linux-kernel - Re: [lkp-robot] [x86/mm] 9e52fc2b50: will-it-scale.per_thread

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Date:   Fri, 29 Sep 2017 15:16:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...capital.net>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...org
Subject: Re: [lkp-robot] [x86/mm]  9e52fc2b50:  will-it-scale.per_thread_ops
 -16% regression

On Fri, Sep 29, 2017 at 03:13:29PM +0200, Peter Zijlstra wrote:
> 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.

My bad, that's only for pages, not tables :/

> 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.