Re: [PATCH v7 0/7] Introduce ZONE_CMA

[Michal Hocko]

On Mon 15-05-17 12:57:15, Joonsoo Kim wrote:
> On Fri, May 12, 2017 at 08:38:15AM +0200, Michal Hocko wrote:
[...]
> > I really do not want to question your "simple test" but page_zonenum is
> > used in many performance sensitive paths and proving it doesn't regress
> > would require testing many different workload. Are you going to do that?
> 
> In fact, I don't think that we need to take care about this
> performance problem seriously. The reasons are that:
> 
> 1. Currently, there is a usable bit in the page flags.
> 2. Even if others consume one usable bit, there still exists spare bit
> in 64b kernel. And, for 32b kernel, the number of the zone can be five
> if both, ZONE_CMA and ZONE_HIGHMEM, are used. And, using ZONE_HIGHMEM
> in 32b system is out of the trend.
> 3. Even if we fall into the latter category, I can optimize it not to
> regress if both the zone, ZONE_MOVABLE and ZONE_CMA, aren't used
> simultaneously with two zone bits in page flags. However, using both
> zones is not usual case.
> 4. This performance problem only affects CMA users and there is also a
> benefit due to removal of many hooks in MM subsystem so net result would
> not be worse.

A lot of fiddling for something that we can address in a different way,
really.

> So, I think that performance would be better in most of cases. It
> would be magianlly worse in rare cases and they could bear with it. Do
> you still think that using ZONE_MOVABLE for CMA memory is
> necessary rather than separate zone, ZONE_CMA?

yes, because the main point is that a new zone is not really needed
AFAICS. Just try to reuse what we already have (ZONE_MOVABLE). And more
over a new zone just pulls a lot of infrastructure which will be never
used.

> > > > But I feel we are looping without much progress. So let me NAK this
> > > > until it is _proven_ that the current code is unfixable nor ZONE_MOVABLE
> > > > can be reused
> > > 
> > > I want to open all the possibilty so could you check that ZONE_MOVABLE
> > > can be overlapped with other zones? IIRC, your rework doesn't allow
> > > it.
> > 
> > My rework keeps the status quo, which is based on the assumption that
> > zones cannot overlap. A longer term plan is that this restriction is
> > removed. As I've said earlier overlapping zones is an interesting
> > concept which is definitely worth pursuing.
> 
> Okay. We did a lot of discussion so it's better to summarise it.
> 
> 1. ZONE_CMA might be a nicer solution than MIGRATETYPE.
> 2. Additional bit in page flags would cause another kind of
> maintenance problem so it's better to avoid it as much as possible.
> 3. Abusing ZONE_MOVABLE looks better than introducing ZONE_CMA since
> it doesn't need additional bit in page flag.
> 4. (Not-yet-finished) If ZONE_CMA doesn't need extra bit in page
> flags with hacky magic and it has no performance regression,
> ??? (it's okay to use separate zone for CMA?)

As mentioned above. I do not see why we should go over additional hops
just to have a zone which is not strictly needed. So if there are no
inherent problems reusing MOVABLE/HIGMEM zone then a separate zone
sounds like a wrong direction.

But let me repeat. I am _not_ convinced that the migratetype situation
is all that bad and unfixable. You have mentioned some issues with the
current approach but none of them seem inherently unfixable. So I would
still prefer keeping the current way. But I am not going to insist if
you _really_ believe that the long term maintenance cost will be higher
than a zone approach and you can reuse MOVABLE/HIGHMEM zones without
disruptive changes. I can help you with the hotplug part of the MOVABLE
zone because that is desirable on its own.

-- 
Michal Hocko
SUSE Labs