Re: [mm/gup] 57efa1fe59: will-it-scale.per_thread_ops -9.2% regression

[John Hubbard <jhubbard@nvidia.com>]

On 6/4/21 12:52 AM, Feng Tang wrote:
...
>>> The perf data doesn't even mention any of the GUP paths, and on the
>>> pure fork path the biggest impact would be:
>>>
>>>   (a) maybe "struct mm_struct" changed in size or had a different cache layout
>>
>> Yes, this seems to be the cause of the regression.
>>
>> The test case is many thread are doing map/unmap at the same time,
>> so the process's rw_semaphore 'mmap_lock' is highly contended.
>>
>> Before the patch (with 0day's kconfig), the mmap_lock is separated
>> into 2 cachelines, the 'count' is in one line, and the other members
>> sit in the next line, so it luckily avoid some cache bouncing. After

Wow! That's quite a fortunate layout to land on by accident. Almost
makes me wonder if mmap_lock should be designed to do that, but it's
probably even better to just keep working on having a less contended
mmap_lock.

I *suppose* it's worth trying to keep this fragile layout in place,
but it is a landmine for anyone who touches mm_struct. And the struct
is so large already that I'm not sure a comment warning would even
be noticed. Anyway...

>> the patch, the 'mmap_lock' is pushed into one cacheline, which may
>> cause the regression.
>>
>> Below is the pahole info:
>>
>> - before the patch
>>
>> 	spinlock_t         page_table_lock;      /*   116     4 */
>> 	struct rw_semaphore mmap_lock;           /*   120    40 */
>> 	/* --- cacheline 2 boundary (128 bytes) was 32 bytes ago --- */
>> 	struct list_head   mmlist;               /*   160    16 */
>> 	long unsigned int  hiwater_rss;          /*   176     8 */
>>
>> - after the patch
>>
>> 	spinlock_t         page_table_lock;      /*   124     4 */
>> 	/* --- cacheline 2 boundary (128 bytes) --- */
>> 	struct rw_semaphore mmap_lock;           /*   128    40 */
>> 	struct list_head   mmlist;               /*   168    16 */
>> 	long unsigned int  hiwater_rss;          /*   184     8 */
>>
>> perf c2c log can also confirm this.
>   
> We've tried some patch, which can restore the regerssion. As the
> newly added member 'write_protect_seq' is 4 bytes long, and putting
> it into an existing 4 bytes long hole can restore the regeression,
> while not affecting most of other member's alignment. Please review
> the following patch, thanks!
> 

So, this is a neat little solution, if we agree that it's worth "fixing".

I'm definitely on the fence, but leaning toward, "go for it", because
I like the "no cache effect" result of using up the hole.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> - Feng
> 
>  From 85ddc2c3d0f2bdcbad4edc5c392c7bc90bb1667e Mon Sep 17 00:00:00 2001
> From: Feng Tang <feng.tang@intel.com>
> Date: Fri, 4 Jun 2021 15:20:57 +0800
> Subject: [PATCH RFC] mm: relocate 'write_protect_seq' in struct mm_struct
> 
> Before commit 57efa1fe5957 ("mm/gup: prevent gup_fast from
> racing with COW during fork), on 64bits system, the hot member
> rw_semaphore 'mmap_lock' of 'mm_struct' could be separated into
> 2 cachelines, that its member 'count' sits in one cacheline while
> all other members in next cacheline, this naturally reduces some
> cache bouncing, and with the commit, the 'mmap_lock' is pushed
> into one cacheline, as shown in the pahole info:
> 
>   - before the commit
> 
> 	spinlock_t         page_table_lock;      /*   116     4 */
> 	struct rw_semaphore mmap_lock;           /*   120    40 */
> 	/* --- cacheline 2 boundary (128 bytes) was 32 bytes ago --- */
> 	struct list_head   mmlist;               /*   160    16 */
> 	long unsigned int  hiwater_rss;          /*   176     8 */
> 
>   - after the commit
> 
> 	spinlock_t         page_table_lock;      /*   124     4 */
> 	/* --- cacheline 2 boundary (128 bytes) --- */
> 	struct rw_semaphore mmap_lock;           /*   128    40 */
> 	struct list_head   mmlist;               /*   168    16 */
> 	long unsigned int  hiwater_rss;          /*   184     8 */
> 
> and it causes one 9.2% regression for 'mmap1' case of will-it-scale
> benchmark[1], as in the case 'mmap_lock' is highly contented (occupies
> 90%+ cpu cycles).
> 
> Though relayouting a structure could be a double-edged sword, as it
> helps some case, but may hurt other cases. So one solution is the
> newly added 'seqcount_t' is 4 bytes long (when CONFIG_DEBUG_LOCK_ALLOC=n),
> placing it into an existing 4 bytes hole in 'mm_struct' will not
> affect most of other members's alignment, while restoring the
> regression.
> 
> [1]. https://lore.kernel.org/lkml/20210525031636.GB7744@xsang-OptiPlex-9020/
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
>   include/linux/mm_types.h | 15 ++++++++-------
>   1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 5aacc1c..5b55f88 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -445,13 +445,6 @@ struct mm_struct {
>   		 */
>   		atomic_t has_pinned;
>   
> -		/**
> -		 * @write_protect_seq: Locked when any thread is write
> -		 * protecting pages mapped by this mm to enforce a later COW,
> -		 * for instance during page table copying for fork().
> -		 */
> -		seqcount_t write_protect_seq;
> -
>   #ifdef CONFIG_MMU
>   		atomic_long_t pgtables_bytes;	/* PTE page table pages */
>   #endif
> @@ -480,7 +473,15 @@ struct mm_struct {
>   		unsigned long stack_vm;	   /* VM_STACK */
>   		unsigned long def_flags;
>   
> +		/**
> +		 * @write_protect_seq: Locked when any thread is write
> +		 * protecting pages mapped by this mm to enforce a later COW,
> +		 * for instance during page table copying for fork().
> +		 */
> +		seqcount_t write_protect_seq;
> +
>   		spinlock_t arg_lock; /* protect the below fields */
> +
>   		unsigned long start_code, end_code, start_data, end_data;
>   		unsigned long start_brk, brk, start_stack;
>   		unsigned long arg_start, arg_end, env_start, env_end;
>