Re: [PATCH 2/2] sysvshm: SHM_LOCK use lru_add_drain_all_async()

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

2012/1/4 Hugh Dickins <hughd@google.com>:
> 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.

hehe, I have big bad reputation about for bad remember capabilities from
my friends. I should have remembered what i implemented. ;-)



>> > (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().

When, I implement lru_add_drain_all_async(), I thought this idea. I don't
dislike both. But if we take vmstat_update() one, I think we need more tricks.
pcp draining in refresh_cpu_vm_stats() delays up to 3 seconds. Why?
round_jiffies_relative() don't silly round to HZ boundary. Instead of, it adds
a few unique offset per each cpus. thus, 3 seconds mean max 3000cpus
don't make zone_{lru_}lock contention. pagevec draining also need same
trick for rescue SGI UV. It might be too pessimistic concern. but
vmstat_update() shouldn't make obsevable lock contention.


> But it would still require your changes to ____pagevec_lru_add_fn(),
> if those turn out to help more than they hurt.

I agree.

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