From: Rusty Russell <rusty@rustcorp.com.au>
To: Alex Shi <alex.shi@intel.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jan Beulich <JBeulich@suse.com>, borislav.petkov@amd.com, arnd@arndb.de,
        akinobu.mita@gmail.com, eric.dumazet@gmail.com, fweisbec@gmail.com,
        rostedt@goodmis.org, hughd@google.com, jeremy@goop.org,
        len.brown@intel.com, tony.luck@intel.com, yongjie.ren@intel.com,
        kamezawa.hiroyu@jp.fujitsu.com, seto.hidetoshi@jp.fujitsu.com,
        penberg@kernel.org, yinghai@kernel.org, tglx@linutronix.de,
        akpm@linux-foundation.org, ak@linux.intel.com, luto@mit.edu,
        avi@redhat.com, dhowells@redhat.com, mingo@redhat.com, riel@redhat.com,
        cpw@sgi.com, steiner@sgi.com, linux-kernel@vger.kernel.org,
        viro@zeniv.linux.org.uk, hpa@zytor.com
Subject: Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of siblings of SMT
In-Reply-To: <4FBDF5E5.7010806@intel.com>
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Date: Thu, 24 May 2012 21:05:36 +0930
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On Thu, 24 May 2012 16:48:37 +0800, Alex Shi <alex.shi@intel.com> wrote:
> On 05/24/2012 04:42 PM, Peter Zijlstra wrote:
> 
> > On Thu, 2012-05-24 at 16:32 +0800, Alex Shi wrote:
> >> So, I use cpumask_t in stack.
> > 
> > cpumask_t is 512 bytes with NR_CPUS=4096, that's generally considered
> > too big to be on stack.
> > 
> > A number of people spend a lot of time removing cpumask_t from stacks a
> > while ago, I'm very sure they'll not be happy if you're going to add it
> > back.
> 
> 
> In my testing, allocate a cpumask_var_t is more worse than cpumask_t.
> So, another choice is using percpu pre-allocatd cpumask for this, but I
> am wondering if it is acceptable. What's suggestion for this point?

Thanks for the ping Peter!

Please don't use cpus_ operations: they're deprecated.  Use cpumask_.
Similarly, avoid cpumask_t.

And yes, if you configure for thousands of CPUs, it's not free!  If it's
a significant, you will want to use a per-cpu cpumask_var_t.

My other thought: your patch seems optimal as far as avoiding IPIs goes,
but I wonder how often it folds down to a single CPU?  That case is
easier to fast-path without using a new cpumask.

Cheers,
Rusty.
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