Re: [PATCH] CMA/HOTPLUG: clear buffer-head lru before page migration

[Gioh Kim]

2014-07-22 ìì 9:15, Minchan Kim ì ê:
> Hello Mel,
>
> On Mon, Jul 21, 2014 at 02:01:46PM +0100, Mel Gorman wrote:
> > On Mon, Jul 21, 2014 at 04:36:51PM +0900, Minchan Kim wrote:
> >
> > I'm not reviewing this in detail at all, didn't even look at the patch
> > but two things popped out at me during the discussion.
> >
> > > > > Anyway, why cannot CMA have the cost without affecting other subsystem?
> > > > > I mean it's okay for CMA to consume more time to shoot out the bh
> > > > > instead of simple all bh_lru invalidation because big order allocation is
> > > > > kinds of slow thing in the VM and everybody already know that and even
> > > > > sometime get failed so it's okay to add more code that extremly slow path.
> > > >
> > > > There are 2 reasons to invalidate entire bh_lru.
> > > >
> > > > 1. I think CMA allocation is very rare so that invalidaing bh_lru affects the system little.
> > > > How do you think about it? My platform does not call CMA allocation often.
> > > > Is the CMA allocation or Memory-Hotplug called often?
> > >
> > > It depends on usecase and you couldn't assume anyting because we couldn't
> > > ask every people in the world. "Please ask to us whenever you try to use CMA".
> > >
> > > The key point is how the patch is maintainable.
> > > If it's too complicate to maintain, maybe we could go with simple solution
> > > but if it's not too complicate, we can go with more smart thing to consider
> > > other cases in future. Why not?
> > >
> > > Another point is that how user can detect where the regression is from.
> > > If we cannot notice the regression, it's not a good idea to go with simple
> > > version.
> >
> > The buffer LRU avoids a lookup of a radix tree. If the LRU hit rate is
> > low then the performance penalty of repeated radix tree lookups is
> > severe but the cost of missing one hot lookup because CMA invalidate it
> > is not.
> >
> > The real cost to be concerned with is the cost of performing the
> > invalidation not the fact a lookup in the LRU was missed. It's because
> > the cost of invalidation is high that this is being pushed to CMA because
> > for CMA an allocation failure can be a functional failure and not just a
> > performance problem.
> >
> > > > 2. Adding code in drop_buffers() can affect the system more that adding code in alloc_contig_range()
> > > > because the drop_buffers does not have a way to distinguish migrate type.
> > > > Even-though the lmbech results that it has almost the same performance.
> > > > But I am afraid that it can be changed.
> > > > As you said if bh_lru size can be changed it affects more than now.
> > > > SO I do not want to touch non-CMA related code.
> > >
> > > I'm not saying to add hook in drop_buffers.
> > > What I suggest is to handle failure by bh_lrus in migrate_pages
> > > because it's not a problem only in CMA.
> >
> > No, please do not insert a global IPI to invalidate buffer heads in the
> > general migration case. It's too expensive for either THP allocations or
> > automatic NUMA migrates. The global IPI cost is justified for rare events
> > where it causes functional problems if it fails to migreate -- CMA, memory
> > hot-remove, memory poisoning etc.
>
> I didn't want to add that flushing in migrate_pages *unconditionlly*.
> Please, look at this patch. It fixes only CMA although it's an issue
> for others. Even, it depends on retry logic of upper layer of
> alloc_contig_range but even cma_alloc(ie, upper layer of alloc_contig_range)
> doesn't have retry logic. :(
> That's why I suggested it in migrate_pages.
>
> Actually, I'd like to go with making migrate_pages's user blind on pcp
> draining stuff by squeezing that inside migrate_pages.
> IOW, current users of migrate pages don't need to be aware of per-cpu
> draining. What they should know is just they should use MIGRATE_SYNC
> for best effort but costly opeartion.
>
> For implemenation, we could use retry logic in migrate_pages.
>
> int migrate_pages(xxx)
> {
>          for (pass = 0; pass < 10 && retry; pass++)
>                  if (retry && pass > 2 && mode == MIGRATE_SYNC)
>                          flush_all_of_percpu_stuff();
> }
>
> migrate_page has migrate_mode and retry logic with 'pass', even
> reason if we want ot filter out MR_CMA|MEMORY_HOTPLUG|MR_MEMORY_FAILURE.
> so that we could handle all of things inside migrate_pages.
>
> Normally, MIGRATE_SYNC would be expensive operation and mostly
> it is used for CMA, memory-hotplug, memory-poisoning so THP and
> automatic NUMA cannot affect so I believe adding IPI to that is not
> a big problem in such trouble condition(ie, retry && pass > 2).


I agree Minchan's point.
I am not sure it is ok to touch the common code such as migrate_pages().

If Mel agrees, I am going to report another patch of flush_all_of_percpu_stuff() like following:

flush_all_of_percpu_stuff()
{
	drop_only_bh_of_migrating_page();
	lru_add_drain_all();
	drain_all_pages();
}

And remove lru_add_drain_all() and drain_all_pages() in CMA/HOTPLUG codes.



> > --
> > Mel Gorman
> > SUSE Labs
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@xxxxxxxxxx  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/