From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
"Minchan Kim" <minchan@kernel.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Hugh Dickins" <hughd@google.com>,
"Johannes Weiner" <hannes@cmpxchg.org>,
"Tim Chen" <tim.c.chen@linux.intel.com>,
"Shaohua Li" <shli@fb.com>,
"Mel Gorman" <mgorman@techsingularity.net>,
"J�r�me Glisse" <jglisse@redhat.com>,
"Michal Hocko" <mhocko@suse.com>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"David Rientjes" <rientjes@google.com>,
"Rik van Riel" <riel@redhat.com>, "Jan Kara" <jack@suse.cz>,
"Dave Jiang" <dave.jiang@intel.com>,
"Aaron Lu" <aaron.lu@intel.com>
Subject: Re: [PATCH -mm] mm, swap: Fix race between swapoff and some swap operations
Date: Tue, 12 Dec 2017 19:27:25 -0800 [thread overview]
Message-ID: <20171213032725.GJ7829@linux.vnet.ibm.com> (raw)
In-Reply-To: <87indbnzga.fsf@yhuang-dev.intel.com>
On Wed, Dec 13, 2017 at 10:17:41AM +0800, Huang, Ying wrote:
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
>
> > On Tue, Dec 12, 2017 at 09:12:20AM +0800, Huang, Ying wrote:
> >> Hi, Pual,
> >>
> >> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> writes:
> >>
> >> > On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote:
> >> >> Andrew Morton <akpm@linux-foundation.org> writes:
> >> >>
> >> >> > On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote:
> >> >> >
> >> >> >> > Why do we need srcu here? Is it enough with rcu like below?
> >> >> >> >
> >> >> >> > It might have a bug/room to be optimized about performance/naming.
> >> >> >> > I just wanted to show my intention.
> >> >> >>
> >> >> >> Yes. rcu should work too. But if we use rcu, it may need to be called
> >> >> >> several times to make sure the swap device under us doesn't go away, for
> >> >> >> example, when checking si->max in __swp_swapcount() and
> >> >> >> add_swap_count_continuation(). And I found we need rcu to protect swap
> >> >> >> cache radix tree array too. So I think it may be better to use one
> >> >> >> calling to srcu_read_lock/unlock() instead of multiple callings to
> >> >> >> rcu_read_lock/unlock().
> >> >> >
> >> >> > Or use stop_machine() ;) It's very crude but it sure is simple. Does
> >> >> > anyone have a swapoff-intensive workload?
> >> >>
> >> >> Sorry, I don't know how to solve the problem with stop_machine().
> >> >>
> >> >> The problem we try to resolved is that, we have a swap entry, but that
> >> >> swap entry can become invalid because of swappoff between we check it
> >> >> and we use it. So we need to prevent swapoff to be run between checking
> >> >> and using.
> >> >>
> >> >> I don't know how to use stop_machine() in swapoff to wait for all users
> >> >> of swap entry to finish. Anyone can help me on this?
> >> >
> >> > You can think of stop_machine() as being sort of like a reader-writer
> >> > lock. The readers can be any section of code with preemption disabled,
> >> > and the writer is the function passed to stop_machine().
> >> >
> >> > Users running real-time applications on Linux don't tend to like
> >> > stop_machine() much, but perhaps it is nevertheless the right tool
> >> > for this particular job.
> >>
> >> Thanks a lot for explanation! Now I understand this.
> >>
> >> Another question, for this specific problem, I think both stop_machine()
> >> based solution and rcu_read_lock/unlock() + synchronize_rcu() based
> >> solution work. If so, what is the difference between them? I guess rcu
> >> based solution will be a little better for real-time applications? So
> >> what is the advantage of stop_machine() based solution?
> >
> > The stop_machine() solution places similar restrictions on readers as
> > does rcu_read_lock/unlock() + synchronize_rcu(), if that is what you
> > are asking.
> >
> > More precisely, the stop_machine() solution places exactly the
> > same restrictions on readers as does preempt_disable/enable() and
> > synchronize_sched().
> >
> > I would expect stop_machine() to be faster than either synchronize_rcu()
> > synchronize_sched(), or synchronize_srcu(), but stop_machine() operates
> > by making each CPU spin with interrupts until all the other CPUs arrive.
> > This normally does not make real-time people happy.
> >
> > An compromise position is available in the form of
> > synchronize_rcu_expedited() and synchronize_sched_expedited(). These
> > are faster than their non-expedited counterparts, and only momentarily
> > disturb each CPU, rather than spinning with interrupts disabled. However,
> > stop_machine() is probably a bit faster.
> >
> > Finally, syncrhonize_srcu_expedited() is reasonably fast, but
> > avoids disturbing other CPUs. Last I checked, not quite as fast as
> > synchronize_rcu_expedited() and synchronize_sched_expedited(), though.
> >
> > You asked! ;-)
>
> Thanks a lot Paul! That exceeds my expectation!
>
> The performance of swapoff() isn't very important, probably it's not
> necessary to accelerate it at the cost of realtime. I think it is
> better to use a rcu or srcu based solution. I think the cost at reader
> side should be almost same between rcu and srcu? To use srcu, we need
> to select CONFIG_SRCU when CONFIG_SWAP is enabled in Kconfig. I think
> that should be OK?
The thing to do is to try SRCU and see if you can see significant
performance degradation. Given that there is swapping involved, I
would be surprised if the added read-side overhead of SRCU was even
measurable, but then again I have been surprised before.
And yes, just select CONFIG_SRCU when you need it.
Thanx, Paul
prev parent reply other threads:[~2017-12-13 3:27 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-07 1:14 [PATCH -mm] mm, swap: Fix race between swapoff and some swap operations Huang, Ying
2017-12-08 0:29 ` Andrew Morton
2017-12-08 1:43 ` Minchan Kim
[not found] ` <87po7pg4jt.fsf@yhuang-dev.intel.com>
2017-12-08 8:26 ` Minchan Kim
2017-12-08 8:41 ` Huang, Ying
2017-12-08 9:10 ` Minchan Kim
2017-12-08 12:32 ` Huang, Ying
2017-12-13 7:15 ` Minchan Kim
2017-12-13 8:52 ` Huang, Ying
2017-12-08 22:09 ` Andrew Morton
2017-12-11 5:30 ` Huang, Ying
2017-12-11 17:04 ` Paul E. McKenney
2017-12-12 1:12 ` Huang, Ying
2017-12-12 17:11 ` Paul E. McKenney
2017-12-13 2:17 ` Huang, Ying
2017-12-13 3:27 ` Paul E. McKenney [this message]
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