linux-kernel - Re: [RFC PATCH 29/30] softirq: Make softirq processing softinterruptible

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Message-ID: <20181016041552.GA27587@codeaurora.org>
Date:   Tue, 16 Oct 2018 09:45:52 +0530
From:   Pavan Kondeti <pkondeti@...eaurora.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "David S . Miller" <davem@...emloft.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>
Subject: Re: [RFC PATCH 29/30] softirq: Make softirq processing
 softinterruptible

Hi Frederic,

On Thu, Oct 11, 2018 at 01:12:16AM +0200, Frederic Weisbecker wrote:
> From: Frederic Weisbecker <fweisbec@...il.com>
> 
> Make do_softirq() re-entrant and allow a vector, being either processed
> or disabled, to be interrupted by another vector. This way a vector
> won't be able to monopolize the CPU for a long while at the expense of
> the others that may rely on some predictable latency, especially on
> softirq disabled sections that used to disable all vectors.
> 
I understand that a long running softirq can be preempted/interrupted by
other softirqs which is not possible today. I have few questions on your
patches.

(1) When softirq processing is pushed to ksoftirqd, then the long running
softirq can still block other softirqs (not in SOFTIRQ_NOW_MASK) for a while.
correct?

(2) When softirqs processing happens asynchronously, a particular softirq
like TASKLET can keep interrupting an already running softirq like TIMER/NET_RX,
correct? In worse case scenario, a long running softirq like NET_RX interrupt
a TIMER softirq. But I guess this is something expected with this. i.e
each softirq is independent and whichever comes recent gets to interrupt the
previously running softirqs.

Thanks,
Pavan

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