Re: perf: add cond_resched() to task_function_call()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 14, 2020 at 03:03:51PM -0400, Barret Rhoden wrote:
> Under rare circumstances, task_function_call() can repeatedly fail and
> cause a soft lockup.
> 
> There is a slight race where the process is no longer running on the cpu
> we targeted by the time remote_function() runs.  The code will simply
> try again.  If we are very unlucky, this will continue to fail, until a
> watchdog fires.  This can happen in a heavily loaded, multi-core virtual
> machine.

Sigh,.. virt again :/

> Reported-by: syzbot+bb4935a5c09b5ff79940@syzkaller.appspotmail.com
> Signed-off-by: Barret Rhoden <brho@google.com>
> ---
>  kernel/events/core.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 55e44417f66d..65c2c05e24c2 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -99,7 +99,7 @@ static void remote_function(void *data)
>   *
>   * returns: @func return value, or
>   *	    -ESRCH  - when the process isn't running
> - *	    -EAGAIN - when the process moved away
> + *	    -ENXIO  - when the cpu the process was on has gone offline
>   */

Hurm.. I don't think that was actually intended behaviour. As long as
the task lives we ought to retry. Luckily I don't think the current code
cares much, it'll loop again on the caller side.

With the exception of perf_cgroup_attach() that is, that might actually
be broken because of this.

>  static int
>  task_function_call(struct task_struct *p, remote_function_f func, void *info)
> @@ -112,11 +112,15 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info)
>  	};
>  	int ret;
>  
> -	do {
> -		ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);
> +	while (1) {
> +		ret = smp_call_function_single(task_cpu(p), remote_function,
> +					       &data, 1);
>  		if (!ret)
>  			ret = data.ret;
> -	} while (ret == -EAGAIN);
> +		if (ret != -EAGAIN)
> +			break;
> +		cond_resched();
> +	}

So how about we make that:

	for (;;) {
		ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);
		ret = !ret ? data.ret : -EAGAIN;

		if (ret != -EAGAIN)
			break;

		cond_resched();
	}

Or something like that, hmmm?