On Thu, May 21, 2020 at 09:46:35AM -0700, Alexander Duyck wrote:
> It is more about not bothering with the extra tracking. We don't
> really need it and having it doesn't really add much in the way of
> value.
Yeah, it can probably go.
> > > > @@ -1863,11 +1892,32 @@ static int __init deferred_init_memmap(void *data)
> > > > goto zone_empty;
> > > >
> > > > /*
> > > > - * Initialize and free pages in MAX_ORDER sized increments so
> > > > - * that we can avoid introducing any issues with the buddy
> > > > - * allocator.
> > > > + * More CPUs always led to greater speedups on tested systems, up to
> > > > + * all the nodes' CPUs. Use all since the system is otherwise idle now.
> > > > */
> > > > + max_threads = max(cpumask_weight(cpumask), 1u);
> > > > +
> > > > while (spfn < epfn) {
> > > > + epfn_align = ALIGN_DOWN(epfn, PAGES_PER_SECTION);
> > > > +
> > > > + if (IS_ALIGNED(spfn, PAGES_PER_SECTION) &&
> > > > + epfn_align - spfn >= PAGES_PER_SECTION) {
> > > > + struct definit_args arg = { zone, ATOMIC_LONG_INIT(0) };
> > > > + struct padata_mt_job job = {
> > > > + .thread_fn = deferred_init_memmap_chunk,
> > > > + .fn_arg = &arg,
> > > > + .start = spfn,
> > > > + .size = epfn_align - spfn,
> > > > + .align = PAGES_PER_SECTION,
> > > > + .min_chunk = PAGES_PER_SECTION,
> > > > + .max_threads = max_threads,
> > > > + };
> > > > +
> > > > + padata_do_multithreaded(&job);
> > > > + nr_pages += atomic_long_read(&arg.nr_pages);
> > > > + spfn = epfn_align;
> > > > + }
> > > > +
> > > > nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> > > > cond_resched();
> > > > }
> > >
> > > This doesn't look right. You are basically adding threads in addition
> > > to calls to deferred_init_maxorder.
> >
> > The deferred_init_maxorder call is there to do the remaining, non-section
> > aligned part of a range. It doesn't have to be done this way.
>
> It is also doing the advancing though isn't it?
Yes. Not sure what you're getting at. There's the 'spfn = epfn_align' before
so nothing is skipped. It's true that the nonaligned part is done outside of
padata when it could be done by a thread that'd otherwise be waiting or idle,
which should be addressed in the next version.
> I think I resolved this with the fix for it I described in the other
> email. We just need to swap out spfn for epfn and make sure we align
> spfn with epfn_align. Then I think that takes care of possible skips.
Right, though your fix looks a lot like deferred_init_mem_pfn_range_in_zone().
Seems better to just use that and not repeat ourselves. Lame that it's
starting at the beginning of the ranges every time, maybe it could be
generalized somehow, but I think it should be fast enough.
> > We could use deferred_init_mem_pfn_range_in_zone() instead of the for_each
> > loop.
> >
> > What I was trying to avoid by aligning down is creating a discontiguous pfn
> > range that get passed to padata. We already discussed how those are handled
> > by the zone iterator in the thread function, but job->size can be exaggerated
> > to include parts of the range that are never touched. Thinking more about it
> > though, it's a small fraction of the total work and shouldn't matter.
>
> So the problem with aligning down is that you are going to be slowed
> up as you have to go single threaded to initialize whatever remains.
> So worst case scenario is that you have a section aligned block and
> you will process all but 1 section in parallel, and then have to
> process the remaining section one max order block at a time.
Yes, aligning up is better.
> > > This should accomplish the same thing, but much more efficiently.
> >
> > Well, more cleanly. I'll give it a try.
>
> I agree I am not sure if it will make a big difference on x86, however
> the more ranges you have to process the faster this approach should be
> as it stays parallel the entire time rather than having to drop out
> and process the last section one max order block at a time.
Right.
