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

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Message-ID: <20181017002601.GB12996@lenoir>
Date:   Wed, 17 Oct 2018 02:26:02 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Pavan Kondeti <pkondeti@...eaurora.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 Pavan,

On Tue, Oct 16, 2018 at 09:45:52AM +0530, Pavan Kondeti wrote:
> 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?

No, Ksoftirqd is treated the same as IRQ tail processing here: a vector can
interrupt another. So for example, a NET_RX softirq running in Ksoftirqd can
be interrupted by a TIMER softirq running in hardirq tail.

> 
> (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.

Exactly, and that's inherent with interrupts in general. The only way to work
around that is to thread each vector independantly but that's a whole different
dimension :-)

Thanks!