linux-kernel - Re: [PATCH] latency improvement in __smp_call_single

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Date:   Thu, 24 Sep 2020 10:42:15 +0200
From:   peterz@...radead.org
To:     George Prekas <prekageo@...zon.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Xu <peterx@...hat.com>,
        Kaitao Cheng <pilgrimtao@...il.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] latency improvement in __smp_call_single_queue

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?