Re: [PATCH] mm: swap: async free swap slot cache entries

[Chris Li]

On Sun, Dec 24, 2023 at 1:13 PM David Rientjes <rientjes@xxxxxxxxxx> wrote:
>
> On Sun, 24 Dec 2023, Chris Li wrote:
>
> > On Sat, Dec 23, 2023 at 7:01 PM David Rientjes <rientjes@xxxxxxxxxx> wrote:
> > >
> > > On Sat, 23 Dec 2023, Chris Li wrote:
> > >
> > > > > How do you quantify the impact of the delayed swap_entry_free()?
> > > > >
> > > > > Since the free and memcg uncharge are now delayed, is there not the
> > > > > possibility that we stay under memory pressure for longer?  (Assuming at
> > > > > least some users are swapping because of memory pressure.)
> > > > >
> > > > > I would assume that since the free and uncharge itself is delayed that in
> > > > > the pathological case we'd actually be swapping *more* until the async
> > > > > worker can run.
> > > >
> > > > Thanks for raising this interesting question.
> > > >
> > > > First of all, the swap_entry_free() does not impact "memory.current".
> > > > It reduces "memory.swap.current". Technically it is the swap pressure
> > > > not memory pressure that suffers the extra delay.
> > > >
> > > > Secondly, we are talking about delaying up to 64 swap entries for a
> > > > few microseconds.
> > >
> > > What guarantees that the async freeing happens within a few microseconds?
> >
> > Linux kernel typically doesn't provide RT scheduling guarantees. You
> > can change microseconds to milliseconds, my following reasoning still
> > holds.
> >
>
> What guarantees that the async freeing happens even within 10s?  Your
> responses are implying that there is some deadline by which this freeing
> absolutely must happen (us or ms), but I don't know of any strong
> guarantees.

I think we are in agreement there, there are no such strong guarantees
in linux scheduling. However, when there are free CPU resources, the
job will get scheduled to execute in a reasonable table time frame. If
it does not, I consider that a bug if the CPU has idle resources and
the pending jobs are not able to run for a long time.
The existing code doesn't have such a guarantee either, see my point
follows. I don't know why you want to ask for such a guarantee.

> If there are no absolute guarantees about when the freeing may now occur,
> I'm asking how the impact of the delayed swap_entry_free() can be
> quantified.

Presumably each application has their own SLO metrics for monitoring
their application behavior. I am happy to take a look if any app has
new SLO violations caused by this change.
If you have one metric in mind, please  name it so we can look at it
together. During my current experiment and the chromebook benchmark, I
haven't noticed such ill effects show up in the other metrics drops in
a statistically significant manner. That is not the same as saying
such drops don't exist at all. Just I haven't noticed or the SLO
watching system hasn't caught it.

> The benefit to the current implementation is that there *are* strong
> guarantees about when the freeing will occur and cannot grow exponentially
> before the async worker can do the freeing.

I don't understand your point. Please help me. In the current code,
for the previous swapin fault that releases the swap slots into the
swap slot caches. Let's say the swap slot remains in the cache for X
seconds until Nth (N < 64) swapin page fault later, the cache is full
and finally all 64 swap slot caches are free. Are you suggesting there
is some kind of guarantee X is less than some fixed bound seconds?
What is that bound then? 10 second? 1 minutes?

BTW, there will be no exponential growth, that is guaranteed. Until
the 64 entries cache were freed. The swapin code will take the direct
free path for the current swap slot in hand. The direct free path
existed before my change.

Chris