated. So one alternative is
> > to do the allocation only if both interrupts and preemption are enabled.
> > As long as most kfree_rcu() invocations can allocate, you get good
> > performance and things work nicely in -rt.
> >
> Agree it is not absolutely necessary :) The easiest way is something like that:
>
> #ifdef CONFIG_PREEMPT_RT
> if (is_there_any_space_in_array())
> put_the_pointer_into_arry()
> finish.
> else
> queue_it_using_rcu_head_helper()
> #endif
>
Sorry, somme correction:
#ifdef CONFIG_PREEMPT_RT
if (is_there_any_space_in_array())
put_the_pointer_into_arry()
else if (not in irq && not in NMI)
alloca_page_and_put_ptr_there();
else
queue_it_using_rcu_head_helper()
#endif
--
Vlad Rezki
