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Date:   Sat, 9 May 2020 09:09:00 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org,
        jiangshanlai@gmail.com, dipankar@in.ibm.com,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, joel@joelfernandes.org, viro@zeniv.linux.org.uk,
        Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH RFC tip/core/rcu] Add shrinker to shift to
 fast/inefficient GP mode
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On Sat, May 09, 2020 at 11:54:40AM +0300, Konstantin Khlebnikov wrote:
> On 08/05/2020 17.46, Paul E. McKenney wrote:
> > On Fri, May 08, 2020 at 12:00:28PM +0300, Konstantin Khlebnikov wrote:
> > > On 07/05/2020 22.09, Paul E. McKenney wrote:
> > > > On Thu, May 07, 2020 at 02:31:02PM -0400, Johannes Weiner wrote:
> > > > > On Thu, May 07, 2020 at 10:09:03AM -0700, Paul E. McKenney wrote:
> > > > > > On Thu, May 07, 2020 at 01:00:06PM -0400, Johannes Weiner wrote:
> > > > > > > On Wed, May 06, 2020 at 05:55:35PM -0700, Andrew Morton wrote:
> > > > > > > > On Wed, 6 May 2020 17:42:40 -0700 "Paul E. McKenney" <paulmck@kernel.org> wrote:
> > > > > > > > 
> > > > > > > > > This commit adds a shrinker so as to inform RCU when memory is scarce.
> > > > > > > > > RCU responds by shifting into the same fast and inefficient mode that is
> > > > > > > > > used in the presence of excessive numbers of RCU callbacks.  RCU remains
> > > > > > > > > in this state for one-tenth of a second, though this time window can be
> > > > > > > > > extended by another call to the shrinker.
> > > > > > > 
> > > > > > > We may be able to use shrinkers here, but merely being invoked does
> > > > > > > not carry a reliable distress signal.
> > > > > > > 
> > > > > > > Shrinkers get invoked whenever vmscan runs. It's a useful indicator
> > > > > > > for when to age an auxiliary LRU list - test references, clear and
> > > > > > > rotate or reclaim stale entries. The urgency, and what can and cannot
> > > > > > > be considered "stale", is encoded in the callback frequency and scan
> > > > > > > counts, and meant to be relative to the VM's own rate of aging: "I've
> > > > > > > tested X percent of mine for recent use, now you go and test the same
> > > > > > > share of your pool." It doesn't translate well to other
> > > > > > > interpretations of the callbacks, although people have tried.
> > > > > > 
> > > > > > Would it make sense for RCU to interpret two invocations within (say)
> > > > > > 100ms of each other as indicating urgency?  (Hey, I had to ask!)
> > > > > 
> > > > > It's the perfect number for one combination of CPU, storage device,
> > > > > and shrinker implementation :-)
> > > > 
> > > > Woo-hoo!!!
> > > > 
> > > > But is that one combination actually in use anywhere?  ;-)
> > > > 
> > > > > > > > > If it proves feasible, a later commit might add a function call directly
> > > > > > > > > indicating the end of the period of scarce memory.
> > > > > > > > 
> > > > > > > > (Cc David Chinner, who often has opinions on shrinkers ;))
> > > > > > > > 
> > > > > > > > It's a bit abusive of the intent of the slab shrinkers, but I don't
> > > > > > > > immediately see a problem with it.  Always returning 0 from
> > > > > > > > ->scan_objects might cause a problem in some situations(?).
> > > > > > > > 
> > > > > > > > Perhaps we should have a formal "system getting low on memory, please
> > > > > > > > do something" notification API.
> > > > > > > 
> > > > > > > It's tricky to find a useful definition of what low on memory
> > > > > > > means. In the past we've used sc->priority cutoffs, the vmpressure
> > > > > > > interface (reclaimed/scanned - reclaim efficiency cutoffs), oom
> > > > > > > notifiers (another reclaim efficiency cutoff). But none of these
> > > > > > > reliably capture "distress", and they vary highly between different
> > > > > > > hardware setups. It can be hard to trigger OOM itself on fast IO
> > > > > > > devices, even when the machine is way past useful (where useful is
> > > > > > > somewhat subjective to the user). Userspace OOM implementations that
> > > > > > > consider userspace health (also subjective) are getting more common.
> > > > > > > 
> > > > > > > > How significant is this?  How much memory can RCU consume?
> > > > > > > 
> > > > > > > I think if rcu can end up consuming a significant share of memory, one
> > > > > > > way that may work would be to do proper shrinker integration and track
> > > > > > > the age of its objects relative to the age of other allocations in the
> > > > > > > system. I.e. toss them all on a clock list with "new" bits and shrink
> > > > > > > them at VM velocity. If the shrinker sees objects with new bit set,
> > > > > > > clear and rotate. If it sees objects without them, we know rcu_heads
> > > > > > > outlive cache pages etc. and should probably cycle faster too.
> > > > > > 
> > > > > > It would be easy for RCU to pass back (or otherwise use) the age of the
> > > > > > current grace period, if that would help.
> > > > > > 
> > > > > > Tracking the age of individual callbacks is out of the question due to
> > > > > > memory overhead, but RCU could approximate this via statistical sampling.
> > > > > > Comparing this to grace-period durations could give information as to
> > > > > > whether making grace periods go faster would be helpful.
> > > > > 
> > > > > That makes sense.
> > > > > 
> > > > > So RCU knows the time and the VM knows the amount of memory. Either
> > > > > RCU needs to figure out its memory component to be able to translate
> > > > > shrinker input to age, or the VM needs to learn about time to be able
> > > > > to say: I'm currently scanning memory older than timestamp X.
> > > > > 
> > > > > The latter would also require sampling in the VM. Nose goes. :-)
> > > > 
> > > > Sounds about right.  ;-)
> > > > 
> > > > Does reclaim have any notion of having continuously scanned for
> > > > longer than some amount of time?  Or could RCU reasonably deduce this?
> > > > For example, if RCU noticed that reclaim had been scanning for longer than
> > > > (say) five grace periods, RCU might decide to speed things up.
> > > > 
> > > > But on the other hand, with slow disks, reclaim might go on for tens of
> > > > seconds even without much in the way of memory pressure, mightn't it?
> > > > 
> > > > I suppose that another indicator would be recent NULL returns from
> > > > allocators.  But that indicator flashes a bit later than one would like,
> > > > doesn't it?  And has false positives when allocators are invoked from
> > > > atomic contexts, no doubt.  And no doubt similar for sleeping more than
> > > > a certain length of time in an allocator.
> > > > 
> > > > > There actually is prior art for teaching reclaim about time:
> > > > > https://lore.kernel.org/linux-mm/20130430110214.22179.26139.stgit@zurg/
> > > > > 
> > > > > CCing Konstantin. I'm curious how widely this ended up being used and
> > > > > how reliably it worked.
> > > > 
> > > > Looking forward to hearing of any results!
> > > 
> > > Well, that was some experiment about automatic steering memory pressure
> > > between containers. LRU timings from milestones itself worked pretty well.
> > > Remaining engine were more robust than mainline cgroups these days.
> > > Memory becomes much cheaper - I hope nobody want's overcommit it that badly anymore.
> > > 
> > > It seems modern MM has plenty signals about memory pressure.
> > > Kswapsd should have enough knowledge to switch gears in RCU.
> > 
> > Easy for me to provide "start fast and inefficient mode" and "stop fast
> > and inefficient mode" APIs for MM to call!
> > 
> > How about rcu_mempressure_start() and rcu_mempressure_end()?  I would
> > expect them not to nest (as in if you need them to nest, please let
> > me know).  I would not expect these to be invoked all that often (as in
> > if you do need them to be fast and scalable, please let me know). >
> > RCU would then be in fast/inefficient mode if either MM told it to be
> > or if RCU had detected callback overload on at least one CPU.
> > 
> > Seem reasonable?
> 
> Not exactly nested calls, but kswapd threads are per numa node.
> So, at some level nodes under pressure must be counted.

Easy enough, especially given that RCU already "counts" CPUs having
excessive numbers of callbacks.  But assuming that the transitions to/from
OOM are rare, I would start by just counting them with a global counter.
If the counter is non-zero, RCU is in fast and inefficient mode.

> Also forcing rcu calls only for cpus in one numa node might be useful.

Interesting.  RCU currently evaluates a given CPU by comparing the
number of callbacks against a fixed cutoff that can be set at boot using
rcutree.qhimark, which defaults to 10,000.  When this cutoff is exceeded,
RCU becomes more aggressive about invoking callbacks on that CPU, for
example, by sacrificing some degree of real-time response.  I believe
that this heuristic would also serve the OOM use case well.

> I wonder if direct-reclaim should at some stage simply wait for RCU QS.
> I.e. call rcu_barrier() or similar somewhere before invoking OOM.

The rcu_oom_count() function in the patch starting this thread returns the
total number of outstanding callbacks queued on all CPUs.  So one approach
would be to invoke this function, and if the return value was truly
huge (taking size of memory and who knows that all else into account),
do the rcu_barrier() to wait for RCU to clear its current backlog.

On the NUMA point, it would be dead easy for me to supply a function
that returned the number of callbacks on a given CPU, which would allow
you to similarly evaluate a NUMA node, a cgroup, or whatever.

> All GFP_NOFAIL users should allow direct-reclaim, thus this loop
> in page_alloc shouldn't block RCU and doesn't need special care.

I must defer to you guys on this.  The main caution is the duration of
direct reclaim.  After all, if it is too long, the kfree_rcu() instance
would have been better of just invoking synchronize_rcu().

 							Thanx, Paul

> > > > > > But, yes, it would be better to have an elusive unambiguous indication
> > > > > > of distress.  ;-)
> > > > > 
> > > > > I agree. Preferably something more practical than a dialogue box
> > > > > asking the user on how well things are going for them :-)
> > > > 
> > > > Indeed, that dialog box should be especially useful for things like
> > > > light bulbs running Linux.  ;-)
> > > > 
> > > > 							Thanx, Paul
> > > >