linux-kernel - Re: [PATCH V2] mm/cma: using per-CMA locks to improve concurrent allocation performance

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Message-ID: <CAGsJ_4yobqDst0wwLAqgADa=q0OyUu5-U6iMYWbaQq6GyixPwQ@mail.gmail.com>
Date: Mon, 10 Feb 2025 16:44:58 +1300
From: Barry Song <21cnbao@...il.com>
To: yangge1116@....com
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, david@...hat.com, baolin.wang@...ux.alibaba.com, 
	aisheng.dong@....com, liuzixing@...on.cn
Subject: Re: [PATCH V2] mm/cma: using per-CMA locks to improve concurrent
 allocation performance

On Mon, Feb 10, 2025 at 2:56 PM <yangge1116@....com> wrote:
>
> From: yangge <yangge1116@....com>
>
> For different CMAs, concurrent allocation of CMA memory ideally should not
> require synchronization using locks. Currently, a global cma_mutex lock is
> employed to synchronize all CMA allocations, which can impact the
> performance of concurrent allocations across different CMAs.
>
> To test the performance impact, follow these steps:
> 1. Boot the kernel with the command line argument hugetlb_cma=30G to
>    allocate a 30GB CMA area specifically for huge page allocations. (note:
>    on my machine, which has 3 nodes, each node is initialized with 10G of
>    CMA)
> 2. Use the dd command with parameters if=/dev/zero of=/dev/shm/file bs=1G
>    count=30 to fully utilize the CMA area by writing zeroes to a file in
>    /dev/shm.
> 3. Open three terminals and execute the following commands simultaneously:
>    (Note: Each of these commands attempts to allocate 10GB [2621440 * 4KB
>    pages] of CMA memory.)
>    On Terminal 1: time echo 2621440 > /sys/kernel/debug/cma/hugetlb1/alloc
>    On Terminal 2: time echo 2621440 > /sys/kernel/debug/cma/hugetlb2/alloc
>    On Terminal 3: time echo 2621440 > /sys/kernel/debug/cma/hugetlb3/alloc
>
> We attempt to allocate pages through the CMA debug interface and use the
> time command to measure the duration of each allocation.
> Performance comparison:
>              Without this patch      With this patch
> Terminal1        ~7s                     ~7s
> Terminal2       ~14s                     ~8s
> Terminal3       ~21s                     ~7s
>
> To slove problem above, we could use per-CMA locks to improve concurrent
> allocation performance. This would allow each CMA to be managed
> independently, reducing the need for a global lock and thus improving
> scalability and performance.
>
> Signed-off-by: yangge <yangge1116@....com>

An allocation from one CMA region should not be blocked by an allocation from
another CMA region, especially since we may have multiple CMA regions or
even per-NUMA CMA regions.

Reviewed-by: Barry Song <baohua@...nel.org>

> ---
>
> V2:
> - update code and message suggested by Barry.
>
>  mm/cma.c | 7 ++++---
>  mm/cma.h | 1 +
>  2 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/mm/cma.c b/mm/cma.c
> index 34a4df2..a0d4d2f 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -34,7 +34,6 @@
>
>  struct cma cma_areas[MAX_CMA_AREAS];
>  unsigned int cma_area_count;
> -static DEFINE_MUTEX(cma_mutex);
>
>  static int __init __cma_declare_contiguous_nid(phys_addr_t base,
>                         phys_addr_t size, phys_addr_t limit,
> @@ -175,6 +174,8 @@ static void __init cma_activate_area(struct cma *cma)
>
>         spin_lock_init(&cma->lock);
>
> +       mutex_init(&cma->alloc_mutex);
> +
>  #ifdef CONFIG_CMA_DEBUGFS
>         INIT_HLIST_HEAD(&cma->mem_head);
>         spin_lock_init(&cma->mem_head_lock);
> @@ -813,9 +814,9 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>                 spin_unlock_irq(&cma->lock);
>
>                 pfn = cmr->base_pfn + (bitmap_no << cma->order_per_bit);
> -               mutex_lock(&cma_mutex);
> +               mutex_lock(&cma->alloc_mutex);
>                 ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA, gfp);
> -               mutex_unlock(&cma_mutex);
> +               mutex_unlock(&cma->alloc_mutex);
>                 if (ret == 0) {
>                         page = pfn_to_page(pfn);
>                         break;
> diff --git a/mm/cma.h b/mm/cma.h
> index df7fc62..41a3ab0 100644
> --- a/mm/cma.h
> +++ b/mm/cma.h
> @@ -39,6 +39,7 @@ struct cma {
>         unsigned long   available_count;
>         unsigned int order_per_bit; /* Order of pages represented by one bit */
>         spinlock_t      lock;
> +       struct mutex alloc_mutex;
>  #ifdef CONFIG_CMA_DEBUGFS
>         struct hlist_head mem_head;
>         spinlock_t mem_head_lock;
> --
> 2.7.4
>
>

Thanks
Barry