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

[Huang, Ying]

Uladzislau Rezki <urezki@xxxxxxxxx> writes:

> On Thu, Nov 19, 2020 at 09:40:29AM +0800, Huang, Ying wrote:
>> 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?
>> 
> Hmm.. looking at it one more time i do not see what batching solves.

Without batch protection, we may release the lock and CPU anytime during
looping if "vmap_lazy_nr < resched_threshold".  Too many vmalloc/vfree
may be done during that.  So I think we can restrict it.  Batching can
improve the performance of purging itself too.

> Anyway we need to have some threshold(what we do have), that regulates
> a priority between vmalloc()/vfree().
>
> What we can do more with it are:
>
> - purging should be just performed asynchronously in workqueue context.
> Giving the fact, that now we also do a merge of outstanding areas, the
> data structure(rb-tree) will not be so fragmented.

Async works only if there are idle CPU time on other CPUs.  And it may
punish other innocent workloads instead of the heavy vmalloc/vfree
users.  So we should be careful about that.

> - lazy_max_pages() can slightly be decreased. If there are existing
> workloads which suffer from such long value. It would be good to get
> real complains and evidence.
>
>> > 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?
>> 
> If we set lazy_max_pages() to vague value such as 100, the performance
> will be just destroyed.

Sorry, my original words weren't clear enough.  What I really want to
suggest is to control the length of the purging list instead of reduce
lazy_max_pages() directly.  That is, we can have a "atomic_t
nr_purge_item" to record the length of the purging list and start
purging if (vmap_lazy_nr > lazy_max_pages && nr_purge_item >
max_purge_item).  vmap_lazy_nr is to control the virtual address space,
nr_purge_item is to control the batching purging latency.  "100" is just
an example, the real value should be determined according to the test
results.

Best Regards,
Huang, Ying