linux-kernel - Re: [PATCH v3] do_wait: make PIDTYPE

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Date:   Wed, 10 Mar 2021 16:40:57 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Jim Newsome <jnewsome@...project.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] do_wait: make PIDTYPE_PID case O(1) instead of O(n)

Jim Newsome <jnewsome@...project.org> writes:

> do_wait is an internal function used to implement waitpid, waitid,
> wait4, etc. To handle the general case, it does an O(n) linear scan of
> the thread group's children and tracees.
>
> This patch adds a special-case when waiting on a pid to skip these scans
> and instead do an O(1) lookup. This improves performance when waiting on
> a pid from a thread group with many children and/or tracees.
>
> Signed-off-by: James Newsome <jnewsome@...project.org>
> ---
>  kernel/exit.c | 53 +++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 43 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 04029e35e69a..c2438d4ba262 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1439,9 +1439,34 @@ void __wake_up_parent(struct task_struct *p, struct task_struct *parent)
>  			   TASK_INTERRUPTIBLE, p);
>  }
>  
> +// Optimization for waiting on PIDTYPE_PID. No need to iterate through child
> +// and tracee lists to find the target task.

Minor nit:  C++ style comments look very out of place in this file
            which uses old school C /* */ comment delimiters for
            all of it's block comments.
                      
> +static int do_wait_pid(struct wait_opts *wo)
> +{
> +	struct task_struct *target = pid_task(wo->wo_pid, PIDTYPE_PID);
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This is subtle change in behavior.

Today on the task->children list we only place thread group leaders.

Which means that your do_wait_pid wait for thread of someone else's
process and that is a change in behavior.

So the code either needs a thread_group_leader filter on target before
the ptrace=0 case or we need to use "pid_task(wo->wo_pid, PIDTYPE_TGID)"
and "pid_task(wo->wo_pid, PIDTYPE_PID)" for the "ptrace=1" case.

I would like to make thread_group_leaders go away so I would favor two
pid_task calls.  But either will work right now.

Eric

                                     
> +	int retval;
> +
> +	if (!target)
> +		return 0;
> +	if (current == target->real_parent ||
> +	    (!(wo->wo_flags & __WNOTHREAD) &&
> +	     same_thread_group(current, target->real_parent))) {
> +		retval = wait_consider_task(wo, /* ptrace= */ 0, target);
> +		if (retval)
> +			return retval;
> +	}
> +	if (target->ptrace && (current == target->parent ||
> +			       (!(wo->wo_flags & __WNOTHREAD) &&
> +				same_thread_group(current, target->parent)))) {
> +		retval = wait_consider_task(wo, /* ptrace= */ 1, target);
> +		if (retval)
> +			return retval;
> +	}
> +	return 0;
> +}
> +
>  static long do_wait(struct wait_opts *wo)
>  {
> -	struct task_struct *tsk;
>  	int retval;
>  
>  	trace_sched_process_wait(wo->wo_pid);
> @@ -1463,19 +1488,27 @@ static long do_wait(struct wait_opts *wo)
>  
>  	set_current_state(TASK_INTERRUPTIBLE);
>  	read_lock(&tasklist_lock);
> -	tsk = current;
> -	do {
> -		retval = do_wait_thread(wo, tsk);
> -		if (retval)
> -			goto end;
>  
> -		retval = ptrace_do_wait(wo, tsk);
> +	if (wo->wo_type == PIDTYPE_PID) {
> +		retval = do_wait_pid(wo);
>  		if (retval)
>  			goto end;
> +	} else {
> +		struct task_struct *tsk = current;
>  
> -		if (wo->wo_flags & __WNOTHREAD)
> -			break;
> -	} while_each_thread(current, tsk);
> +		do {
> +			retval = do_wait_thread(wo, tsk);
> +			if (retval)
> +				goto end;
> +
> +			retval = ptrace_do_wait(wo, tsk);
> +			if (retval)
> +				goto end;
> +
> +			if (wo->wo_flags & __WNOTHREAD)
> +				break;
> +		} while_each_thread(current, tsk);
> +	}
>  	read_unlock(&tasklist_lock);
>  
>  notask: