Re: [PATCH v2 4/5] mm, kswapd: replace kswapd compaction with waking up kcompactd

[Vlastimil Babka]

On 03/02/2016 02:57 PM, Joonsoo Kim wrote:
> 2016-03-02 19:04 GMT+09:00 Vlastimil Babka <vbabka@xxxxxxx>:
> > On 03/02/2016 07:33 AM, Joonsoo Kim wrote:
> > > Why you did the test with THP? THP interferes result of main test so
> > > it would be better not to enable it.
> >
> >
> > Hmm I've always left it enabled. It makes for a more realistic interference
> > and would also show unintended regressions in that closely related area.
>
> But, it makes review hard because complex analysis is needed to
> understand the result.
>
> Following is the example.
>
> "The compaction stalls
> (direct compaction) in the interfering kernel builds (probably THP's) also
> decreased somewhat to kcompactd activity, yet THP alloc successes improved a
> bit."
>
> So, why do we need this comment to understand effect of this patch? If you did
> a test without THP, it would not be necessary.

I see. Next time I'll do a run with THP disabled.

> > > And, this patch increased compaction activity (10 times for migrate
> > > scanned)
> > > may be due to resetting skip block information.
> >
> >
> > Note that kswapd compaction activity was completely non-existent for reasons
> > outlined in the changelog.
> > > Isn't is better to disable it
> > > for this patch to work as similar as possible that kswapd does and
> > > re-enable it
> > > on next patch? If something goes bad, it can simply be reverted.
> > >
> > > Look like it is even not mentioned in the description.
> >
> >
> > Yeah skip block information is discussed in the next patch, which mentions
> > that it's being reset and why. I think it makes more sense, as when kswapd
>
> Yes, I know.
> What I'd like to say here is that you need to care current_is_kswapd() in
> this patch. This patch unintentionally change the back ground compaction thread
> behaviour to restart compaction by every 64 trials because calling
> curret_is_kswapd()
> by kcompactd would return false and is treated as direct reclaim.

Oh, you mean this path to reset the skip bits. I see. But if skip bits 
are already reset by kswapd when waking kcompactd, then effect of 
another (rare) reset in kcompactd itself will be minimal?

> Result of patch 4
> and patch 5 would be same.

It's certainly possible to fold patch 5 into 4. I posted them separately 
mainly to make review more feasible. But the differences in results are 
already quite small.