Re: [RFC v4 PATCH 3/5] mm/rmqueue_bulk: alloc without touching individual page structure

[Aaron Lu]

On Thu, Oct 18, 2018 at 12:20:55PM +0100, Mel Gorman wrote:
> On Wed, Oct 17, 2018 at 10:23:27PM +0800, Aaron Lu wrote:
> > > RT has had problems with cpu_relax in the past but more importantly, as
> > > this delay for parallel compactions and allocations of contig ranges,
> > > we could be stuck here for very long periods of time with interrupts
> > 
> > The longest possible time is one CPU accessing pcp->batch number cold
> > cachelines. Reason:
> > When zone_wait_cluster_alloc() is called, we already held zone lock so
> > no more allocations are possible. Waiting in_progress to become zero
> > means waiting any CPU that increased in_progress to finish processing
> > their allocated pages. Since they will at most allocate pcp->batch pages
> > and worse case are all these page structres are cache cold, so the
> > longest wait time is one CPU accessing pcp->batch number cold cache lines.
> > 
> > I have no idea if this time is too long though.
> > 
> 
> But compact_zone calls zone_wait_and_disable_cluster_alloc so how is the
> disabled time there bound by pcp->batch?

My mistake, I misunderstood spin_lock_irqsave() and thought lock would
need be acquired before irq is disabled...

So yeah, your concern of possible excessive long irq disabled time here
is true.