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

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Date:   Wed, 13 Sep 2023 21:49:07 +0900
From:   Suleiman Souhlal <suleiman@...gle.com>
To:     Peter Zijlstra <peterz@...radead.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>
Subject: NOHZ interaction between IPI-less kick_ilb() and nohz_csd_func().

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