Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation

[David Woodhouse]

On Tue, 2026-08-25 at 10:19 -0700, Paul E. McKenney wrote:
> On Tue, Aug 25, 2026 at 06:05:54PM +0100, David Woodhouse wrote:
> > On Tue, 2026-08-25 at 09:47 -0700, Paul E. McKenney wrote:
> > > 
> > > On the tail latencies...
> > > 
> > > The easiest way to reduce them is to require that preemption be disabled
> > > across srcu_read_lock_atomic()/srcu_read_unlock_atomic() regions and
> > > across all calls to synchronize_srcu_atomic().  Without that, the problem
> > > is that the scheduler does not know that the spinning is pointless,
> > > and we cannot use the blocking primitives that we could otherwise use
> > > to tell it what is going on.
> > > 
> > > So, is it feasible to simply require preemption be disabled as called
> > > out above?
> > 
> > I'd experimented with disabling it around the GP driver loop in
> > synchronize_srcu_atomic() as seen in
> > https://git.infradead.org/?p=users/dwmw2/linux.git;a=commitdiff;h=07165e79340e
> > and that didn't seem to change anything (which seems reasonable, as
> > it's the *waiters* that were descheduled, not the threads driving the
> > actual GP). So your suggestion that we do it around the whole function
> > certainly makes sense too. I'll test it.
> > 
> > I do wonder if we're really doing the right thing here by selfishly
> > blocking preemption because we want a specific tail latency to remain
> > low in a contended system. Maybe we should allow preemption and trust
> > that the right thing will happen? Maybe the p100 isn't the right
> > benchmark to be chasing... I'm looking at it because Sean expressed
> > concerns about it, but it's not the only consideration.
> 
> My concern is algorithmic, not benchmark optimization.
> 
> Suppose that there is only one CPU, or, alternatively, that one of the
> atomic SRCU readers is pinned to the same CPU occupied by the (higher
> priority) task running synchronize_srcu_atomic().  In this case, the
> call to synchronize_srcu_atomic() uselessly burns CPU time until its
> priority decays, real-time throttling kicks in, or in some configurations,
> maybe never.

I certainly have no problem with a blanket preempt_disable() around
both sides for algorithmic reasons. As long as we aren't *just* doing
it for the selfish reasons I described. 

> Requiring preemption be disabled across both the atomic SRCU readers
> and the synchronize_srcu_atomic() avoids this, at least when running on
> bare metal.  My (perhaps naive) hope is that guest OSes get some use
> out of those cpu_relax() calls.

Yeah, an overcommited guest vCPU should be able to get preempted there
by the hypervisor, allowing other vCPUs to run.

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