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

[Huang, Ying]

Uladzislau Rezki <urezki@xxxxxxxxx> writes:

> 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

Yes.  And in theory there are vfree() storm, that is, thousands vfree()
can be called in short time.  But I don't think that's practical use
case.

> 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.

Yes.  That can accelerate freeing if necessary.

>>
>> 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>

That's the typical long latency avoidance method.

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

I think we can do some measurement to determine it?

> 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.

Em...  For example, if we set it to 100, then the number of the TLB
flushes can be reduced to 1% of the un-optimized implementation
already.  Do you think so?

Best Regards,
Huang, Ying