On Tue, Jul 28, 2026 at 09:21:55AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 28, 2026 at 10:57:57AM +0530, Shrikanth Hegde wrote:
> > Hi Paul, Christophe,
> >
> > On 7/27/26 11:59 PM, Christophe Leroy (CS GROUP) wrote:
> > > Hi Shrikanth,
> > >
> >
> > >
> > > That's right with CONFIG_PREEMPT_DYNAMIC=y
> > >
> > > With CONFIG_PREEMT_DYNAMIC=n:
> > > - CONFIG_PREEMPT (full) implies CONFIG_PREEMPT_RCU
> > > - CONFIG_PREEMPT_LAZY (lazy) doesn't imply CONFIG_PREEMPT_RCU
> > >
> > > because of kernel/rcu/Kconfig,
> > >
> > > config PREEMPT_RCU
> > > bool
> > > default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
> > > select TREE_RCU
> > >
> > > Christophe
> > >
> > >
> >
> > Right.
> >
> > Looks like __rcu_read_unlock can set need_resched bit if it find irq were
> > disabled
> > and rcu_read_unlock_special.s is set. I don't have much clue about it.
> > So, after seeing __rcu_read_unlock for PREEMPT_RCU I have these concerns
> > now.
>
> This is because irq-disabled regions of code act as RCU readers.
> For non-preemptible RCU (even in CONFIG_PREEMPT_LAZY=y kernels), this
> comes for free. In contrast, preemptible RCU must explicitly (and,
> as you noticed, rather painfully) handle the irq-disabled case:
>
> rcu_read_lock();
> do_something();
> local_irq_disable();
> do_something_else();
> rcu_read_unlock();
> do_yet_another_thing();
> local_irq_enable();
>
> Here, the RCU reader extends all the way from the rcu_read_lock() to
> the final local_irq_enable(). And this is why RCU checks for IRQs being
> disabled in __rcu_read_unlock().
>
> > In current upstream, lazy is a preemption mode with preempt_enable being a
> > preemptible point,
> >
> > 1. If PREEMPT_LAZY=y and PREEMPT_DYNAMIC=n, then PREEMPT_RCU=n. That seems
> > wrong. It is supposed to
> > do the preemption checks, since preempt_count is actual count with lazy
> > preemption.
>
> Sorry, but no, not wrong at all. This is the way that it is supposed
> to be. In this configuration, RCU readers explicitly disable preemption.
> This gets us minimal RCU overhead on architectures such as x86 and arm64
> that have prohibited CONFIG_PREEMPT_NONE and CONFIG_PREEMPT_VOLUNTARY.
> Not zero overhead, but minimal overhead given the constraints imposed by
> CONFIG_PREEMPT_LAZY. This also prevents preemption of RCU readers, which
> is extremely important on datacenter servers that have good and sufficient
> quantities of memory, but which are not well-endowed with memory.
Now, if you would like to be able to manually choose between non-preemptible
and preemptible RCU when when PREEMPT_LAZY=y, *that* we could discuss.
But it looks like you are looking for low overhead, and thus wanting to
avoid preemptible RCU.
Thanx, Paul
> > 2. PREEMPT_LAZY=y and PREEMPT_DYNAMIC=n and PREEMPT_LAZY=y and
> > PREEMPT_DYNAMIC=y are supposed
> > to be similar w.r.t to performance.
>
> RCU never made that promise. ;-)
>
> With PREMPT_DYNAMIC=y, RCU has no choice but to be preemptible, at
> least unless and until someone decides to make RCU's preemptibility be
> changeable at runtime (which I do not recommend). And we really do need
> non-preemptible RCU for server workloads.
>
> So it is not just that RCU never made that promise, it is that RCU cannot
> reasonably make that promise.
>
> > Promise of PREEMPT_DYNAMIC was the
> > ability to
> > switch at runtime based on the user workload and his/her preference.
> > But with PREEMPT_LAZY=y and PREEMPT_DYNAMIC=y, there is additional cost
> > due to PREEMPT_RCU.
> > lazy being the middle ground w.r.t for decent performance, either
> > PREEMPT_RCU should either kick in
> > for both cases or bail out for both.
>
> Absolutely not!!! Please see above.
>
> > Before 7.0
> > 3. the same concerns would be true to PREEMPT_VOLUNTARY and PREEMPT_NONE
> > with PREEMPT_DYNAMIC=y.
> > Those model never wanted a preemption, but PREEMPT_RCU could have forced
> > a need_resched to be
> > set. Which is also likely wrong. No?
>
> You lost me on this one. Please give me an explicit sequence of events
> that would cause things to go wrong in this case.
>
> Thanx, Paul
