`On Mon, Dec 16, 2019 at 01:25:18PM -0500, Johannes Weiner wrote:> On Fri, Dec 13, 2019 at 08:40:31PM +0000, Roman Gushchin wrote:> > On Fri, Dec 13, 2019 at 02:21:56PM -0500, Johannes Weiner wrote:> > > When memory.low is overcommitted - i.e. the children claim more> > > protection than their shared ancestor grants them - the allowance is> > > distributed in proportion to each siblings's utilized protection:> > > > > > 	low_usage = min(low, usage)> > > 	elow = parent_elow * (low_usage / siblings_low_usage)> > > > > > However, siblings_low_usage is not the sum of all low_usages. It sums> > > up the usages of *only those cgroups that are within their memory.low*> > > That means that low_usage can be *bigger* than siblings_low_usage, and> > > consequently the total protection afforded to the children can be> > > bigger than what the ancestor grants the subtree.> > > > > > Consider three groups where two are in excess of their protection:> > > > > >   A/memory.low = 10G> > >   A/A1/memory.low = 10G, A/memory.current = 20G> > >   A/A2/memory.low = 10G, B/memory.current = 20G> > >   A/A3/memory.low = 10G, C/memory.current =  8G> > > > > >   siblings_low_usage = 8G (only A3 contributes)> > >   A1/elow = parent_elow(10G) * low_usage(20G) / siblings_low_usage(8G) = 25G> > > > > > The 25G are then capped to A1's own memory.low setting, i.e. 10G. The> > > same is true for A2. And A3 would also receive 10G. The combined> > > protection of A1, A2 and A3 is 30G, when A limits the tree to 10G.> > > > > > What does this mean in practice? A1 and A2 would still be in excess of> > > their 10G allowance and would be reclaimed, whereas A3 would not. As> > > they eventually drop below their protection setting, they would be> > > counted in siblings_low_usage again and the error would right itself.> > > > > > When reclaim is applied in a binary fashion - cgroup is reclaimed when> > > it's above its protection, otherwise it's skipped - this could work> > > actually work out just fine - although it's not quite clear to me why> > > we'd introduce this error in the first place.> > > > This complication is not simple an error, it protects cgroups under> > their low limits if there is unprotected memory.> > > > So, here is an example:> > > >       A      A/memory.low = 2G, A/memory.current = 4G> >      / \> >     B   C    B/memory.low = 3G  B/memory.current = 2G> >              C/memory.low = 1G  C/memory.current = 2G> > > > as now:> > > > B/elow = 2G * 2G / 2G = 2G == B/memory.current> > C/elow = 2G * 1G / 2G = 1G < C/memory.current> > > > with this fix:> > > > B/elow = 2G * 2G / 3G = 4/3 G < B/memory.current> > C/elow = 2G * 1G / 3G = 2/3 G < C/memory.current> > > > So in other words, currently B won't be scanned at all, because> > there is 1G of unprotected memory in C. With your patch both B and C> > will be scanned.> > Looking at the B and C numbers alone: C is bigger than what it claims> for protection and B is smaller than what it claims for protection.> > However, A doesn't provide 4G to its children. It provides 2G to be> distributed between the two. So how can B claim 3G and be exempted> from reclaim?First, what if the memory pressure comes from memory.high/max set on A?Second, it's up to semantics we define. Looking at it from the other side:there is clearly 1G of memory in C which is not protected no matter what.B wants it's memory to be fully protected, but it's limited by the competitionon the parent level. Now we try to satisfy B's requirements until we can.Should we treat B and C equally from scratch?I think both approaches is acceptable, but if we're switching from one optionto another, let's make it clear.> > But more importantly, it isn't in either case! The end result is the> same in both implementations. Because as soon as C is reclaimed down> to below 1G, A is still in excess of its memory.low (because it's> overcommitted!), and they both will be reclaimed proportionally.I do not disagree: the introduction of the proportional reclaimmade this complication (partially?) obsolete. But originally it wasrequired to make target distribution correct.> > From the example in the current code:> >  * For example, if there are memcgs A, A/B, A/C, A/D and A/E:>  *>  *     A      A/memory.low = 2G, A/memory.current = 6G>  *    //\\>  *   BC  DE   B/memory.low = 3G  B/memory.current = 2G>  *            C/memory.low = 1G  C/memory.current = 2G>  *            D/memory.low = 0   D/memory.current = 2G>  *            E/memory.low = 10G E/memory.current = 0>  *>  * and the memory pressure is applied, the following memory distribution>  * is expected (approximately):>  *>  *     A/memory.current = 2G>  *>  *     B/memory.current = 1.3G>  *     C/memory.current = 0.6G>  *     D/memory.current = 0>  *     E/memory.current = 0> > Even though B starts out within whatever it claims to be its> protection, A is overcommitted and so B and C converge on their> proportional share of the parent's allowance.> > So to go back to the example chosen above:> > >       A      A/memory.low = 2G, A/memory.current = 4G> >      / \> >     B   C    B/memory.low = 3G  B/memory.current = 2G> >              C/memory.low = 1G  C/memory.current = 2G> > With either implementation we'd expect the distribution to be about> 1.5G and 0.5G for B and C, respectively.> > And they'd have to be, too. Otherwise the semantics would be> completely unpredictable to anyone trying to configure this.> > So I think mixing proportional distribution with absolute thresholds> like this makes the implementation unnecessarily hard to reason> about. It's also clearly buggy as pointed out in the changelog.> > > > However, since> > > 1bc63fb1272b ("mm, memcg: make scan aggression always exclude> > > protection"), reclaim pressure is scaled to how much a cgroup is above> > > its protection. As a result this calculation error unduly skews> > > pressure away from A1 and A2 toward the rest of the system.> > > > It could be that with 1bc63fb1272b the target memory distribution> > will be fine. However the patch will change the memory pressure in B and C> > (in the example above). Maybe it's ok, but at least it should be discussed> > and documented.> > I'll try to improve the changelog based on this, thanks for filling in> the original motivation. But I do think it's a change we want to make.Absolutely, I'm not against the change. I just want to make sure we willput all details into the changelog.Thanks!`