linux-kernel - Re: [PATCH v4 01/11] powerpc/mm: Adds counting method to monitor lockless pgtable walks

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Message-ID: <673bcb94b7752e086cc4133fb6cceb24394c02c0.camel@linux.ibm.com>
Date:   Mon, 30 Sep 2019 15:42:42 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     John Hubbard <jhubbard@...dia.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, kvm-ppc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        YueHaibing <yuehaibing@...wei.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Keith Busch <keith.busch@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Paul Mackerras <paulus@...ba.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Allison Randal <allison@...utok.net>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Ganesh Goudar <ganeshgr@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v4 01/11] powerpc/mm: Adds counting method to monitor
 lockless pgtable walks

On Mon, 2019-09-30 at 10:57 -0700, John Hubbard wrote:
> > As I told before, there are cases where this function is called from
> > 'real mode' in powerpc, which doesn't disable irqs and may have a
> > tricky behavior if we do. So, encapsulate the irq disable in this
> > function can be a bad choice.
> 
> You still haven't explained how this works in that case. So far, the
> synchronization we've discussed has depended upon interrupt disabling
> as part of the solution, in order to hold off page splitting and page
> table freeing.

The irqs are already disabled by another mechanism (hw): MSR_EE=0.
So, serialize will work as expected.

> Simply skipping that means that an additional mechanism is required...which
> btw might involve a new, ppc-specific routine, so maybe this is going to end
> up pretty close to what I pasted in after all...
> > Of course, if we really need that, we can add a bool parameter to the
> > function to choose about disabling/enabling irqs.
> > > * This is really a core mm function, so don't hide it away in arch layers.
> > >     (If you're changing mm/ files, that's a big hint.)
> > 
> > My idea here is to let the arch decide on how this 'register' is going
> > to work, as archs may have different needs (in powerpc for example, we
> > can't always disable irqs, since we may be in realmode).
> > 
> > Maybe we can create a generic function instead of a dummy, and let it
> > be replaced in case the arch needs to do so.
> 
> Yes, that might be what we need, if it turns out that ppc can't use this
> approach (although let's see about that).
> 

I initially used the dummy approach because I did not see anything like
serialize in other archs. 

I mean, even if I put some generic function here, if there is no
function to use the 'lockless_pgtbl_walk_count', it becomes only a
overhead.

> 
> thanks,

Thank you!

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