linux-kernel - Re: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment

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Date:   Fri, 14 Sep 2018 16:07:14 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     will.deacon@....com, aneesh.kumar@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, npiggin@...il.com,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux@...linux.org.uk,
        heiko.carstens@...ibm.com,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment

On Fri, 14 Sep 2018 15:02:07 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, Sep 14, 2018 at 12:28:24PM +0200, Martin Schwidefsky wrote:
> 
> > I spent some time to get s390 converted to the common mmu_gather code.
> > There is one thing I would like to request, namely the ability to
> > disable the page gather part of mmu_gather. For my prototype patch
> > see below, it defines the negative HAVE_RCU_NO_GATHER_PAGES Kconfig
> > symbol that if defined will remove some parts from common code.
> > Ugly but good enough for the prototype to convey the idea.
> > For the final solution we better use a positive Kconfig symbol and
> > add that to all arch Kconfig files except for s390.  
> 
> In a private thread ealier Linus raised the point that the batching and
> freeing of lots of pages at once is probably better for I$.

That would be something to try. For now I would like to do a conversion
that more or less preserves the old behavior. You know these pesky TLB
related bugs..

> > +config HAVE_RCU_NO_GATHER_PAGES
> > +	bool  
> 
> I have a problem with the name more than anything else; this name
> suggests it is the RCU table freeing that should not batch, which is not
> the case, you want the regular page gather gone, but very much require
> the RCU table gather to batch.
> 
> So I would like to propose calling it:
> 
> config HAVE_MMU_GATHER_NO_GATHER
> 
> Or something along those lines.
 
Imho a positive config option like HAVE_MMU_GATHER_PAGES would make the
most sense. It has the downside that it needs to be added to all
arch/*/Kconfig files except for s390. 

But I am not hung-up on a name, whatever does not sound to awful will do
for me. HAVE_MMU_GATHER_NO_GATHER would be ok.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.