Re: [RFC] perf/core: Fixes hung issue on perf stat command during cpu hotplug

[Arnaldo Carvalho de Melo]

Em Wed, Aug 26, 2020 at 03:02:36PM +0530, Kajol Jain escreveu:
> Commit 2ed6edd33a21 ("perf: Add cond_resched() to task_function_call()")
> added assignment of ret value as -EAGAIN in case function
> call to 'smp_call_function_single' fails.
> For non-zero ret value, it did 
> 'ret = !ret ? data.ret : -EAGAIN;', which always
> assign -EAGAIN to ret and make second if condition useless.
> 
> In scenarios like when executing a perf stat with --per-thread option, and 
> if any of the monitoring cpu goes offline, the 'smp_call_function_single'
> function could return -ENXIO, and with the above check,
> task_function_call hung and increases CPU
> usage (because of repeated 'smp_call_function_single()')
> 
> Recration scenario:
> 	# perf stat -a --per-thread && (offline a CPU )

Peter, this is kernel stuff, can you take a look?

- Arnaldo
 
> Patch here removes the tertiary condition added as part of that 
> commit and added a check for NULL and -EAGAIN.
> 
> Fixes: 2ed6edd33a21("perf: Add cond_resched() to task_function_call()")
> Signed-off-by: Kajol Jain <kjain@xxxxxxxxxxxxx>
> Reported-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/events/core.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 5bfe8e3c6e44..330c53f7df9c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -115,9 +115,9 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info)
>  	for (;;) {
>  		ret = smp_call_function_single(task_cpu(p), remote_function,
>  					       &data, 1);
> -		ret = !ret ? data.ret : -EAGAIN;
> -
> -		if (ret != -EAGAIN)
> +		if (!ret)
> +			ret = data.ret;
> +		else if (ret != -EAGAIN)
>  			break;
>  
>  		cond_resched();
> -- 
> 2.26.2
> 

-- 

- Arnaldo