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Date: Fri, 29 Sep 2017 15:13:29 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kernel test robot <xiaolong.ye@intel.com>,
        Ingo Molnar <mingo@kernel.org>, Juergen Gross <jgross@suse.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Jork Loeser <Jork.Loeser@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, lkp@01.org
Subject: Re: [lkp-robot] [x86/mm]  9e52fc2b50:  will-it-scale.per_thread_ops
 -16% regression
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On Fri, Sep 29, 2017 at 02:24:03PM +0200, Vitaly Kuznetsov wrote:
> 1) In case the system is under extreme memory pressure and
> __get_free_page() is failing in tlb_remove_table() we'll be doing
> smp_call_function() for _each_ call (avoiding batching). We may want to
> have a pre-allocated pool.

MMU_GATHER_BUNDLE should avoid it being for _every_ call.

Also, note that tlb_gather is preemptible, so pre-alloc is 'difficult'
and you will run out, esp. when memory is right.

> 2) The default MAX_TABLE_BATCH is static (it is equal to the number of
> pointer we can fit into one page - sizeof(struct mmu_table_batch) ==
> 509), we may want to adjust it for very big systems.

That would then put more stress on the memory allocator because you're
then asking for higher order pages.