Re: [PATCH/RFC] core/nfsd: allow kernel threads to use task_work.

[Chuck Lever]

On Tue, Nov 28, 2023 at 09:05:21AM +1100, NeilBrown wrote:
> 
> I have evidence from a customer site of 256 nfsd threads adding files to
> delayed_fput_lists nearly twice as fast they are retired by a single
> work-queue thread running delayed_fput().  As you might imagine this
> does not end well (20 million files in the queue at the time a snapshot
> was taken for analysis).
> 
> While this might point to a problem with the filesystem not handling the
> final close efficiently, such problems should only hurt throughput, not
> lead to memory exhaustion.

I have this patch queued for v6.8:

https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/commit/?h=nfsd-next&id=c42661ffa58acfeaf73b932dec1e6f04ce8a98c0


> For normal threads, the thread that closes the file also calls the
> final fput so there is natural rate limiting preventing excessive growth
> in the list of delayed fputs.  For kernel threads, and particularly for
> nfsd, delayed in the final fput do not impose any throttling to prevent
> the thread from closing more files.

I don't think we want to block nfsd threads waiting for files to
close. Won't that be a potential denial of service?


> A simple way to fix this is to treat nfsd threads like normal processes
> for task_work.  Thus the pending files are queued for the thread, and
> the same thread finishes the work.
> 
> Currently KTHREADs are assumed never to call task_work_run().  With this
> patch that it still the default but it is implemented by storing the
> magic value TASK_WORKS_DISABLED in ->task_works.  If a kthread, such as
> nfsd, will call task_work_run() periodically, it sets ->task_works
> to NULL to indicate this.
> 
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> 
> I wonder which tree this should go through assuming everyone likes it.
> VFS maybe??
> 
> Thanks.
> 
>  fs/file_table.c           | 2 +-
>  fs/nfsd/nfssvc.c          | 4 ++++
>  include/linux/sched.h     | 1 +
>  include/linux/task_work.h | 4 +++-
>  kernel/fork.c             | 2 +-
>  kernel/task_work.c        | 7 ++++---
>  6 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/file_table.c b/fs/file_table.c
> index de4a2915bfd4..e79351df22be 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -445,7 +445,7 @@ void fput(struct file *file)
>  	if (atomic_long_dec_and_test(&file->f_count)) {
>  		struct task_struct *task = current;
>  
> -		if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
> +		if (likely(!in_interrupt())) {
>  			init_task_work(&file->f_rcuhead, ____fput);
>  			if (!task_work_add(task, &file->f_rcuhead, TWA_RESUME))
>  				return;
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index 66ca50b38b27..c047961262ca 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -13,6 +13,7 @@
>  #include <linux/fs_struct.h>
>  #include <linux/swap.h>
>  #include <linux/siphash.h>
> +#include <linux/task_work.h>
>  
>  #include <linux/sunrpc/stats.h>
>  #include <linux/sunrpc/svcsock.h>
> @@ -941,6 +942,7 @@ nfsd(void *vrqstp)
>  	}
>  
>  	current->fs->umask = 0;
> +	current->task_works = NULL; /* Declare that I will call task_work_run() */
>  
>  	atomic_inc(&nfsdstats.th_cnt);
>  
> @@ -955,6 +957,8 @@ nfsd(void *vrqstp)
>  
>  		svc_recv(rqstp);
>  		validate_process_creds();
> +		if (task_work_pending(current))
> +			task_work_run();
>  	}
>  
>  	atomic_dec(&nfsdstats.th_cnt);
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 292c31697248..c63c2bedbf71 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1117,6 +1117,7 @@ struct task_struct {
>  	unsigned int			sas_ss_flags;
>  
>  	struct callback_head		*task_works;
> +#define	TASK_WORKS_DISABLED	((void*)1)
>  
>  #ifdef CONFIG_AUDIT
>  #ifdef CONFIG_AUDITSYSCALL
> diff --git a/include/linux/task_work.h b/include/linux/task_work.h
> index 795ef5a68429..3c74e3de81ed 100644
> --- a/include/linux/task_work.h
> +++ b/include/linux/task_work.h
> @@ -22,7 +22,9 @@ enum task_work_notify_mode {
>  
>  static inline bool task_work_pending(struct task_struct *task)
>  {
> -	return READ_ONCE(task->task_works);
> +	struct callback_head *works = READ_ONCE(task->task_works);
> +
> +	return works && works != TASK_WORKS_DISABLED;
>  }
>  
>  int task_work_add(struct task_struct *task, struct callback_head *twork,
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..903b29804fe1 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2577,7 +2577,7 @@ __latent_entropy struct task_struct *copy_process(
>  	p->dirty_paused_when = 0;
>  
>  	p->pdeath_signal = 0;
> -	p->task_works = NULL;
> +	p->task_works = args->kthread ? TASK_WORKS_DISABLED : NULL;
>  	clear_posix_cputimers_work(p);
>  
>  #ifdef CONFIG_KRETPROBES
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 95a7e1b7f1da..ffdf4b0d7a0e 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -49,7 +49,8 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>  
>  	head = READ_ONCE(task->task_works);
>  	do {
> -		if (unlikely(head == &work_exited))
> +		if (unlikely(head == &work_exited ||
> +			     head == TASK_WORKS_DISABLED))
>  			return -ESRCH;
>  		work->next = head;
>  	} while (!try_cmpxchg(&task->task_works, &head, work));
> @@ -157,7 +158,7 @@ void task_work_run(void)
>  		work = READ_ONCE(task->task_works);
>  		do {
>  			head = NULL;
> -			if (!work) {
> +			if (!work || work == TASK_WORKS_DISABLED) {
>  				if (task->flags & PF_EXITING)
>  					head = &work_exited;
>  				else
> @@ -165,7 +166,7 @@ void task_work_run(void)
>  			}
>  		} while (!try_cmpxchg(&task->task_works, &work, head));
>  
> -		if (!work)
> +		if (!work || work == TASK_WORKS_DISABLED)
>  			break;
>  		/*
>  		 * Synchronize with task_work_cancel(). It can not remove
> -- 
> 2.42.1
> 

-- 
Chuck Lever