Subject: Re: [PATCH] net: rps: fix data stall after hotplug

From: [email-masked]
Date: Mon, 23 Mar 2015 22:16:12 -0000

>> On Thu, 2015-03-19 at 14:50 -0700, Eric Dumazet wrote:
>>
>>> Are you seeing this race on x86 ?
>>>
>>> If IPI are not reliable on your arch, I am guessing you should fix
>>> them.
>>>
>>> Otherwise, even without hotplug you'll have hangs.
>>
>> Please try instead this patch :
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index
>> 5d43e010ef870a6ab92895297fe18d6e6a03593a..baa4bff9a6fbe0d77d7921865c038060cb5efffd
>> 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -4320,9 +4320,8 @@ static void net_rps_action_and_irq_enable(struct
>> softnet_data *sd)
>>  		while (remsd) {
>>  			struct softnet_data *next = remsd->rps_ipi_next;
>>
>> -			if (cpu_online(remsd->cpu))
>> -				smp_call_function_single_async(remsd->cpu,
>> -							   &remsd->csd);
>> +			smp_call_function_single_async(remsd->cpu,
>> +						       &remsd->csd);
>>  			remsd = next;
>>  		}
>>  	} else
>>
>>
> Thanks for the patch Eric. We are seeing this race on ARM.
> I will try this and update.
>

Unfortunately, I am not able to reproduce data stall now with or without
the patch. Could you tell me more about the patch and what issue you were
suspecting?

Based on the code, it looks like we BUG out on our arch if we try to call
an IPI on an offline CPU. Since this condition is never hit, I feel that
the IPI might not have failed.

void smp_send_reschedule(int cpu)
{
        BUG_ON(cpu_is_offline(cpu));
        smp_cross_call_common(cpumask_of(cpu), IPI_RESCHEDULE);
}

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