Re: [PATCH 01/10] mm, pcp: avoid to drain PCP when process exit

[Mel Gorman]

On Wed, Sep 20, 2023 at 02:18:47PM +0800, Huang Ying wrote:
> In commit f26b3fa04611 ("mm/page_alloc: limit number of high-order
> pages on PCP during bulk free"), the PCP (Per-CPU Pageset) will be
> drained when PCP is mostly used for high-order pages freeing to
> improve the cache-hot pages reusing between page allocation and
> freeing CPUs.
> 
> But, the PCP draining mechanism may be triggered unexpectedly when
> process exits.  With some customized trace point, it was found that
> PCP draining (free_high == true) was triggered with the order-1 page
> freeing with the following call stack,
> 
>  => free_unref_page_commit
>  => free_unref_page
>  => __mmdrop
>  => exit_mm
>  => do_exit
>  => do_group_exit
>  => __x64_sys_exit_group
>  => do_syscall_64
> 
> Checking the source code, this is the page table PGD
> freeing (mm_free_pgd()).  It's a order-1 page freeing if
> CONFIG_PAGE_TABLE_ISOLATION=y.  Which is a common configuration for
> security.
> 
> Just before that, page freeing with the following call stack was
> found,
> 
>  => free_unref_page_commit
>  => free_unref_page_list
>  => release_pages
>  => tlb_batch_pages_flush
>  => tlb_finish_mmu
>  => exit_mmap
>  => __mmput
>  => exit_mm
>  => do_exit
>  => do_group_exit
>  => __x64_sys_exit_group
>  => do_syscall_64
> 
> So, when a process exits,
> 
> - a large number of user pages of the process will be freed without
>   page allocation, it's highly possible that pcp->free_factor becomes
>   > 0.
> 
> - after freeing all user pages, the PGD will be freed, which is a
>   order-1 page freeing, PCP will be drained.
> 
> All in all, when a process exits, it's high possible that the PCP will
> be drained.  This is an unexpected behavior.
> 
> To avoid this, in the patch, the PCP draining will only be triggered
> for 2 consecutive high-order page freeing.
> 
> On a 2-socket Intel server with 224 logical CPU, we tested kbuild on
> one socket with `make -j 112`.  With the patch, the build time
> decreases 3.4% (from 206s to 199s).  The cycles% of the spinlock
> contention (mostly for zone lock) decreases from 43.6% to 40.3% (with
> PCP size == 361).  The number of PCP draining for high order pages
> freeing (free_high) decreases 50.8%.
> 
> This helps network workload too for reduced zone lock contention.  On
> a 2-socket Intel server with 128 logical CPU, with the patch, the
> network bandwidth of the UNIX (AF_UNIX) test case of lmbench test
> suite with 16-pair processes increase 17.1%.  The cycles% of the
> spinlock contention (mostly for zone lock) decreases from 50.0% to
> 45.8%.  The number of PCP draining for high order pages
> freeing (free_high) decreases 27.4%.  The cache miss rate keeps 0.3%.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

However, I want to note that batching on exit is not necessarily
unexpected. For processes that are multi-TB in size, the time to exit
can actually be quite large and batching is of benefit but optimising
for exit is rarely a winning strategy. The pattern of "all allocs on CPU
B and all frees on CPU B" or "short-lived tasks triggering a premature
drain" is a bit more compelling but not worth a changelog rewrite.
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 4106fbc5b4b3..64d5ed2bb724 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -676,12 +676,15 @@ enum zone_watermarks {
>  #define high_wmark_pages(z) (z->_watermark[WMARK_HIGH] + z->watermark_boost)
>  #define wmark_pages(z, i) (z->_watermark[i] + z->watermark_boost)
>  
> +#define	PCPF_PREV_FREE_HIGH_ORDER	0x01
> +

The meaning of the flag and its intent should have been documented.

-- 
Mel Gorman
SUSE Labs