linux-kernel - Re: [PATCH 10/10] softirq: Remove __ARCH_SET_SOFTIRQ

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Date:   Tue, 3 Apr 2018 10:42:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        Helge Deller <deller@....de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "James E . J . Bottomley" <jejb@...isc-linux.org>,
        heiko.carstens@...ibm.com
Subject: Re: [PATCH 10/10] softirq: Remove __ARCH_SET_SOFTIRQ_PENDING

On Tue, Apr 03, 2018 at 07:52:25AM +0200, Martin Schwidefsky wrote:
> On Thu, 29 Mar 2018 20:08:36 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Thu, Mar 29, 2018 at 04:53:43PM +0200, Martin Schwidefsky wrote:
> > > The lowcore optimization for softirq_pending field is not really needed,
> > > just nice to have. But if there is a strong reason to make a common
> > > definition for it we can certainly do that.  
> > 
> > A slightly related question; would it make sense to move all kernel
> > static per-cpu stuff into lowcore, or is that asking for too much
> > trickery?
>  
> The space in lowcore is quite limited, for zArch the structure is 8K with
> many pre-defined fields. I fear that putting all of the static per-cpu
> stuff in there is too much. 
> 
> So far I used the lowcore as optimization for selected per-cpu fields
> which are performance relevant.

Fair enough; and yes 8k isn't much. Thanks for the info.