Re: [PATCH 5/7] mm: move zone iterator outside of deferred_init_maxorder()

[Alexander Duyck]

On Wed, May 6, 2020 at 3:39 PM Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> wrote:
>
> On Tue, May 05, 2020 at 08:27:52AM -0700, Alexander Duyck wrote:
> > As it turns out that deferred_free_range will be setting the
> > migratetype for the page. In a sparse config the migratetype bits are
> > stored in the section bitmap. So to avoid cacheline bouncing it would
> > make sense to section align the tasks so that they only have one
> > thread touching one section rather than having the pageblock_flags
> > getting bounced between threads.
>
> That's a good point, I'll change the alignment.
>
> I kicked off some runs on the Skylake bare metal system to check how this did
> and the performance stayed the same, but see below.
>
> > It should also reduce the overhead
> > for having to parallelize the work in the first place since a section
> > is several times larger than a MAX_ORDER page and allows for more
> > batching of the work.
>
> I think you may be assuming that threads work in MAX_ORDER batches, maybe
> because that's the job's min_chunk, but padata works differently.  The
> min_chunk is a lower bound that establishes the smallest amount of work that
> makes sense for a thread to do in one go, so in this case it's useful to
> prevent starting large numbers of threads to initialize a tiny amount of pages.
>
> Internally padata uses total job size and min chunk to arrive at the chunk
> size, which on big machines will be much larger than min_chunk.  The idea is
> the chunk size should be large enough to minimize multithreading overhead but
> small enough to permit load balancing between threads.
>
> This is probably why the results didn't change much when aligning by section,
> but that doesn't mean other systems won't benefit.

Okay, that makes sense.

> > > Maybe it's better to leave deferred_init_maxorder alone and adapt the
> > > multithreading to the existing implementation.  That'd mean dealing with the
> > > pesky opaque index somehow, so deferred_init_mem_pfn_range_in_zone() could be
>
> I should have been explicit, was thinking of @i from
> () when mentioning the opaque index.

Okay, that makes sense. However in reality you don't need to split
that piece out. All you really are doing is splitting up the
first_init_pfn value over multiple threads so you just need to make
use of deferred_init_mem_pfn_range_in_zone() to initialize it.

> > > generalized to find it in the thread function based on the start/end range, or
> > > it could be maintained as part of the range that padata passes to the thread
> > > function.
> >
> > You may be better off just implementing your threads to operate like
> > deferred_grow_zone does. All your worker thread really needs then is
> > to know where to start performing the page initialization and then it
> > could go through and process an entire section worth of pages. The
> > other bit that would have to be changed is patch 6 so that you combine
> > any ranges that might span a single section instead of just splitting
> > the work up based on the ranges.
>
> How are you thinking of combining them?  I don't see a way to do it without
> storing an arbitrary number of ranges somewhere for each thread.

So when you are putting together your data you are storing a starting
value and a length. All you end up having to do is make certain that
the size + start pfn is section aligned. Then if you jump to a new
section you have the option of either adding to the size of your
current section or submitting the range and starting with a new start
pfn in a new section. All you are really doing is breaking up the
first_deferred_pfn over multiple sections. What I would do is section
align end_pfn, and then check the next range from the zone. If the
start_pfn of the next range is less than end_pfn you merge the two
ranges by just increasing the size, otherwise you could start a new
range.

The idea is that you just want to define what the valid range of PFNs
are, and if there are sizable holes you skip over them. You would
leave most of the lifting for identifying exactly what PFNs to
initialize to the pfn_range_in_zone iterators since they would all be
read-only accesses anyway.

> > If you are referring to the mo_pfn you shouldn't even need to think
> > about it.
>
> (clarified "opaque index" above)

Thanks.

> > All it is doing is guaranteeing you are processing at least
> > a full max order worth of pages. Without that the logic before was
> > either process a whole section, or just process all of memory
> > initializing it before it started freeing it. I found it made things
> > much more efficient to process only up to MAX_ORDER at a time as you
> > could squeeze that into the L2 cache for most x86 processors at least
> > and it reduced the memory bandwidth by quite a bit.
>
> Yes, that was clever, we should keep doing it that way.

Thanks.