Re: [PATCH 2/2] mm/vmalloc: rework the drain logic

[Uladzislau Rezki]

On Wed, Nov 18, 2020 at 10:44:13AM +0800, huang ying wrote:
> On Tue, Nov 17, 2020 at 9:04 PM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
> >
> > On Tue, Nov 17, 2020 at 10:37:34AM +0800, huang ying wrote:
> > > On Tue, Nov 17, 2020 at 6:00 AM Uladzislau Rezki (Sony)
> > > <urezki@xxxxxxxxx> wrote:
> > > >
> > > > A current "lazy drain" model suffers from at least two issues.
> > > >
> > > > First one is related to the unsorted list of vmap areas, thus
> > > > in order to identify the [min:max] range of areas to be drained,
> > > > it requires a full list scan. What is a time consuming if the
> > > > list is too long.
> > > >
> > > > Second one and as a next step is about merging all fragments
> > > > with a free space. What is also a time consuming because it
> > > > has to iterate over entire list which holds outstanding lazy
> > > > areas.
> > > >
> > > > See below the "preemptirqsoff" tracer that illustrates a high
> > > > latency. It is ~24 676us. Our workloads like audio and video
> > > > are effected by such long latency:
> > >
> > > This seems like a real problem.  But I found there's long latency
> > > avoidance mechanism in the loop in __purge_vmap_area_lazy() as
> > > follows,
> > >
> > >         if (atomic_long_read(&vmap_lazy_nr) < resched_threshold)
> > >             cond_resched_lock(&free_vmap_area_lock);
> > >
> > I have added that "resched threshold" because of on my tests i could
> > simply hit out of memory, due to the fact that a drain work is not up
> > to speed to process such long outstanding list of vmap areas.
> 
> OK.  Now I think I understand the problem.  For free area purging,
> there are multiple "producers" but one "consumer", and it lacks enough
> mechanism to slow down the "producers" if "consumer" can not catch up.
> And your patch tries to resolve the problem via accelerating the
> "consumer".
>
Seems, correct. But just in case one more time:

the cond_resched_lock was added once upon a time to get rid of long
preemption off time. Due to dropping the lock, "producers" can start
generate further vmap area, so "consumer" can not catch up. Seems

Later on, a resched threshold was added. It is just a simple protection
threshold, passing which, a freeing is prioritized back over allocation,
so we guarantee that we do not hit out of memory.

>
> That isn't perfect, but I think we may have quite some opportunities
> to merge the free areas, so it should just work.
> 
Yes, merging opportunity should do the work. But of course there are
exceptions.

> And I found the long latency avoidance logic in
> __purge_vmap_area_lazy() appears problematic,
> 
>          if (atomic_long_read(&vmap_lazy_nr) < resched_threshold)
>              cond_resched_lock(&free_vmap_area_lock);
> 
> Shouldn't it be something as follows?
> 
>          if (i >= BATCH && atomic_long_read(&vmap_lazy_nr) <
> resched_threshold) {
>              cond_resched_lock(&free_vmap_area_lock);
>              i = 0;
>          } else
>              i++;
> 
> This will accelerate the purging via batching and slow down vmalloc()
> via holding free_vmap_area_lock.  If it makes sense, can we try this?
> 
Probably we can switch to just using "batch" methodology:

<snip>
    if (!(i++ % batch_threshold))
        cond_resched_lock(&free_vmap_area_lock);
<snip>

The question is, which value we should use as a batch_threshold: 100, 1000, etc.

Apart of it and in regard to CONFIG_KASAN_VMALLOC, it seems that we are not
allowed to drop the free_vmap_area_lock at all. Because any simultaneous
allocations are not allowed within a drain region, so it should occur in
disjoint regions. But i need to double check it.

>
> And, can we reduce lazy_max_pages() to control the length of the
> purging list?  It could be > 8K if the vmalloc/vfree size is small.
>
We can adjust it for sure. But it will influence on number of global
TLB flushes that must be performed.

Thanks.

--
Vlad Rezki