linux-kernel - Re: [patch 12/19] softirq: Add RT specific softirq accounting

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Date:   Thu, 19 Nov 2020 23:52:27 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        linux-um@...ts.infradead.org, Russell King <linux@...linux.org.uk>,
        Marc Zyngier <maz@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch 12/19] softirq: Add RT specific softirq accounting

On Thu, Nov 19, 2020 at 07:34:13PM +0100, Thomas Gleixner wrote:
> On Thu, Nov 19 2020 at 13:18, Frederic Weisbecker wrote:
> > On Fri, Nov 13, 2020 at 03:02:19PM +0100, Thomas Gleixner wrote:
> >> RT requires the softirq to be preemptible and uses a per CPU local lock to
> >> protect BH disabled sections and softirq processing. Therefore RT cannot
> >> use the preempt counter to keep track of BH disabled/serving.
> >> 
> >> Add a RT only counter to task struct and adjust the relevant macros in
> >> preempt.h.
> >
> > You may want to describe a bit the reason for this per task counter.
> > It's not intuitive at this stage.
> 
> Something like this:
> 
>  RT requires the softirq processing and local bottomhalf disabled regions
>  to be preemptible. Using the normal preempt count based serialization is
>  therefore not possible because this implicitely disables preemption.
> 
>  RT kernels use a per CPU local lock to serialize bottomhalfs. As
>  local_bh_disable() can nest the lock can only be acquired on the
>  outermost invocation of local_bh_disable() and released when the nest
>  count becomes zero. Tasks which hold the local lock can be preempted so
>  its required to keep track of the nest count per task.
> 
>  Add a RT only counter to task struct and adjust the relevant macros in
>  preempt.h.
> 
> Thanks,

Very good, thanks!