On Mon, Mar 16, 2015 at 09:49:56AM -0400, Eric B Munson wrote: > 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. > For kernel development, yes. But this change affects users. Cc'ing the linux-rt-users mailing list (which I did) is appropriate in this case. -- Steve -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . 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