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

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Message-ID: <874lrlzjpo.fsf@vitty.brq.redhat.com>
Date:   Fri, 29 Sep 2017 16:02:27 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

Peter Zijlstra <peterz@...radead.org> writes:

> 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.
>> 

(purely teoretical thought) what I meant to say is in tlb_remove_table()
we may try to get new batch from some pre-allocated (on boot) pool and
revert to __get_free_page() when it's empty. This may make sense
combined with the next idea, allocating more than 1 page.

>> > 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.

Of course, but the question is: what's cheaper -- try to alloc e.g. 8
pages or do 8 smp_call_function() calls?

But adding such complexity to the code would require a good
justification, of course.

-- 
  Vitaly