Messages in this thread | | | Date | Tue, 3 Jan 2012 21:17:30 -0800 (PST) | From | Hugh Dickins <> | Subject | Re: [PATCH 2/2] sysvshm: SHM_LOCK use lru_add_drain_all_async() |
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On Tue, 3 Jan 2012, KOSAKI Motohiro wrote: > (1/3/12 8:51 PM), Hugh Dickins wrote: > > > > 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.
Why ashamed? The shmctl man-page documents "The caller must fault in any pages that are required to be present after locking is enabled." That's just how it behaves.
> > (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.
In case you got distracted before you found it, mm/vmstat.c's
static DEFINE_PER_CPU(struct delayed_work, vmstat_work); int sysctl_stat_interval __read_mostly = HZ;
static void vmstat_update(struct work_struct *w) { refresh_cpu_vm_stats(smp_processor_id()); schedule_delayed_work(&__get_cpu_var(vmstat_work), round_jiffies_relative(sysctl_stat_interval)); }
would be why, I think. And that implies to me that your lru_add_drain_all_async() is not necessary, you'd get just as good an effect, more cheaply, by doing a local lru_add_drain() before the refresh in vmstat_update().
But it would still require your changes to ____pagevec_lru_add_fn(), if those turn out to help more than they hurt.
Hugh
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