Messages in this thread | | | Date | Tue, 03 Jan 2012 21:19:01 -0500 | From | KOSAKI Motohiro <> | Subject | Re: [PATCH 2/2] sysvshm: SHM_LOCK use lru_add_drain_all_async() |
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(1/3/12 8:51 PM), Hugh Dickins wrote: > On Sun, 1 Jan 2012, kosaki.motohiro@gmail.com wrote: >> From: KOSAKI Motohiro<kosaki.motohiro@jp.fujitsu.com> >> >> shmctl also don't need synchrounous pagevec drain. This patch replace it with >> lru_add_drain_all_async(). >> >> Signed-off-by: KOSAKI Motohiro<kosaki.motohiro@jp.fujitsu.com> > > Let me answer this 2/2 first since it's easier. > > I'm going to thank you for bringing this lru_add_drain_all() > to my attention, I had not noticed it; but Nak the patch itself. > > The reason being, that particular lru_add_drain_all() serves no > useful purpose, so let's delete it instead of replacing it. I believe > that it serves no purpose for SHM_LOCK and no purpose for SHM_UNLOCK. > > I'm dabbling in this area myself, since you so cogently pointed out that > I'd tried to add a cond_resched() to scan_mapping_unevictable_pages() > (which is a helper for SHM_UNLOCK here) while it's under spinlock. > > In testing my fix for that, I find that there has been no attempt to > keep the Unevictable count accurate on SysVShm: SHM_LOCK pages get > marked unevictable lazily later as memory pressure discovers them - > which perhaps mirrors the way in which SHM_LOCK makes no attempt to > instantiate pages, unlike mlock.
Ugh, you are right. I'm recovering my remember gradually. Lee implemented immediate lru off logic at first and I killed it to close a race. I completely forgot. So, yes, now SHM_LOCK has no attempt to instantiate pages. I'm ashamed.
> > Since nobody has complained about that in the two years since we've > had an Unevictable count in /proc/meminfo, I don't see any need to > add code (it would need more than just your change here; would need > more even than calling scan_mapping_unevictable_pages() at SHM_LOCK > time - though perhaps along with your 1/2 that could handle it) and > overhead to satisfy a need that nobody has. > > I'll delete that lru_add_drain_all() in my patch, okay?
Sure thing. :)
> (But in writing this, realize I still don't quite understand why > the Unevictable count takes a second or two to get back to 0 after > SHM_UNLOCK: perhaps I've more to discover.)
Interesting. I'm looking at this too.
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