Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: [resend][PATCH v2] mlock() doesn't wait to finish lru_add_drain_all() | Date | Tue, 13 Oct 2009 10:17:48 +0900 (JST) |
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Hi
> On Fri, 9 Oct 2009 11:21:55 +0900 (JST) > KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> 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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