[PATCH v2 02/28] vmscan: take at least one pass with shrinkers

Joonsoo Kim iamjoonsoo.kim at lge.com
Mon Apr 8 09:01:31 UTC 2013

On Mon, Apr 08, 2013 at 12:47:14PM +0400, Glauber Costa wrote:
> On 04/08/2013 12:42 PM, Joonsoo Kim wrote:
> > Hello, Glauber.
> > 
> > On Fri, Mar 29, 2013 at 01:13:44PM +0400, Glauber Costa wrote:
> >> In very low free kernel memory situations, it may be the case that we
> >> have less objects to free than our initial batch size. If this is the
> >> case, it is better to shrink those, and open space for the new workload
> >> then to keep them and fail the new allocations.
> >>
> >> More specifically, this happens because we encode this in a loop with
> >> the condition: "while (total_scan >= batch_size)". So if we are in such
> >> a case, we'll not even enter the loop.
> >>
> >> This patch modifies turns it into a do () while {} loop, that will
> >> guarantee that we scan it at least once, while keeping the behaviour
> >> exactly the same for the cases in which total_scan > batch_size.
> > 
> > Current user of shrinker not only use their own condition, but also
> > use batch_size and seeks to throttle their behavior. So IMHO,
> > this behavior change is very dangerous to some users.
> > 
> > For example, think lowmemorykiller.
> > With this patch, he always kill some process whenever shrink_slab() is
> > called and their low memory condition is satisfied.
> > Before this, total_scan also prevent us to go into lowmemorykiller, so
> > killing innocent process is limited as much as possible.
> > 
> shrinking is part of the normal operation of the Linux kernel and
> happens all the time. Not only the call to shrink_slab, but actual
> shrinking of unused objects.
> I don't know therefore about any code that would kill process only
> because they have reached shrink_slab.
> In normal systems, this loop will be executed many, many times. So we're
> not shrinking *more*, we're just guaranteeing that at least one pass
> will be made.

This one pass guarantee is a problem for lowmemory killer.

> Also, anyone looking at this to see if we should kill processes, is a
> lot more likely to kill something if we tried to shrink but didn't, than
> if we successfully shrunk something.

lowmemory killer is hacky user of shrink_slab interface. It kill a process
if system memory goes under some level. It check that condition every time
it is called. Without this patch, it cannot check that condition
if total_scan < batch_size. But with this patch, it check that condition
more frequently, so there will be side-effect.


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