On 9/24/2020 3:42 AM, pet...@infradead.org wrote:
> On Wed, Sep 23, 2020 at 10:00:41AM -0500, George Prekas wrote:
>> If an interrupt arrives between llist_add and
>> send_call_function_single_ipi in the following code snippet, then the
>> remote CPU will not receive the IPI in a timely manner and subsequent
>> SMP calls even from other CPUs for other functions will be delayed:
>>
>> if (llist_add(node, &per_cpu(call_single_queue, cpu)))
>> send_call_function_single_ipi(cpu);
>>
>> Note: llist_add returns 1 if it was empty before the operation.
>>
>> CPU 0 | CPU 1 | CPU 2
>> __smp_call_single_q(2,f1) | __smp_call_single_q(2,f2) |
>> llist_add returns 1 | |
>> interrupted | llist_add returns 0 |
>> ... | branch not taken |
>> ... | |
>> resumed | |
>> send_call_function_single_ipi | |
>> | | f1
>> | | f2
>>
>> The call from CPU 1 for function f2 will be delayed because CPU 0 was
>> interrupted.
>
> Do you happen to have any actual numbers and a use-case where this was
> relevant?
Hi Peter,
I encountered this problem while developing a device driver that used
smp_call_function_single to communicate with other cores. I observed
latency spikes and after investigation I figured out the problem
described above.
I have written a simple device driver to validate the above fix. It does
smp_call_function_single and measures the latency. I can post it here if
it is appropriate. The latency impact is equal to the duration of the
CPU 0's interruption.
--
George
