linux-kernel - Re: [RFC 2/2] timers: Make sure irq_work is handled when no pending timers

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Message-ID: <875yyae018.ffs@nanos.tec.linutronix.de>
Date:   Sat, 19 Jun 2021 10:22:43 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     linux-rt-users@...r.kernel.org, frederic@...nel.org,
        mtosatti@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC 2/2] timers: Make sure irq_work is handled when no pending timers

On Sat, Jun 19 2021 at 00:47, Thomas Gleixner wrote:
> On Thu, Jun 10 2021 at 14:59, Nicolas Saenz Julienne wrote:
> As we since then made work queues RT aware and it's possible to queue
> work from the irq_work IPI context, the obvious solution is to delegate
> this to a work queue.
>
> If we do a proper analysis of the affected irq work callbacks then this
> probably makes a lot of sense independent of RT. There are only a few
> really urgent irq work items which need to be handled immediately in the
> IPI.
>
> RT is special, but as we have demonstrated over time it's not _that_
> special. It just needs a proper analysis and a real good argument why
> something has to be special for RT and does not fit into the common
> case. Or to demonstrate that the common case approach of 'do it right
> away' is pointless or even harmfull.

I skimmed most of the ~40 irq_work instances.

Most of them have no urgency at all. And out of those non-urgent cases
the majority does not even have the requirement to run on the current
CPU, so they can be pushed off to a global work queue which moves them
away from NOHZ full CPUs completely.

That has nothing to do with RT, that's a benefit in general.

Thanks,

        tglx