On Sat, Nov 16, 2024 at 1:34 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Sat, Nov 16, 2024 at 1:13 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Sat, Nov 16, 2024 at 11:42 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Nov 15, 2024 at 05:48:53PM -0800, Alexei Starovoitov wrote:
> > > > +static inline struct page *try_alloc_page_noprof(int nid)
> > > > +{
> > > > + /* If spin_locks are not held and interrupts are enabled, use normal path. */
> > > > + if (preemptible())
> > > > + return alloc_pages_node_noprof(nid, GFP_NOWAIT | __GFP_ZERO, 0);
> > >
> > > This isn't right for PREEMPT_RT, spinlock_t will be preemptible, but you
> > > very much do not want regular allocation calls while inside the
> > > allocator itself for example.
> >
> > I'm aware that spinlocks are preemptible in RT.
> > Here is my understanding of why the above is correct...
> > - preemptible() means that IRQs are not disabled and preempt_count == 0.
> >
> > - All page alloc operations are protected either by
> > pcp_spin_trylock() or by spin_lock_irqsave(&zone->lock, flags)
> > or both together.
> >
> > - In non-RT spin_lock_irqsave disables IRQs, so preemptible()
> > check guarantees that we're not holding zone->lock.
> > The page alloc logic can hold pcp lock when try_alloc_page() is called,
> > but it's always using pcp_trylock, so it's still ok to call it
> > with GFP_NOWAIT. pcp trylock will fail and zone->lock will proceed
> > to acquire zone->lock.
> >
> > - In RT spin_lock_irqsave doesn't disable IRQs despite its name.
> > It calls rt_spin_lock() which calls rcu_read_lock()
> > which increments preempt_count.
>
> The maze of ifdef-s beat me :(
> It doesn't increment in PREEMPT_RCU.
> Need an additional check then. hmm.
Like:
if (preemptible() && !rcu_preempt_depth())
return alloc_pages_node_noprof(nid, GFP_NOWAIT | __GFP_ZERO, 0);
Not pretty, but should do.
wdyt?
