linux-kernel - Re: NOHZ interaction between IPI-less kick_ilb() and nohz_csd

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

Message-ID: <CABCjUKCRXeT2eExm6+y7cZCX4yEbj5MbjFnCHv4xvRPL5ce13g@mail.gmail.com>
Date:   Wed, 13 Sep 2023 23:31:49 +0900
From:   Suleiman Souhlal <suleiman@...gle.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Vineeth Pillai <vineethrp@...gle.com>,
        Youssef Esmat <youssefesmat@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: NOHZ interaction between IPI-less kick_ilb() and nohz_csd_func().

(I forgot to also add Vincent...)

On Wed, Sep 13, 2023 at 9:49 PM Suleiman Souhlal <suleiman@...gle.com> wrote:
>
> Hello,
>
> I noticed that on x86 machines that have MWAIT, with NOHZ, when the
> kernel decides to kick the idle load balance on another CPU in
> kick_ilb(), there's an optimization that makes it avoid using an IPI
> and instead exploit the fact that the remote CPU is MWAITing on the
> thread_info flags, by just setting TIF_NEED_RESCHED, in
> call_function_single_prep_ipi().
> However, on the remote CPU, in nohz_csd_func(), we end up not raising
> the sched softirq due to NEED_RESCHED being set, so the ILB doesn't
> end up getting done.
>
> Is this intended?
>
> Thanks,
> -- Suleiman