Re: [PATCH] mm: throttle LRU pages skipping on rmap_lock contention

[Minchan Kim <minchan@kernel.org>]

Bump up.

On Thu, May 26, 2022 at 10:08:44AM -0700, Minchan Kim wrote:
> On Thu, May 12, 2022 at 12:55:16PM -0700, Minchan Kim wrote:
> > On Wed, May 11, 2022 at 07:05:23PM -0700, Andrew Morton wrote:
> > > On Wed, 11 May 2022 15:57:09 -0700 Minchan Kim <minchan@kernel.org> wrote:
> > > 
> > > > > 
> > > > > Could we burn much CPU time pointlessly churning though the LRU?  Could
> > > > > it mess up aging decisions enough to be performance-affecting in any
> > > > > workload?
> > > > 
> > > > Yes, correct. However, we are already churning LRUs by several
> > > > ways. For example, isolate and putback from LRU list for page
> > > > migration from several sources(typical example is compaction)
> > > > and trylock_page and sc->gfp_mask not allowing page to be
> > > > reclaimed in shrink_page_list.
> > > 
> > > Well.  "we're already doing a risky thing so it's OK to do more of that
> > > thing"?
> > 
> > I meant the aging is not rocket science.
> > 
> > 
> > > 
> > > > > 
> > > > > Something else?
> > > > 
> > > > One thing I am worry about was the granularity of the churning.
> > > > Example above was page granuarity churning so might be execuse
> > > > but this one is address space's churning, especically for file LRU
> > > > (i_mmap_rwsem) which might cause too many rotating and live-lock
> > > > in the end(keey rotating in small LRU with heavy memory pressure).
> > > > 
> > > > If it could be a problem, maybe we use sc->priority to stop
> > > > the skipping on a certain level of memory pressure.
> > > > 
> > > > Any thought? Do we really need it?
> > > 
> > > Are we able to think of a test which might demonstrate any worst case? 
> > > Whip that up and see what the numbers say?
> > 
> > Yeah, let me create a worst test case to see how it goes.
> > 
> > A thread keep reading a file-backed vma with 2xRAM file but other threads
> > keep changing other vmas mapped at the same file so heavy i_mmap_rwsem
> > contention in aging path.
> 
> Forking new thread
> 
> I checked what happens the worst case. I am not sure how the worst
> case is realistic but would be great to have safety net.
> 
> From 5ccc8b170af5496f803243732e96b131419d7462 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Thu, 19 May 2022 19:48:12 -0700
> Subject: [PATCH] mm: throttle LRU pages skipping on rmap_lock contention
> 
> On heavy contention on rmap_lock(e.g., i_mmap_rwsem), VM can keep
> skipping LRU pages so reclaim efficiency(steal/scanning) would drop
> from 48% to 27% and workingset would be reclaimed faster than old
> so workingset_refault rate increased to 240%.
> 
> We need a safe net to throttle the skipping LRU pages. This patch
> throttle the skipping policy using (DEF_PRIRORITY - 2) magic value
> VM has used for indicating non-light memory pressure.
> IOW, let's skip rmap_lock contendeded pages only when
> only when sc->priority >= (DEF_PRIRORITY - 2).
> 
> The test scenario to see the worst case:
> 
> 1. A thread mmap a big file(e.g., 2x times of RAM) and keep touching
>    the address space up to three times.
> 2. B thread keeps doing mmap/munmap with the same file to cause
>    heavy lock contention in i_mmap_rwsem until the A thread finish
>    the job.
> 3. measure vmstat and thread A's elapsed time.
> 
> Thread's elapsed time:
> 
> 1. vanilla
> 24.64sec(5.04%)
> 
> 2. rmap_skip(i.e., mm-dont-be-stuck-to-rmap-lock-on-reclaim-path.patch)
> 25.20sec(4.16%)
> 
> 3. priority(2 + this patch)
> 23.62sec(6.61%)
> 
> Vmstat Comparison:
> 				     vanilla    rmap_skip    priority
> 	     allocstall_movable          582         9772       14643
> 		     pgactivate          232        25865        4906
>       		   pgdeactivate           78        17265         651
>         	     pgmajfault           58        10639        1376
>     		 pgsteal_kswapd     15947857     15133195    15095445
>     		 pgsteal_direct       105439       583092      943195
>      	          pgscan_kswapd     24647536     52768898    28103170
>      		  pgscan_direct      8398139      3767100     7966353
> 	workingset_refault_file     12582926     12248353    12565934
> 
> B test scenario
> 
> 1. A thread mmap a big file(e.g., 2x times of RAM) and keep touching
>    the address space up to three times.
> 2. B thread keeps doing mmap/munmap with the same file to cause
>    heavy lock contention in i_mmap_rwsem until the A thread finish
>    the job.
> 3. C thread keep reading other big file using read(2) syscall
> 4. measure vmstat and thread A's elapsed time.
> 
> 1. vanilla
> 27.24sec(5.29%)
> 
> 2. rmap_skip
> 33.54sec(3.20%)
> 
> 3. priority
> 28.68sec(1.26%)
> 
> Vmstat Comparison:
> 				     vanilla    rmap_skip    priority
> 	     allocstall_movable        15262        81258       21644
>         	     pgactivate      3042004      3086906     3502959
>       		   pgdeactivate      2307849      8959162     3605768
>         	     pgmajfault          566         1059	  557
>     		 pgsteal_kswapd     17557735     30861283    18385674
>     		 pgsteal_direct       955389      6353527     1233605
>      		  pgscan_kswapd     31622695     59670433    35372575
> 		  pgscan_direct      4924052     13939254     4310247
> 	workingset_refault_file     13466538     32193161    14588019
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  include/linux/rmap.h | 5 +++--
>  mm/rmap.c            | 6 ++++--
>  mm/vmscan.c          | 6 ++++--
>  3 files changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 9ec23138e410..2893da3f1cd3 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -296,7 +296,8 @@ static inline int page_try_share_anon_rmap(struct page *page)
>   * Called from mm/vmscan.c to handle paging out
>   */
>  int folio_referenced(struct folio *, int is_locked,
> -			struct mem_cgroup *memcg, unsigned long *vm_flags);
> +			struct mem_cgroup *memcg, unsigned long *vm_flags,
> +			bool rmap_try_lock);
>  
>  void try_to_migrate(struct folio *folio, enum ttu_flags flags);
>  void try_to_unmap(struct folio *, enum ttu_flags flags);
> @@ -418,7 +419,7 @@ void page_unlock_anon_vma_read(struct anon_vma *anon_vma);
>  
>  static inline int folio_referenced(struct folio *folio, int is_locked,
>  				  struct mem_cgroup *memcg,
> -				  unsigned long *vm_flags)
> +				  unsigned long *vm_flags, bool rmap_try_lock)
>  {
>  	*vm_flags = 0;
>  	return 0;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d4cf3ea1b616..a75c7f7a0392 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -888,6 +888,7 @@ static bool invalid_folio_referenced_vma(struct vm_area_struct *vma, void *arg)
>   * @is_locked: Caller holds lock on the folio.
>   * @memcg: target memory cgroup
>   * @vm_flags: A combination of all the vma->vm_flags which referenced the folio.
> + * @rmap_try_lock: bail out if the rmap lock is contended
>   *
>   * Quick test_and_clear_referenced for all mappings of a folio,
>   *
> @@ -895,7 +896,8 @@ static bool invalid_folio_referenced_vma(struct vm_area_struct *vma, void *arg)
>   * the function bailed out due to rmap lock contention.
>   */
>  int folio_referenced(struct folio *folio, int is_locked,
> -		     struct mem_cgroup *memcg, unsigned long *vm_flags)
> +		     struct mem_cgroup *memcg, unsigned long *vm_flags,
> +		     bool rmap_try_lock)
>  {
>  	int we_locked = 0;
>  	struct folio_referenced_arg pra = {
> @@ -906,7 +908,7 @@ int folio_referenced(struct folio *folio, int is_locked,
>  		.rmap_one = folio_referenced_one,
>  		.arg = (void *)&pra,
>  		.anon_lock = folio_lock_anon_vma_read,
> -		.try_lock = true,
> +		.try_lock = rmap_try_lock,
>  	};
>  
>  	*vm_flags = 0;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ac168f4b0492..f0987e027aba 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1381,7 +1381,8 @@ static enum page_references folio_check_references(struct folio *folio,
>  	unsigned long vm_flags;
>  
>  	referenced_ptes = folio_referenced(folio, 1, sc->target_mem_cgroup,
> -					   &vm_flags);
> +					   &vm_flags,
> +					   sc->priority >= DEF_PRIORITY - 2);
>  	referenced_folio = folio_test_clear_referenced(folio);
>  
>  	/*
> @@ -2497,7 +2498,8 @@ static void shrink_active_list(unsigned long nr_to_scan,
>  
>  		/* Referenced or rmap lock contention: rotate */
>  		if (folio_referenced(folio, 0, sc->target_mem_cgroup,
> -				     &vm_flags) != 0) {
> +				     &vm_flags,
> +				     sc->priority >= DEF_PRIORITY - 2) != 0) {
>  			/*
>  			 * Identify referenced, file-backed active pages and
>  			 * give them one more trip around the active list. So
> -- 
> 2.36.1.124.g0e6072fb45-goog
>