Re: [PATCH RFC tip/core/rcu] Add shrinker to shift to fast/inefficient GP mode

From: Konstantin Khlebnikov
Date: Fri May 08 2020 - 05:00:45 EST

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@xxxxxxxxxx> 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.


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