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    SubjectRe: [resend][PATCH v2] mlock() doesn't wait to finish lru_add_drain_all()

    > On Fri, 9 Oct 2009 11:21:55 +0900 (JST)
    > KOSAKI Motohiro <> wrote:
    > > Recently, Mike Galbraith reported mlock() makes hang-up very long time in
    > > his system. Peter Zijlstra explainted the reason.
    > >
    > > Suppose you have 2 cpus, cpu1 is busy doing a SCHED_FIFO-99 while(1),
    > > cpu0 does mlock()->lru_add_drain_all(), which does
    > > schedule_on_each_cpu(), which then waits for all cpus to complete the
    > > work. Except that cpu1, which is busy with the RT task, will never run
    > > keventd until the RT load goes away.
    > >
    > > This is not so much an actual deadlock as a serious starvation case.
    > >
    > > His system has two partions using cpusets and RT-task partion cpu doesn't
    > > have any PCP cache. thus, this result was pretty unexpected.
    > >
    > > The fact is, mlock() doesn't need to wait to finish lru_add_drain_all().
    > > if mlock() can't turn on PG_mlock, vmscan turn it on later.
    > >
    > > Thus, this patch replace it with lru_add_drain_all_async().
    > So why don't we just remove the lru_add_drain_all() call from sys_mlock()?

    There are small reason. the administrators and the testers (include me)
    look at Mlock field in /proc/meminfo.
    They natually expect Mlock field match with actual number of mlocked pages
    if the system don't have any stress. Otherwise, we can't make mlock test case ;)

    > How did you work out why the lru_add_drain_all() is present in
    > sys_mlock() anyway? Neither the code nor the original changelog tell
    > us. Who do I thwap for that? Nick and his reviewers. Sigh.

    [Umm, My dictionaly don't tell me the meaning of "thwap". An meaning of
    an imitative word strongly depend on culture. Thus, I probably
    misunderstand this paragraph.]

    I've understand the existing reason by looooooong time review.

    > There are many callers of lru_add_drain_all() all over the place. Each
    > of those is vulnerable to the same starvation issue, is it not?

    There are.

    > If so, it would be better to just fix up lru_add_drain_all(). Afaict
    > all of its functions can be performed in hard IRQ context, so we can
    > use smp_call_function()?

    There is a option. but it have one downside, it require lru_add_pvecs
    related function call irq_disable().

    __lru_cache_add() is often called from page fault path. then we need
    performance mesurement.

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