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

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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