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    SubjectRe: [PATCH 2/2] sysvshm: SHM_LOCK use lru_add_drain_all_async()
    2012/1/4 Hugh Dickins <>:
    > 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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