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Subject: Re: [RFC v2] perf/core: Fixes hung issue on perf stat command during
 cpu hotplug
To:     Kajol Jain <kjain@linux.ibm.com>, acme@kernel.org,
        peterz@infradead.org
Cc:     jolsa@redhat.com, linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org, maddy@linux.ibm.com,
        mingo@redhat.com, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, namhyung@kernel.org,
        daniel@iogearbox.net, srikar@linux.vnet.ibm.com
References: <20200826145411.489169-1-kjain@linux.ibm.com>
From:   Barret Rhoden <brho@google.com>
Message-ID: <19fac9f1-4662-b695-3bf3-60ddd7133594@google.com>
Date:   Wed, 26 Aug 2020 11:14:25 -0400
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Hi -

On 8/26/20 10:54 AM, 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.
> 
> 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>
> ---
>   kernel/events/core.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Changelog:
> - Remove addition of else in the first patch for
>    if(ret != -EAGAIN) condition.
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 856d98c36f56..fe104fee097a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -115,8 +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)
                   ^
Minor nit, you need a space after the if.

Also, the comment for the function says it returns @func's return val or 
-ESRCH.  You could also add -ENXIO to that.

Thanks for the fix.

Reviewed-by: Barret Rhoden <brho@google.com>

> +			ret = data.ret;
>   		if (ret != -EAGAIN)
>   			break;
>   
>