linux-kernel - Re: [PATCH v3 2/2] kernel/fork: group allocation/free of per-cpu counters for mm struct

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Date:   Wed, 23 Aug 2023 23:28:43 -0700
From:   Dennis Zhou <dennis@...nel.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     linux-kernel@...r.kernel.org, tj@...nel.org, cl@...ux.com,
        akpm@...ux-foundation.org, shakeelb@...gle.com,
        vegard.nossum@...cle.com, linux-mm@...ck.org
Subject: Re: [PATCH v3 2/2] kernel/fork: group allocation/free of per-cpu
 counters for mm struct

On Wed, Aug 23, 2023 at 07:06:09AM +0200, Mateusz Guzik wrote:
> A trivial execve scalability test which tries to be very friendly
> (statically linked binaries, all separate) is predominantly bottlenecked
> by back-to-back per-cpu counter allocations which serialize on global
> locks.
> 
> Ease the pain by allocating and freeing them in one go.
> 
> Bench can be found here:
> http://apollo.backplane.com/DFlyMisc/doexec.c
> 
> $ cc -static -O2 -o static-doexec doexec.c
> $ ./static-doexec $(nproc)
> 
> Even at a very modest scale of 26 cores (ops/s):
> before:	133543.63
> after:	186061.81 (+39%)
> 
> While with the patch these allocations remain a significant problem,
> the primary bottleneck shifts to page release handling.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>

Same message as for 1/2. I'm happy with this, just a minor reflow. I'll
take this for-6.6 unless there are other comments / objections to that.

I'll run a few tests myself too tomorrow just for validation.

Reviewed-by: Dennis Zhou <dennis@...nel.org>

Thanks,
Dennis

> ---
>  kernel/fork.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index d2e12b6d2b18..4f0ada33457e 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -909,8 +909,6 @@ static void cleanup_lazy_tlbs(struct mm_struct *mm)
>   */
>  void __mmdrop(struct mm_struct *mm)
>  {
> -	int i;
> -
>  	BUG_ON(mm == &init_mm);
>  	WARN_ON_ONCE(mm == current->mm);
>  
> @@ -925,9 +923,8 @@ void __mmdrop(struct mm_struct *mm)
>  	put_user_ns(mm->user_ns);
>  	mm_pasid_drop(mm);
>  	mm_destroy_cid(mm);
> +	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
>  
> -	for (i = 0; i < NR_MM_COUNTERS; i++)
> -		percpu_counter_destroy(&mm->rss_stat[i]);
>  	free_mm(mm);
>  }
>  EXPORT_SYMBOL_GPL(__mmdrop);
> @@ -1252,8 +1249,6 @@ static void mm_init_uprobes_state(struct mm_struct *mm)
>  static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	struct user_namespace *user_ns)
>  {
> -	int i;
> -
>  	mt_init_flags(&mm->mm_mt, MM_MT_FLAGS);
>  	mt_set_external_lock(&mm->mm_mt, &mm->mmap_lock);
>  	atomic_set(&mm->mm_users, 1);
> @@ -1301,17 +1296,14 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	if (mm_alloc_cid(mm))
>  		goto fail_cid;
>  
> -	for (i = 0; i < NR_MM_COUNTERS; i++)
> -		if (percpu_counter_init(&mm->rss_stat[i], 0, GFP_KERNEL_ACCOUNT))
> -			goto fail_pcpu;
> +	if (percpu_counter_init_many(mm->rss_stat, 0, GFP_KERNEL_ACCOUNT, NR_MM_COUNTERS))
> +		goto fail_pcpu;
>  
>  	mm->user_ns = get_user_ns(user_ns);
>  	lru_gen_init_mm(mm);
>  	return mm;
>  
>  fail_pcpu:
> -	while (i > 0)
> -		percpu_counter_destroy(&mm->rss_stat[--i]);
>  	mm_destroy_cid(mm);
>  fail_cid:
>  	destroy_context(mm);
> -- 
> 2.41.0
>