[v3,0/3] Reduce TLB flushes under some specific conditions

  To Huang Ying,

I tried to apply migrc to swap. Fortunately I couldn't see any
regression but no performance improvement either. I thought it's
meaningless to make code bigger without observing any benefit. So I
won't include the part. Thoughts?

To Nadav Amit,

I tried to split this patch set to as many as possible for better
review. However, it was very hard to make each patch meaningfully and
stably work because it works very tightly coupled to one another. So I
ended in combining those patches to one again. Instead, I tried my best
to add sufficient comments in code. Any opinion would be appreciated.

Hi everyone,

While I'm working with CXL memory, I have been facing migraion overhead
esp. TLB shootdown on promotion or demotion between different tiers.
Yeah.. most TLB shootdowns on migration through hinting fault can be
avoided thanks to Huang Ying's work, commit 4d4b6d66db ("mm,unmap: avoid
flushing TLB in batch if PTE is inaccessible").

However, it's only for ones using hinting fault. I thought it'd be much
better if we have a general mechanism to reduce # of TLB flushes and
TLB misses, that we can apply to any type of migration. I tried it only
for tiering migration for now tho.

I'm suggesting a mechanism to reduce TLB flushes by keeping source and
destination of folios participated in the migrations until all TLB
flushes required are done, only if those folios are not mapped with
write permission PTE entries at all. I worked Based on v6.6-rc5.

Can you believe it? I saw the number of TLB full flush reduced about
80%, iTLB miss reduced about 50% and the performance improved a little
bit with the workload I tested with, XSBench. However, I believe that it
would help more with other ones or any real ones. It'd be appreciated to
let me know if I'm missing something.

	Byungchul

---

Changes from RFC v2:
	1. Remove additional occupation in struct page. To do that,
	   unioned with lru field for migrc's list and added a page
	   flag. I know page flag is a thing that we don't like to add
	   but no choice because migrc should distinguish folios under
	   migrc's control from others. Instead, I force migrc to be
	   used only on 64 bit system to mitigate you guys from getting
	   angry.

	2. Remove meaningless internal object allocator that I
	   introduced to minimize impact onto the system. However, a ton
	   of tests showed there was no difference.

	3. Stop migrc from working when the system is in high memory
	   pressure like about to perform direct reclaim. At the
	   condition where the swap mechanism is heavily used, I found
	   the system suffered from regression without this control.

	4. Exclude folios that pte_dirty() == true from migrc's interest
	   so that migrc can work simpler.

	5. Combine several patches that work tightly coupled to one.

	6. Add sufficient comments for better review.

	7. Manage migrc's request in per-node manner (from globally).

	8. Add TLB miss improvement in commit message.

	9. Test with more CPUs(4 -> 16) to see bigger improvement.

Changes from RFC:

	1. Fix a bug triggered when a destination folio at the previous
	   migration becomes a source folio at the next migration,
	   before the folio gets handled properly so that the folio can
	   play with another migration. There was inconsistency in the
	   folio's state. Fixed it.

	2. Split the patch set into more pieces so that the folks can
	   review better. (Feedbacked by Nadav Amit)

	3. Fix a wrong usage of barrier e.g. smp_mb__after_atomic().
	   (Feedbacked by Nadav Amit)

	4. Tried to add sufficient comments to explain the patch set
	   better. (Feedbacked by Nadav Amit)

Byungchul Park (3):
  mm/rmap: Recognize non-writable TLB entries during TLB batch flush
  mm: Defer TLB flush by keeping both src and dst folios at migration
  mm, migrc: Add a sysctl knob to enable/disable MIGRC mechanism

 arch/x86/include/asm/tlbflush.h |   9 +
 arch/x86/mm/tlb.c               |  98 ++++++++++
 include/linux/mm.h              |  25 +++
 include/linux/mm_types.h        |  49 +++++
 include/linux/mm_types_task.h   |   4 +-
 include/linux/mmzone.h          |   7 +
 include/linux/page-flags.h      |   7 +
 include/linux/sched.h           |   9 +
 include/trace/events/mmflags.h  |   9 +-
 init/Kconfig                    |  14 ++
 mm/internal.h                   |  14 ++
 mm/memory.c                     |  13 ++
 mm/migrate.c                    | 310 ++++++++++++++++++++++++++++++++
 mm/page_alloc.c                 |  29 ++-
 mm/rmap.c                       | 115 +++++++++++-
 15 files changed, 703 insertions(+), 9 deletions(-)
  

On 10/30/23 00:25, Byungchul Park wrote:
> I'm suggesting a mechanism to reduce TLB flushes by keeping source and
> destination of folios participated in the migrations until all TLB
> flushes required are done, only if those folios are not mapped with
> write permission PTE entries at all. I worked Based on v6.6-rc5.

There's a lot of common overhead here, on top of the complexity in general:

 * A new page flag
 * A new cpumask_t in task_struct
 * A new zone list
 * Extra (temporary) memory consumption

and the benefits are ... "performance improved a little bit" on one
workload.  That doesn't seem like a good overall tradeoff to me.

There will certainly be workloads that, before this patch, would have
little or no memory pressure and after this patch would need to do reclaim.

Also, looking with my arch/x86 hat on, there's really nothing
arch-specific here.  Please try to keep stuff out of arch/x86 unless
it's very much arch-specific.

The connection between the arch-generic TLB flushing and
__flush_tlb_local() seems quite tenuous.  __flush_tlb_local() is, to me,
quite deep in the implementation and there are quite a few ways that a
random TLB flush might not end up there.  In other words, I'm not saying
that this is broken, but it's not clear at all to me how it functions
reliably.
  

> On Oct 30, 2023, at 7:55 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> 
> !! External Email
> 
> On 10/30/23 00:25, Byungchul Park wrote:
>> I'm suggesting a mechanism to reduce TLB flushes by keeping source and
>> destination of folios participated in the migrations until all TLB
>> flushes required are done, only if those folios are not mapped with
>> write permission PTE entries at all. I worked Based on v6.6-rc5.
> 
> There's a lot of common overhead here, on top of the complexity in general:
> 
> * A new page flag
> * A new cpumask_t in task_struct
> * A new zone list
> * Extra (temporary) memory consumption
> 
> and the benefits are ... "performance improved a little bit" on one
> workload.  That doesn't seem like a good overall tradeoff to me.

I almost forgot that I did (and embarrassingly did not follow) a TLB
flush deferring mechanism mechanism before [*], which was relatively
generic. I did not look at the migration case, but it could have been
relatively easily added - I think.

Feel free to plagiarize if you find it suitable. Note that some of
the patch-set is not relevant (e.g., 20/20 has already been fixed,
3/20 was merged.)

[*] https://lore.kernel.org/linux-mm/20210131001132.3368247-1-namit@vmware.com/
  

On Mon, Oct 30, 2023 at 10:55:07AM -0700, Dave Hansen wrote:
> On 10/30/23 00:25, Byungchul Park wrote:
> > I'm suggesting a mechanism to reduce TLB flushes by keeping source and
> > destination of folios participated in the migrations until all TLB
> > flushes required are done, only if those folios are not mapped with
> > write permission PTE entries at all. I worked Based on v6.6-rc5.
> 
> There's a lot of common overhead here, on top of the complexity in general:
> 
>  * A new page flag
>  * A new cpumask_t in task_struct
>  * A new zone list
>  * Extra (temporary) memory consumption
> 
> and the benefits are ... "performance improved a little bit" on one
> workload.  That doesn't seem like a good overall tradeoff to me.
> 
> There will certainly be workloads that, before this patch, would have
> little or no memory pressure and after this patch would need to do reclaim.

'if (gain - cost) > 0 ?'" is a difficult problem. I think the followings
are already big benefit in general:

	1. big reduction of IPIs #
	2. big reduction of TLB flushes #
	3. big reduction of TLB misses #

Of course, I or we need to keep trying to see a better number in
end-to-end performance.

> Also, looking with my arch/x86 hat on, there's really nothing
> arch-specific here.  Please try to keep stuff out of arch/x86 unless
> it's very much arch-specific.

Okay. I will try to keep it out of arch code. I should give up an
optimization that can be achieved by working on arch code tho.

	Byungchul
  

On Mon, Oct 30, 2023 at 10:55:07AM -0700, Dave Hansen wrote:
> On 10/30/23 00:25, Byungchul Park wrote:
> > I'm suggesting a mechanism to reduce TLB flushes by keeping source and
> > destination of folios participated in the migrations until all TLB
> > flushes required are done, only if those folios are not mapped with
> > write permission PTE entries at all. I worked Based on v6.6-rc5.
> 
> There's a lot of common overhead here, on top of the complexity in general:
> 
>  * A new page flag
>  * A new cpumask_t in task_struct
>  * A new zone list
>  * Extra (temporary) memory consumption
> 
> and the benefits are ... "performance improved a little bit" on one
> workload.  That doesn't seem like a good overall tradeoff to me.

I tested it under limited conditions to get stable results e.g. not to
use hyper-thread, dedicate cpu times to the test and so on. However, I'm
conviced that this patch set is more worth developing than you think it
is. Lemme share the results I've just got after changing # of CPUs
participated in the test, 16 -> 80, in the system with 80 CPUs. This is
just for your information - not that stable tho.

	Byungchul

---

Architecture - x86_64                                               
QEMU - kvm enabled, host cpu                                        
Numa - 2 nodes (80 CPUs 1GB, no CPUs 8GB)                           
Linux Kernel - v6.6-rc5, numa balancing tiering on, demotion enabled
Benchmark - XSBench -p 50000000 (-p option makes the runtime longer)

mainline kernel
===============

   The 1st try)
   =====================================
   Threads:     64                      
   Runtime:     233.118 seconds         
   =====================================
   numa_pages_migrated 758334           
   pgmigrate_success 1724964            
   nr_tlb_remote_flush 305706           
   nr_tlb_remote_flush_received 18598543
   nr_tlb_local_flush_all 19092         
   nr_tlb_local_flush_one 4518717       
   
   The 2nd try)
   =====================================
   Threads:     64                      
   Runtime:     221.725 seconds         
   =====================================
   numa_pages_migrated 633209           
   pgmigrate_success 2156509            
   nr_tlb_remote_flush 261977           
   nr_tlb_remote_flush_received 14289256
   nr_tlb_local_flush_all 11738         
   nr_tlb_local_flush_one 4520317       
   
mainline kernel + migrc
=======================

   The 1st try)
   =====================================
   Threads:     64                     
   Runtime:     212.522 seconds        
   ====================================
   numa_pages_migrated 901264          
   pgmigrate_success 1990814           
   nr_tlb_remote_flush 151280          
   nr_tlb_remote_flush_received 9031376
   nr_tlb_local_flush_all 21208        
   nr_tlb_local_flush_one 4519595      
   
   The 2nd try)
   =====================================
   Threads:     64                     
   Runtime:     204.410 seconds        
   ====================================
   numa_pages_migrated 929260          
   pgmigrate_success 2729868           
   nr_tlb_remote_flush 166722          
   nr_tlb_remote_flush_received 8238273
   nr_tlb_local_flush_all 13717        
   nr_tlb_local_flush_one 4519582
  

On 30.10.23 23:55, Byungchul Park wrote:
> On Mon, Oct 30, 2023 at 10:55:07AM -0700, Dave Hansen wrote:
>> On 10/30/23 00:25, Byungchul Park wrote:
>>> I'm suggesting a mechanism to reduce TLB flushes by keeping source and
>>> destination of folios participated in the migrations until all TLB
>>> flushes required are done, only if those folios are not mapped with
>>> write permission PTE entries at all. I worked Based on v6.6-rc5.
>>
>> There's a lot of common overhead here, on top of the complexity in general:
>>
>>   * A new page flag
>>   * A new cpumask_t in task_struct
>>   * A new zone list
>>   * Extra (temporary) memory consumption
>>
>> and the benefits are ... "performance improved a little bit" on one
>> workload.  That doesn't seem like a good overall tradeoff to me.
>>
>> There will certainly be workloads that, before this patch, would have
>> little or no memory pressure and after this patch would need to do reclaim.
> 
> 'if (gain - cost) > 0 ?'" is a difficult problem. I think the followings
> are already big benefit in general:
> 
> 	1. big reduction of IPIs #
> 	2. big reduction of TLB flushes #
> 	3. big reduction of TLB misses #
> 
> Of course, I or we need to keep trying to see a better number in
> end-to-end performance.

You'll have to show convincing, real numbers, for use cases people care 
about, to even motivate why people should consider looking at this in 
more detail.

If you can't measure it and only speculate, nobody cares.

The numbers you provided were so far not convincing, and it's 
questionable if the single benchmark you are presenting represents a 
reasonable real workload that ends up improving *real* workloads. A 
better description of the whole benchmark and why it represents a real 
workload behavior might help.