Re: [PATCH] perf/core: Fix hung issue on perf stat command during cpu hotplug

[kajoljain <kjain@linux.ibm.com>]

On 8/27/20 12:17 PM, Kajol Jain wrote:
> 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 )
> 
> Patch here removes the tertiary condition added as part of that
> commit and added a check for NULL and -EAGAIN.

Hi Peter,
     Please let me know if you have any comment on this patch.

Thanks,
Kajol Jain
> 
> Fixes: 2ed6edd33a21("perf: Add cond_resched() to task_function_call()")
> Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
> Reported-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> Reviewed-by: Barret Rhoden <brho@google.com>
> Tested-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
>  kernel/events/core.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> Changelog:
> - Remove RFC tag
> - Resolve some nits issues like space after if and
>   added -ENXIO in comment msg of function 'task_function_call'
>   as suggested by Barret Rhoden.
> 
> Link to the RFC: https://lkml.org/lkml/2020/8/26/896
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 5bfe8e3c6e44..cef646084198 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -99,7 +99,7 @@ static void remote_function(void *data)
>   * retry due to any failures in smp_call_function_single(), such as if the
>   * task_cpu() goes offline concurrently.
>   *
> - * returns @func return value or -ESRCH when the process isn't running
> + * returns @func return value or -ESRCH or -ENXIO when the process isn't running
>   */
>  static int
>  task_function_call(struct task_struct *p, remote_function_f func, void *info)
> @@ -115,7 +115,8 @@ 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)
> +			ret = data.ret;
>  
>  		if (ret != -EAGAIN)
>  			break;
>