Re: [PATCH] mm/memcontrol.c: drains percpu charge caches in memory.reclaim

[Yosry Ahmed]

On Thu, Nov 10, 2022 at 11:35 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Thu, Nov 10, 2022 at 6:42 AM Michal Koutný <mkoutny@xxxxxxxx> wrote:
> >
> > Hello Jialin.
> >
> > On Thu, Nov 10, 2022 at 02:53:16PM +0800, Lu Jialin <lujialin4@xxxxxxxxxx> wrote:
> > > When user use memory.reclaim to reclaim memory, after drain percpu lru
> > > caches, drain percpu charge caches for given memcg stock in the hope
> > > of introducing more evictable pages.
> >
> > Do you have any data on materialization of this hope?
> >
> > IIUC, the stock is useful for batched accounting to page_counter but it
> > doesn't represent real pages. I.e. your change may reduce the
> > page_counter value but it would not release any pages. Or have I missed
> > a way how it helps with the reclaim?
>
> +1
>
> It looks like we just overcharge the memcg if the number of allocated
> pages are less than the charging batch size, so that upcoming
> allocations can go through a fast accounting path and consume from the
> precharged stock. I don't understand how draining this charge may help
> reclaim.
>
> OTOH, it will reduce the page counters, so if userspace is relying on
> memory.current to gauge how much reclaim they want to do, it will make
> it "appear" like the usage dropped. If userspace is using other
> signals (refaults, PSI, etc), then we would be more-or-less tricking
> it into thinking we reclaimed pages when we actually didn't. In that
> case we didn't really reclaim anything, we just dropped memory.current
> slightly, which wouldn't matter to the user in this case, as other
> signals won't change.

In fact, we wouldn't be tricking anyone because this will have no
effect on the return value of memory.reclaim. We would just be causing
a side effect of very slightly reducing memory.current. Not sure if
this really helps.

>
> The difference in perceived usage coming from draining the stock IIUC
> has an upper bound of 63 * PAGE_SIZE (< 256 KB with 4KB pages), I
> wonder if this is really significant anyway.
>
> >
> > Thanks,
> > Michal