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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <85f7c7c5-2ab0-d5df-45a7-989be1286ef9@amazon.com>
Date:   Thu, 24 Sep 2020 10:04:43 -0500
From:   George Prekas <prekageo@...zon.com>
To:     <peterz@...radead.org>
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 9/24/2020 3:42 AM, peterz@...radead.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