linux-kernel - Re: [PATCH 1/3] smp: Process pending softirqs in flush_smp_call_function_from

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Message-ID: <20210201193504.oxceui2y52zsxqmv@linutronix.de>
Date:   Mon, 1 Feb 2021 20:35:04 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 1/3] smp: Process pending softirqs in
 flush_smp_call_function_from_idle()

On 2021-01-23 21:10:25 [+0100], To linux-block@...r.kernel.org wrote:
> send_call_function_single_ipi() may wake an idle CPU without sending an
> IPI. The woken up CPU will process the SMP-functions in
> flush_smp_call_function_from_idle(). Any raised softirq from within the
> SMP-function call will not be processed.
> Should the CPU have no tasks assigned, then it will go back to idle with
> pending softirqs and the NOHZ will rightfully complain.
> 
> Process pending softirqs on return from flush_smp_call_function_queue().
> 
> Fixes: b2a02fc43a1f4 ("smp: Optimize send_call_function_single_ipi()")
> Reported-by: Jens Axboe <axboe@...nel.dk>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

A gentle ping.
This isn't just a requirement for the series: rps_trigger_softirq() is
invoked from smp_call_function_single_async() and raises a softirq.

Sebastian