On Mon, 16 Mar 2015, Vlastimil Babka wrote: > [CC += linux-api@] > > Since this is a kernel-user-space API change, please CC linux-api@. > The kernel source file Documentation/SubmitChecklist notes that all > Linux kernel patches that change userspace interfaces should be CCed > to linux-api@xxxxxxxxxxxxxxx, so that the various parties who are > interested in API changes are informed. For further information, see > https://urldefense.proofpoint.com/v2/url?u=https-3A__www.kernel.org_doc_man-2Dpages_linux-2Dapi-2Dml.html&d=AwIC-g&c=96ZbZZcaMF4w0F4jpN6LZg&r=aUmMDRRT0nx4IfILbQLv8xzE0wB9sQxTHI3QrQ2lkBU&m=GUotTNnv26L0HxtXrBgiHqu6kwW3ufx2_TQpXIA216c&s=IFFYQ7Zr-4SIaF3slOZqiSP_noyva42kCwVRxxDm5wo&e= Added to the Cc list, thanks. > > > On 03/13/2015 09:19 PM, Michal Hocko wrote: > >On Fri 13-03-15 15:09:15, Eric B Munson wrote: > >>On Fri, 13 Mar 2015, Rik van Riel wrote: > >> > >>>On 03/13/2015 01:26 PM, Eric B Munson wrote: > >>> > >>>>--- a/mm/compaction.c > >>>>+++ b/mm/compaction.c > >>>>@@ -1046,6 +1046,8 @@ typedef enum { > >>>> ISOLATE_SUCCESS, /* Pages isolated, migrate */ > >>>> } isolate_migrate_t; > >>>> > >>>>+int sysctl_compact_unevictable; > > A comment here would be useful I think, as well as explicit default > value. Maybe also __read_mostly although I don't know how much that > matters. I am going to sit on V6 for a couple of days incase anyone from rt wants to chime in. But these will be in V6. > > I also wonder if it might be confusing that "compact_memory" is a > write-only trigger that doesn't even show under "sysctl -a", while > "compact_unevictable" is a read/write setting. But I don't have a > better suggestion right now. Does allow_unevictable_compaction sound better? It feels too much like variable naming conventions from other languages which seems to encourage verbosity to me, but does indicate a difference from compact_memory. > > >>>>+ > >>>> /* > >>>> * Isolate all pages that can be migrated from the first suitable block, > >>>> * starting at the block pointed to by the migrate scanner pfn within > >>> > >>>I suspect that the use cases where users absolutely do not want > >>>unevictable pages migrated are special cases, and it may make > >>>sense to enable sysctl_compact_unevictable by default. > >> > >>Given that sysctl_compact_unevictable=0 is the way the kernel behaves > >>now and the push back against always enabling compaction on unevictable > >>pages, I left the default to be the behavior as it is today. > > > >The question is _why_ we have this behavior now. Is it intentional? > > It's there since 748446bb6 ("mm: compaction: memory compaction > core"). Commit c53919adc0 ("mm: vmscan: remove lumpy reclaim") > changes the comment in __isolate_lru_page() handling of unevictable > pages to mention compaction explicitly. It could have been > accidental in 748446bb6 though, maybe it just reused > __isolate_lru_page() for compaction - it seems that the skipping of > unevictable was initially meant to optimize lumpy reclaim. > > >e46a28790e59 (CMA: migrate mlocked pages) is a precedence in that > > Well, CMA and realtime kernels are probably mutually exclusive enough. > > >direction. Vlastimil has then changed that by edc2ca612496 (mm, > >compaction: move pageblock checks up from isolate_migratepages_range()). > >There is no mention about mlock pages so I guess it was more an > >unintentional side effect of the patch. At least that is my current > >understanding. I might be wrong here. > > Although that commit did change unintentionally more details that I > would have liked (unfortunately), I think you are wrong on this one. > ISOLATE_UNEVICTABLE is still passed from > isolate_migratepages_range() which is used by CMA, while the > compaction variant isolate_migratepages() does not pass it. So it's > kept CMA-specific as before. > > >The thing about RT is that it is not usable with the upstream kernel > >without the RT patchset AFAIU. So the default should be reflect what is > >better for the standard kernel. RT loads have to tune the system anyway > >so it is not so surprising they would disable this option as well. We > >should help those guys and do not require them to touch the code but the > >knob is reasonable IMHO. > > > >Especially when your changelog suggests that having this enabled by > >default is beneficial for the standard kernel. > > I agree, but if there's a danger of becoming too of a bikeshed > topic, I'm fine with keeping the default same as current behavior > and changing it later. Or maybe we should ask some -rt mailing list > instead of just Peter and Thomas? According to the rt wiki, there is no -rt development list so lkml is it. I will change the default to 1 for V6 if I don't hear otherwise by the time I get back around to spinning V6.
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